Green Bioprocessing and Applications of Microalgae-derived Biopolymers

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Green bioprocessing and applications of microalgae-derived biopolymers
as a renewable feedstock: Circular bioeconomy approach
Anwesha Khanra, Shrasti Vasistha, Monika Prakash Rai, Wai
Yan Cheah, Kuan Shiong Khoo, Kit Wayne Chew, Lai Fatt Chuah, Pau
Loke Show
PII:
DOI:
Reference:
S2352-1864(22)00310-8
https://doi.org/10.1016/j.eti.2022.102872
ETI 102872
To appear in:
Environmental Technology & Innovation
Received date : 28 May 2022
Revised date : 28 July 2022
Accepted date : 8 August 2022
Please cite this article as: A. Khanra, S. Vasistha, M.P. Rai et al., Green bioprocessing and
applications of microalgae-derived biopolymers as a renewable feedstock: Circular bioeconomy
approach. Environmental Technology & Innovation (2022), doi:
https://doi.org/10.1016/j.eti.2022.102872.
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© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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A manuscript submitted to Environmental Technology and Innovation
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SI: Green Energy Technologies, Advanced Nanomaterials for Selective Detection and
Removal of Hazardous Materials in Industrial Residue
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Green Bioprocessing and Applications of Microalgae-derived Biopolymers as a
Renewable Feedstock: Circular Bioeconomy Approach
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Anwesha Khanraa, Shrasti Vasisthaa, Monika Prakash Raib*, Wai Yan Cheahc, Kuan Shiong
Khood, Kit Wayne Chewe, Lai Fatt Chuahf, Pau Loke Showg,h,i*
a
IMS School of Biosciences, IMS University Courses Campus, Ghaziabad.
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Amity Institute of Biotechnology, Amity University Uttar Pradesh, Noida, Sector 125, 201313
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c
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and Humanities, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan,
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Malaysia.
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d
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Malaysia.
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e
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Sunsuria, 43900 Sepang, Selangor, Malaysia.
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f
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g
Centre of Research in Development, Social and Environment (SEEDS), Faculty of Social Sciences
Faculty of Applied Sciences, UCSI University, UCSI Heights, 56000 Cheras, Kuala Lumpur,
School of Energy and Chemical Engineering, Xiamen University Malaysia, Jalan Sunsuria, Bandar
Malaysia Marine Department Northern Region, 11700, Gelugor, Penang, Malaysia.
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Zhejiang Provincial Key Laboratory for Subtropical Water Environment and Marine Biological
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Resources Protection, Wenzhou University, Wenzhou 325035, China.
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h
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602105.
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i
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University of Nottingham Malaysia.
Department of Chemical and Environmental Engineering, Faculty of Science and Engineering,
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*
Corresponding authors:
Professor Dr. Monika Prakash Rai: mprai@amity.edu
Amity Institute of Biotechnology, Amity University UttarPradesh, Noida, Sector 125, 201313
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Department of Sustainable Engineering, Saveetha School of Engineering, SIMATS, Chennai, India
Professor Ir. Ts. Dr. Pau Loke Show: PauLoke.Show@nottingham.edu.my
Zhejiang Provincial Key Laboratory for Subtropical Water Environment and Marine Biological
Resources Protection, Wenzhou University, Wenzhou 325035, China.
Department of Sustainable Engineering, Saveetha School of Engineering, SIMATS, Chennai, India
602105.
Department of Chemical and Environmental Engineering, Faculty of Science and Engineering,
University of Nottingham Malaysia, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia.
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Abstract
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which immediately requires utmost attention towards the replacement of prosaic petroleum-
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based polymer assimilation. Thus, greening the innovative route of microalgae derived
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biopolymers has attained significant interest as an improved and sustainable approach towards
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the worldwide circular bioeconomy. In this context, the use of as synthesized biopolymers from
8
microalgae attributing as a potential feedstock has bestowed for a biodegradable solution to
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reduce the greenhouse gases emission and rapid biomass productivity with metabolic
10
flexibility. However, the confront of high microalgae cultivation cost and low metabolites’
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accumulation have triggered the advancement of microalgae metabolic cultivation strategy.
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Hence, the current review portrays to propose a novel multi-phasic fed batch light depleted low-
13
cost wastewater cultivation approach and a clear mechanistic phenomenon for accelerating the
14
biopolymer production. This review also provides a comprehensive summary on several
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microalgae strains which are capable for biopolymer synthesis and various effective extraction
16
techniques to isolate the biopolymers. The future endeavour and challenges on the microalgae
17
circular bioeconomy which involves the current issues regarding the cell harvesting method,
18
scale up and bioprocessing cost of microalgae cultivation have been highlighted. The
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applications for microalgae derived biopolymer in industrial and nutraceutical sectors have also
20
been emphasized. This review is expected to bring new insights to the industrial stakeholders
21
for further advancement of microalgae-based biopolymer field economically, and eventually
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contributing towards environmental sustainability.
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Keywords: Biopolymer, Circular bioeconomy, Green bioprocessing, Microalgae, Multiphasic
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fed batch
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An expeditious pace of expanding industrialization and urbanization is a global vulnerability
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1.
Introduction
Global economy has created an alarming hurdle on the environment owing to the rapid
3
inflation of industrialization and urbanization throughout the entire World (Koul et al., 2022;
4
Koul & Taak, 2018). The world population is going to enhance in an incredible manner and
5
anticipated to put pressure for the supply of clean water, renewable energy and essential
6
bioproducts (Vyas et al., 2022). Hence, the metamorphic transition from such linear economy
7
towards circular bioeconomy is enlightening as a paramount necessary to embellish a
8
sustainable pathway for commercial production of valuable metabolites (Shah et al., 2022).
9
Therefore, in this quest, the photosynthetic green microalgae, having a vast metabolic
10
flexibility, maintain a synergy to remove the pollutants from wastewater and significantly
11
progresses the facile synthesis of high value compounds that are potentially applicable for
12
nutraceuticals, pharmaceuticals, cosmetics, and several other industrial purposes (Barati et al.,
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2021; Hussain et al., 2021; Jaiswal et al., 2022; Khanra et al., 2021; You et al., 2022). The
14
release of heavy metals, organic dyes, cleaning agents and radioactive reactants etc. from the
15
industries, is adversely affecting the ecosystem and biomagnified through the food chain
16
(Sharma et al., 2020; Singh et al., 2021). Hence, these incipient bottlenecks exhibit a critical
17
issue that will necessitate the limits on its usage, reusability, communal wisdom and feasibility,
18
for environmental sustainability. Therefore, addressing the alluring challenges in universal
19
water pollution and requirements, the technocrats are showing their intense awareness to
20
explore an innovative, economically viable, biodegradable, resilient, chemical free and energy
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efficient wastewater treatment technique by employing microalgae biomass (Cheah et al., 2018;
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Law et al., 2022; Mohsenpour et al., 2021; Purba et al., 2022).
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Oxygenic photosynthetic microalgae, generally utilize solar energy for converting the
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atmospheric CO2 into storage carbon and oxygen, treating as an ordinary carbon sink (Cheah et
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al., 2016; Cheah et al., 2015; Vasistha et al., 2021). The functional metabolism of microalgae
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is influenced by different trophic modes’ ability which are photoautotrophic, mixotrophic and
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heterotrophic, depending on their physicochemical stoichiometry and environmental growth
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condition (Khanra et al., 2021; Wang et al., 2022). Among them, organic carbon and light
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mediated mixotrophic cultivation affords an additional benefit to the overall microalgae system
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for boosting the biomass and synthesis of valuable co-products, over inherent photoautotrophy
6
(Khanra et al., 2021; Ma et al., 2022; Vasistha et al., 2021). However, the expensiveness of
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organic carbon persists as a major obstacle to the environmental community (Khanra et al.,
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2021). Henceforth, by considering this fact, an effort to make the entire bioprocessing more
9
feasible, the microalgae strains need to be grown in nutrients-rich wastewater for biomass
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enhancement and further the production of valuable resources (Vasistha et al., 2021). However,
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there are few limitations, for instance the nutrients bio-availability in wastewaters, presence of
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contaminants that may inhibit microalgae growth, presence of other organisms that predate
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microalgae, dark colour of wastewater limiting light penetration in the culture as well as
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presence of pathogenic bacteria. Microalgae is able to assimilate the nutrients and upsurge the
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pH of wastewater (Sharma et al., 2020). Phycoremediated effluent can also be applied in
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agricultural sector for irrigation. Nevertheless, the colour of the wastewater depends on its
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source thus proper dilution or pretreatment is usually required to eliminate the dark colour of
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wastewater prior to microalgae cultivation.
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Literatures depicted that the microalgae proliferation on several waste media lessens the
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cost of adding extra nutrients, constructing the entire process more advantageous in occasion
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of economic suitability (Goswami et al., 2022; Ummalyma & Singh, 2022). Though, it is well-
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known that microalgae are profoundly utilized to detoxify the organic and inorganic heavy
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metals present in the wastewater resources (Verma et al., 2016). However, if the wastewater
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contains heavy metals, phycoremediation of wastewater requires appropriate heavy metal
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removal. Hence, selection of wastewater and evaluation of toxic metals and carcinogenic
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substrates are required prior to wastewater phycoremediation or microalgae cultivation (Sharma
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et al., 2020). The effective range of surface area and pore size of these heavy metals could be
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usually accessible for the adsorption by microalgae due to their self-assembled and well-
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organized structural configuration (Devadas et al., 2021). This unique characteristic feature of
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microalgae cells allows them to develop specific biomolecule which could be used as a
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biocompatible adsorbent towards pollutants’ degradation for treating the wastewater efficiently.
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Moreover, microalgae are able to biofix the CO2 at 50 times more than the species of plants
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(Sharma et al., 2020). Microalgae have immense capability to generate an average of
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approximately 280 tons of dry cell biomass per 1 ha per year, providing that the solar radiation
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at 9% of the time duration. Microalgae strains can sequester 513 tons of CO2 during their
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growth. As flue gases comprise of CO2 of 3-30%, thus microalgae are promising to absorb such
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high concentrations of CO2.
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The energy consumption through our entire planet is growing by 2.3% since 2018, with
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an enhancing crisis for all fuel-based sources and among them, fossil fuel alone lights about
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70% of the consumption development. In view of that, the petrochemicals, particularly
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monomers aimed at the plastic based sector, are evolving as an attractive opinion to meet the
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global oil demand and along with that, for the production of carbon-neutral progress (Energy,
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2020). Previous literature depicted that the total plastic production has surpassed at around 8300
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million tonnes (MT) since 1950s, 380 MT in 2015, and at least 79% of which is even now
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existed in the natural atmosphere mainly oceans or landfill (Geyer et al., 2017). The
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mismanaged plastic wastes, including emergent pollutants such as micro and nano-plastics are
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demonstrated to be present in every conceivable environment affecting biodiversity, economic
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and human well-being through worldwide. Therefore, the alternative renewable resources are
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required for sustainable polymer production (Karan et al., 2019). Algae-based biopolymer
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production can address many of the issues as algal biomass production systems can be located
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in non-arable land or offshore marine farms (Devadas et al., 2021). Additionally, microalgae
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are also beneficial in several other applications in food industry, oil industry and wastewater
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remediation purpose etc. (Mohan et al., 2020). Apart from that, microalgae-derived
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polysaccharides are conventionally used in several parts of the entire world. Hence, we envision
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that the polymer, being as the substrates would be much more beneficial to deal with modern
6
aspect. Therefore, there is a promising need to address the utilization of environment-friendly
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polymer by enhancing the economic viability in microalgae based biorefineries, in
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amalgamation with wastewater born microalgae biomass towards biopolymer synthesis. This
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strategy could be able to maintain the overall environmental sustainability by reducing the cost
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and carbon emission for generating a circular bioeconomy approach.
Very recently, the craving for the production of biopolymers has appeared as one of the most
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fascinating larger biomolecules to be an exceptionally biodegradable, economical, reusable and
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green adsorbent which attributes to fulfil the daily life requirements (Devadas et al., 2021;
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Kartik et al., 2021; Yaashikaa et al., 2022). The market value of biopolymers is expected to
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enhance at least 17% annually up to 2022, reaching 7200 million USD (Banu et al., 2022). The
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major benefit is associated with their high structural and better chemical durability due to the
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arrangement of functional groups and bioavailability, if compared to chemically assimilated
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biopolymers. The conventional biopolymers used at present are derived from petrochemical
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feedstocks with plant-derived monomers, bio-mediated monomers in combination with
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polymers and the plant extracted biomass-based feedstocks (Devadas et al., 2021). However,
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the restraint of the above-mentioned first and second generation of biopolymers endure the
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usage of petrochemical-derived plastic blend, which could be able to determine for the
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fractional degradation of plastics into microplastics, addressing to cause an environmental
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destruction (Mal et al., 2022). Moreover, the third-generation of biopolymer assimilation
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emphasizes to exploit natural resources derived from plant-based material, so as to assure the
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easy degradation capability of such biopolymers without parting any bits of microplastics that
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could accrue in the environment. Generally, the natural sources arriving from terrestrial crops
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are utilized to generate the biopolymers, however, this approach is also not considered for a
4
sustainable solution towards long term applications, as food versus fuel dilemma persists. There
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is a compelling requirement for lands and huge consumption of nutrients in a great extent.
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Therefore, the microalgae strains are considered as a potential feedstock for biopolymer
7
production due to its high growth rate, high yield of products and ease of cultivation using even
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non-arable land itself. In addition, recent literatures have illustrated that the microalgae derived
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biopolymers are having a strong mechanical property for manifesting their tailored region,
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compared to petroleum-based polymer (Devadas et al., 2021; Lutzu et al., 2021). The as-
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synthesized biopolymers from microalgae grown green biomass improve an intermolecular
12
force to cooperate with the organic or inorganic hazardous metals, existed in the waste stream
13
(Lutzu et al., 2021). Thereby, the degradation rate of harsh contaminants using microalgae
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which could be able to ameliorate the environmental remediation. A cradle to grave approach
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for the comparison of fossil derived biodegradable polymer and microalgae derived
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biodegradable biopolymer has been elucidated in Fig 1.
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Fig 1 Overview of cradle to grave approach of fossil derived polymer vs microalgae
derived biopolymer
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The removal of contaminants from wastewater medium is frequently enhanced with hampering
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the microalgae growth. Therefore, there is a convincing need for a prospective cultivation
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strategy to improve the biopolymer synthesis. It is being anticipated that the manipulation of
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metabolic pathways of microalgae and coupling of cultivation system with wastewater
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effluents, rich in carbon and nitrogen sources will plausibly help to enhance the biopolymer
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content with significant cost reduction. Apart from the nutrient sources, light illumination is
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also a chief environmental influence which is equally accountable for variation in cell biomass
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increment and biopolymers’ production in microalgae (Khan et al., 2022). Earlier report
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suggested that a phase wise cultural strategy, has been observed as foremost for an expansion
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of biomolecule productivity under nutrient deplete and replete condition (Khanra et al., 2021).
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Hence, presently, a paradigm shift of existed bi-phasic cultivation strategy towards an
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impressive multiphasic culture is required to accumulate essential bioproducts without
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cooperating the microalgae growth. Previous report on multiphasic cultivation strategy has
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elucidated by availing nutrient limited and enriched conditions while, light radiation is also
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emphasizing as an effective environmental factor and driving force to regulate the modification
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of C/N ratio to trigger the biomass as well as biopolymer synthesis (Chew et al., 2019). C/N is
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regarded as a prime factor in wastewater resources, however, the manipulation of light supply
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with the modulation of carbon and nitrogen ratio (C/N) has not been discussed yet to trigger the
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biopolymer yield. Henceforth, this phenomenon has kindled us to provide a possible way for
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further improvement of biopolymer yield by involving such a novel phase-wise intermittent
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fed-batch cultivation strategy with simultaneous light attenuation approach.
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Therefore, our current review aimed to highlight the present state-of-art progress of the
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production of microalgae-derived biopolymers, via green processing and wastewater
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cultivation, for numerous applications by maintaining a circular bioeconomy aspect.
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0Furthermore, the challenges of large-scale biopolymer production issue and operating cost for
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designing the entire bioprocessing was also broadly illustrated to provide an insight into current
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knowledge and future direction in microalgal associated biopolymer accumulation.
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2.
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The upmost factor that provides microalgae as pivotal importance in biopolymer sector is its
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high biomass productivity, compared to other traditional food crops (Devadas et al., 2021).
7
Numerous microalgae strains are found capable for producing the biopolymers in a wide range.
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The characteristic features of few microalgae strains have been depicted below. Apart from
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that, an analysis of various microalgae strains and their efficiency in biopolymer production has
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illustrated in Table 1.
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2.1 Chlorella sp.
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Microalgae strains as potential factory for biopolymer production
Chlorella sp. is the most commonly recognized unicellular green microalgae have a fast
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growth rate and is designated as the oldest plant since 3.5 billion years in the entire planet
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(Almeida et al., 2017). The genus Chlorella sp. belongs to class trebouxiophyceae, order
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chlorellales, and family chlorellaceae. These particular small sized microalgae grow
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indigenously in fresh water habitats. Chlorella sp. is highly utilized in nutraceuticals as it
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contains the maximum amount of cellular protein content of up to 60%. Chlorella sp. can be
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efficiently used as a raw material for biopolymer synthesis as the protein content present in this
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particular candidate is considered to form a complex heteropolymer in contrast with long chain
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monomers of synthetic polymers (Chia et al., 2020). In contrast, Chlorella sp. comprise of rigid
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cell wall which is able to protect the anaerobic degradation by the other microbes (Kartik et al.,
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2021). Chlorella sp. blend exhibits a very strong tensile strength of approximately 35.1 kgf/cm2
23
which can be an essential resource for biopolymer synthesis. Furthermore, if the temperature
24
enhances, this particular microalga Chlorella sp. indicates to describe the three stages of
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degradation initially due to the existence of cysteine, present in the microalgae cell. Such
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degradation can be detected by the weight-loss of carbohydrates and proteins derived from
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microalgae.
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2.2 Nannochloropsis sp.
The genera Nannochloropsis has been evolved as one of the most popular microalgae
5
strains usually for bioenergy production, since decades ago. They are generally unicellular,
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planktonic green microalga belonging to order eustigmatales, class eustigmatophyceae, phylum
7
ocrophyta. This particular monoalgal is able to enhance their growth rate at pH above 8.0. As
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this genus is having maximum amount of lipid biomolecules (aprox. 37-60% DCW), hence,
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this is suitable for biofuel production. Apart from the lipid content, this microalga is
10
contributing an enhanced quantity of carbohydrates too (Ishika et al., 2021; Ma et al., 2016). In
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order to explore the physico-chemical properties, this potential alga contains maximum amount
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of carbon and oxygen that could plausibly helpful for pollutants’ degradation towards
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wastewater remediation. By considering the presence of such essential metabolites, we envisage
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the potential of Nannochloropsis sp. for feasible biopolymer production.
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2.3 Botryococcus brounii
Botryococcus brounii belongs to class chlorophyta, family Botryococcacee and order
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treboxiales. This strain possesses an efficient and strong heterotrophic metabolism for
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enhancing the production of lipids and hydrocarbons. This characteristic feature has addressed
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the great potential to be used as a requisite of raw materials towards the development of
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numerous bioproducts. Apart from that, this particular strain is having a great capacity for
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wastewater remediation (Hidalgo et al., 2015). An enhancement of hydrocarbon content present
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in this microalga emphasizes the existence of aliphatic biopolymer in the prospective cell walls
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of microalga. The secretion of such extracellular products plausibly helps during the milking
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pathway of biopolymer production. However, this monoalgal is containing less biomass
2
productivity in contrast to other microalgae strains. Hence, a cultivation strategy will be
3
developed for boosting the biomass as well as the essential bioproducts.
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2.4 Spirulina sp.
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Spirulina sp., most popularly known as blue green microalgae or cyanobacteria, belongs
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to the class cyanophyceae, family oscillatoriaceae which grows usually in fresh water habitat.
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This cyanobacterial strain is having an ideal distinguished feature of containing microscopically
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spiral morphometric behaviour with trichomes width (5-7µm). Spirulina sp. maintains high
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protein content ranging from 46-63% dry cell weight. Due to the small sized morphological
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nature, this particular strain does not require any extraction step for cell harvesting. Hence,
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Spirulina sp. can be utilized widely as a cost-effective feedstock. A very nominal amount of
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lipids and pigment have been found in this potential strain. Apart from these behavioural
13
approaches, this strain has the capacity to blend with other microalgae strains as well as the
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other compounds. Earlier literature demonstrated that Spirullina platensis mixed with polyvinyl
15
alcohol (PVA) attributed for the formation of a strong composite with efficient tensile strength
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(Kusrini et al., 2018a). This unique feature boosted the synthesis of biopolymers.
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Table 1. Different kinds of biopolymers derived from various microalgae strains and their
efficiency
Microalgae
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Biopolymer
Yield (%)
References
PHA
55
(Nishioka et al., 2001)
Dunaliella sp.
Exopolysaccharide
89
(Mishra & Jha, 2009)
-
(Johnsson & Steuer,
2018)
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Synechococcus sp
Scenesdesmus almeriensis
Starch based
biopolymer
Microalgae consortium
PHA
31
11
(Rahman et al., 2015)
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Nostoc
PHB
21.5
Chlorogloeopsis fritschii
PCC
P3HB
15
Nostoc muscorum
P(3HB-CO-3HV)
Spirulina
Poly butylene
succinate (PBS)
80
(Zhu et al., 2017)
PE
87
(Onen Cinar et al.,
2020)
70
(Saha et al., 2021)
-
(Sayin et al., 2020)
Poly urethane (PU)
Corallina elongata
Polylactide PLA
Cystoseria compressa
1
2
(Toh et al., 2008)
(Mallick et al., 2007)
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Chlorella
Galaxaura oblongata
31.4
(Haase et al., 2012)
3. Different types of microalgae derived biopolymers
The term ‘biopolymer’ and ‘biodegradable polymer’ are not conceptually identical. In
4
view of that, biopolymers are regarded as renewable, sustainable and natural one, whereas, the
5
biodegradable polymers are synthesized from their nature of biodegradability. The natural
6
biopolymers may or may not be able to biodegrade. The biodegradable capacity of biopolymer
7
depends on the raw materials’ utilization, temperature and other physico chemical parameters
8
used during the production process (Rai et al., 2021). Usually, the biopolymers synthesized
9
from numerous biological resources include proteins, polysaccharides, lipids, polyesters and
10
other secondary metabolites are used as natural. On another hand, the fossil derived renewable
11
polymer-based resources including poly caprolactone (PCL), polybutylene succinate (PBS) etc.
12
can easily be broken by the microalgae and hence, these are known as biodegradable polymers
13
(Muthuraj et al., 2018). According to Mathiot et al. 2019, it has been reported that starch,
14
constituted by polysaccharides, amylose, amylopectin etc. is also regarded as biodegradable
15
one, as they are assimilated from agricultural crops and microalgae strains (Mathiot et al.,
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2019). Among all the as-synthesized biopolymers from microalgae, PLA, PHB, and PBAT are
2
recognized as most common biodegradable, bio-based polymeric substances, generated from
3
cyanobacteria and microalgae by fermentation as well as post harvesting processing of entire
4
biomass (Mohan et al., 2022). A comprehensive elucidation of numerous kinds of biopolymers
5
have been depicted below.
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3.1 Microalgae polysaccharides
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In order to explore the typical conceptualization of microalgae biorefinery and waste
8
valorization, the de-oiled microalgae biomass can be utilized towards the extraction of
9
biopolymers followed after the removal of pigments and proteins (Rai et al., 2021). Hence, we
10
believe that such kind of proposed closed loop phenomenon of algae biorefinery could be able
11
to maintain a circular bioeconomy approach by revolutionizing and decarbonizing the
12
microalgae biomass.
13
3.1.1 Cellulose
Cellulose and hemicellulose are known as structural based polysaccharides, existed in
15
several microalgae strains. In view of that, around 47.5% of cellulosic substances have been
16
studied in Chlorella vulgaris (Mohan et al., 2022) and likewise, Nannochloropsis gaditana
17
existed the cellulose materials at around 80% of the total biomass content (Scholz et al., 2014).
18
Further, the cellulose nanocrystals, assimilated from de-fatted biomass of marine microalga
19
Dunaliella tertiolecta were originated to become an actual nucleating content that acted as a
20
potential biofiller. Though the extraction of microalgae cellulosic materials are quite incipient,
21
but the microalgae biorefinery concept gives better hope for the lessening of environmental
22
effects and energy strength for producing cellulose in compared with prevailing industrialized
23
progressions (Mondal et al., 2021; Ross et al., 2021). However, earlier literatures illustrated that
24
the microalgae derived cellulose and hemicellulose have lower crystallinity capacity (≤92%) in
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contrast with Cladophora (≤95%), which has demonstrated for the maximum crystallinity index
2
(Lee et al., 2017; Mihranyan, 2011; Ross et al., 2021).
3
3.1.2 Starch
An extensively dispersed polysaccharide, present in most of the microalgae cells,
5
mainly Chlorophycean, the starch content in microalgae can surpass 50%, used for bioethanol
6
production after starch assimilation under nutrient deplete or replete condition (da Maia et al.,
7
2020). Furthermore, the marine microalga Tetraselmis subcordiformis also produced about
8
54.3% of starch content under nutrients’ restricted condition with less irradiance (Yao et al.,
9
2012). On the other hand, the green microalga Chlorella sorokiniana utilized starch content as
10
a prime energy storage under nitrogen stressed condition. In mixotrophic mode of cultivation,
11
the starch level has been reached about 27% after 2 days of cellular inoculation (da Maia et al.,
12
2020; Mathiot et al., 2019). The report of Mathiot et al., 2019, it was indicated that the
13
accumulation of starch-based substrates in microalgae were further scaled up for
14
commercialization and biomass production was demonstrated for high plasticization capability
15
by mixing with glycerol (Mathiot et al., 2019).
16
3.1.3 Extracellular polymeric substances
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Extracellular Polymeric Substances (EPS) are recognized as biopolymers of high
18
molecular weight, assimilated by microalgae with some procedures like excretion, secretion
19
and cell lysis. Microalgae derived EPS usually contains lipids, polysaccharides, proteins, and
20
small amounts of DNA etc. (Babiak & Krzemińska, 2021). EPS generally protect the
21
microalgae cells from the environmental limitations and hence, plausibly they help for biofilm
22
production, and assist as energy reserves during nutrients’ depleted condition. EPS also
23
contribute as a habitat and energy resource towards lively microscopic society, surrounding by
24
microalgae known as phycosphere (Ramanan et al., 2015; Ramanan et al., 2016; Xiao & Zheng,
25
2016). The assimilation of EPS is based on abundant features like temperature, nitrogen and
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light illumination. However, the constitution of EPS, present in the cell matrix perturb partially
2
the structural conformation and integrity of the matrix (Babiak & Krzemińska, 2021; Xiao &
3
Zheng, 2016). But still, EPS is behaving as an essential component in biogranulation process
4
which can be used for wastewater remediation. On the other hand, EPS is also attributed for the
5
development of valuable co-products that could be applied several pharmaceutical,
6
nutraceutical and health sectors (Xiao & Zheng, 2016).
7
4.
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of
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Biosynthesis of bio-based polymeric substances
The term “microalgae-derived biopolymer” is produced from natural origin, the
9
microalgae. Biopolymer can be identified as amphiphiles, long adhesion proteins, extracellular
10
proteins, amyloids, extracellular polysaccharides, membrane vesicles, nucleic acids,
11
lipopolysaccharides, filamentous phages, glycoproteins, capsular polysaccharides, and pili
12
(Banu et al., 2019; Mohan et al., 2022). Microalgae are present in anaerobic and aerobic
13
granular sludge system which could help in wastewater treatment analysis. Therefore, we can
14
say that microalgae as renewable feedstock for protein biomolecules and plethora of active
15
functional groups constitute the biopolymer and as a great candidate for wastewater
16
remediation.
17
3.1 Basic routes for Biopolymer Synthesis
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As per the literatures’ survey, it has been investigated that there are basically three
19
routes opted for the synthesis of biopolymers (Bellini et al., 2022; Jose et al., 2022; Umesh et
20
al., 2022). The biosynthetic routes for the biopolymers are depicted in Fig 2. Route No.1
21
illustrates the synthesis of biopolymeric products by using the microbes fermented with green
22
microalgae biomass. On the other hand, route No.2 involves the production of biopolymeric
23
based substances by the cell factories inside each microalgae cell. Further, the blending of
24
microalgae biomass with other additives attributes the biopolymer assimilation addressing as
25
the route No.3. Apart from the basic routes for biopolymer synthesis, the microalgae derived
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enzymes are also used to transit the microalgal oriented green biomass towards biopolymer
2
production.
3
Route No.1: Fermentation of microalgae biomass using other microbial strains
4
At present, the fragmentation of microalgae biomass is feasible for the extraction of several
5
bio-based metabolites including carbohydrates, proteins, lipids and other extracellular products,
6
prior to fermentation process. The use of subcritical hydrothermal process with water enabled
7
to decompose the microalgae biomass and subsequently fermentation for the production of
8
polyhydroalkanoates (PHA). Comparatively, the PHA, generated from crude biomass led
9
almost 77.8% mg/g, while, PHA, developed from extracted starch or cellulose were designated
10
as 5.1% and 3% respectively (Steinbruch et al., 2020). Henceforth, it was analysed that the
11
fermentation of microalgae biomass is efficiently better in contrast of using microalgae
12
fragments.
13
Route No.2: Utilization of microalgae biomass as a substrate for biopolymer enrichment
14
The role of microalgae cultivation for biopolymer synthesis is essential to promote cellular
15
biopolymer production. The microalgae cultivation has been regulated by using two
16
consecutively linked stages which include nitrogen deplete and replete condition (Costa et al.,
17
2018). The first stage is operated in continuous cultivation mode under nitrogen enrichment
18
habitat which accelerates the microalgae cell density. Moreover, the second stage deals with
19
the nitrogen limited condition and at this phase, microalgae cells trigger the biopolymer
20
production. As we all know that the light irradiation is implemented as a prime source for
21
microalgae growth as well as the production of essential by-products. The modulation in light
22
regime and light intensity can lead to boost the specific organic biomolecules including
23
carbohydrates, proteins fats that are treated as the major precursor for biopolymer synthesis.
24
Literature depicted that the rate of PHB has been enhanced by providing the light intensity at
25
around 28µmol/m2/s and xylose to the cultivation medium (Cassuriaga et al., 2018). However,
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recently, the gamma irradiation indicates the improvement of material properties and eco-
2
friendly in nature. Further, UV radiation has also been implemented to trigger the biopolymer
3
assimilation (Kartik et al., 2021). The starch mediated biopolymer was synthesized by using
4
UV radiation. Generally, the free radicals governed by UV radiation react with the starch-based
5
molecules to create the formation of crosslink chains. Therefore, with UV irradiation, the
6
technocrats can process to accumulate the biopolymers with their significant characteristics.
7
Route No.3: Hybrid of microalgae-biopolymer
8
The blending of biopolymer and microalgae demonstrates the third path of biopolymer
9
production. Compression is the most useful technology to produce the hybrid of wherein, the
10
microalgae and other additives are to be places in moulds and then compressed (Ciapponi et al.,
11
2019). Apart from this, another commonly available technology is regarded as solvent casting
12
by which the combination of microalgae and the other chemicals are to be dissolved using a
13
solvent and further dried on surface to create the film production. Kusrini et al. (2018b) reported
14
that the amalgamation of polyvinylalcohol (PVA) and microalgae onto a glass plate which was
15
then air dried for 24 h to procedure biopolymeric films. Onen Cinar et al. (2020) has clarified
16
the detailing about the techniques for making microalgae-biopolymer blends for evaluating the
17
features of composite polymers.
19
20
21
22
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Fig 2. Basic biosynthetic route of biopolymer production
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1
5.
Biopolymer extraction technologies
2
5.1 Solvent mediated extraction techniques
A variety of organic solvents have been opted for the production of biopolymers from
4
green microalgae biomass (Roja et al., 2019). Despite of necessitating other chemical reagents,
5
the solvent-based extraction process is relatively easier and entails less downstream processing,
6
in contrast with complete fermentation. The optimization of physicochemical parameters could
7
be able to lead towards the enhancement of biopolymers. A comparative analysis of various
8
extraction techniques and solvents used in microalgae strains and the efficiency has been
9
illustrated in Table 2. It could be seen that the commonly used solvents are sodium chloride,
10
sodium hypochlorite, methanol and glycerol. Sodium hypochlorite and chloroform are shown
11
effective in extracting PHB, with performance up to 60% being reported. The extraction process
12
relies deeply on microalgae species itself, several mechanical actions like sifting, filtration and
13
centrifugation (Faidi et al., 2019). To overcome the tediousness of unit operations, microalgae
14
are usually screened on the performance of the bioproducts yield, to eliminate the excess
15
experimentation and the time spent. The most promising microalgae will be selected for the
16
production of biopolymers. Total of six microalgae strains to analyse their performance on
17
biopolymer synthesis yield. The cyanobacterial strains including Nostoc sp. Synechocystis sp.
18
and Porphyridium purpureum addressed the biopolymer yield proficiency at around 323, 204,
19
and 83 mg/L, consecutively (Morales-Jiménez et al., 2020).
20
21
Table 2: The extraction efficiency of biopolymer from microalgae strains using different
extraction techniques and solvents
Extraction
methods
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Microalgae
strains
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Chlorella sp.
Chlorella fusca
Nostoc sp.
Solvent
extraction
Solvent
extraction
Solvent
extraction
Solvents
Solvent
cost
($)/kg*
-
Types of
biopolymers
Efficiency
(%)
References
PHB
63%
Methanol/H2SO4
0.85
PHA
17.4%
NaCl/glycerol
0.6
PHB
47.5%
(Kumar et
al., 2020)
(Cassuriaga
et al., 2018)
(Kartik et
al., 2021)
NaOCl, CHCl3
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Solvent
extraction
Spirulina
sp. Solvent
LEB 18
extraction
Scenedesmus sp. Solvent
extraction
Synechococcus
Solvent
elongates
extraction,
microwave
extraction
Synechocystis sp. Solvent
extraction
Synechococcus
subsalsus
Synecocystis sp.
PCC6803
Solvent
extraction
Solvent
extraction
NaCl/glycerol
0.6
PHB
12.4%
NaOCl/Methanol
0.85
PHA
12%
-
PHB
60%
Organic solvents
1.69
PHA
7.02%
NaCl/glycerol
0.6
PHB
30%
NaOCl/Methanol
0.85
PHA
16%
Combination of
organic solvents
2.69
P3HB
38%
NaOCl, CHCl3
(MoralesJiménez et
al., 2020)
(Costa et
al., 2018)
(Kumar et
al., 2020)
(Costa et
al., 2018)
repro
of
Porphyrodium
sp.
*Prices above are estimated based on prices given by companies
[-hexane: Roshan Chemical Industries (Chennai, India)
-methanol: Shandong Baovi Energy Technology Co., ltd. (Shandong Chaina)
-glycerol: Shandong Baovi Energy Technology Co., ltd. (Shandong Chaina)]
6
5.2 Microwave abetted extraction
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1
2
3
4
5
(MoralesJiménez et
al., 2020)
(Costa et
al., 2018)
(Das
&
Maiti,
2021)
Microwave assisted extraction technique is regarded as a green viewpoint to improve
8
the essential products from green biomass. This process can be utilized to separate the
9
biopolymers present inside each of the microalgae cell. There are numerous advantages
10
associated with such microwave mediated extraction, that can be accredited due to the effect of
11
microwave irradiation on ions and dipoles (Mirzadeh et al., 2020; Ponthier et al., 2020). The
12
necessary benefits are depicted as (i) rapid and uniform process; (ii) less experimental times;
13
(iii) short use of solvents and (iv) no need of labour cost. The microwave aided extraction of
14
hybrid carrageenan biopolymer from the red algae named Mastocarpus stellatus, which
15
occasioned for heightened extraction yields (Ponthier et al., 2020). The optimized
16
circumstances for determined biopolymer yields were designated as 150 ℃ for 6 min. The
17
increase of temperature endorsed the effectiveness of biopolymers. Apart from that, the
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industrial benefit of microwave-based extraction is related to the absence of syneresis of
2
biopolymeric gels. Further exploring on the biopolymer synthesis through electromagnetic
3
waves and progress few novel ways to accomplish the high productivity and techno economic
4
competency, are potentially promising.
5
5.3 Ultrasound based extraction
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Ultrasound supported extraction procedure usually depends on the cavitation
7
phenomenon, developed by ultrasound waves. Cavitation makes turbulence which creating the
8
collisions and distresses in microparticles existing in microalgae biomass. Generally, the
9
ultrasound energy is transformed into vibrational energy attributing to disrupt the microalgae
10
cell walls. Hence, this condition boosts the transfer rate and thus, enabling the extraction of bio-
11
based polymers from microalgae (Flórez-Fernández et al., 2017). The few advantages
12
influenced by ultrasound mediated extraction over traditional methods are listed as (i)
13
noteworthy lessening of time for biopolymers’ removal (hours to minutes); (ii) extraction can
14
be carried at room temperatures without affecting the yield; (iii) membrane separation processes
15
are not required as compared to conventional method; (iv) saves material losses and (v)
16
environmentally friendly (Hmelkov et al., 2018). The extraction yield obtained by ultrasound
17
assisted extraction was 33% higher than the conventional method. Likewise, Flórez-Fernández
18
et al. (2019) isolated alginate from the same Sargassum muticum by ultrasound assisted
19
extraction approach minimizing the use of disruptive chemicals. The ultrasound approach
20
reduced the extraction time by 4 times in compared to conventional technique for the removal
21
of alginate from Sargassum muticum. The parameters which affect the isolation process include
22
sonication time, temperature, and ultrasound frequency. It has been notified that at maximum
23
sonication time influences better extraction of biopolymers from microalgae biomass (Flórez-
24
Fernández et al., 2019). Moreover, accelerating the frequency of ultrasound waves also boost
25
the biopolymer production from microalgae.
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1
5.4 Subcritical water extraction method
Subcritical water extraction technology is incipient towards the removal of bioactive
3
products from microalgae biomass. During this process, the water is going to pressurize the
4
well at below the critical pressure which is showing as less than 22.12 MPa and heated above
5
the boiling temperature up to 647.14 K (Gereniu et al., 2018). At present, there has been a
6
noteworthy upsurge to feat advanced extraction techniques to isolate the biopolymers from
7
microalgae biomass. This technique possesses numerous benefits compared to conventional
8
extraction procedures. Water is used as a solvent instead of chemical agents. Further,
9
advantages associated with this technology is depicted as (i) high product yield; (ii) shorter
10
reaction times and (iii) lower energy consumption. According to Saravana et al. (2018), they
11
represented a novel method of utilizing the subcritical water extraction technique to remove the
12
fucoidan biopolymer from Saccharina japonica (Saravana et al., 2018). It has been observed
13
that the isolation of fucoidan in presence of this water extraction method addressed as better
14
efficiency of 4.85% while, the traditional extraction method demonstrated as low removal
15
efficacy of 2.47%. The addition of ionic liquid with such subcritical water extraction technique
16
enhances the dissolution of biopolymers to uplift the removal proficiency. The supplementary
17
usage of ionic liquids in compared with some organic solvents are quiet beneficial owing to
18
high durability, enhanced stability, higher thermal efficacy, negligible generation of vapour
19
pressure etc. (Gereniu et al., 2018; Saravana et al., 2018). In another study, deep eutectic
20
solvents in combination with subcritical water extraction was applied for removing the
21
biopolymer from Saccharina japonica (Saravana et al., 2018). The established process
22
indicated a great efficiency and productivity compared to other conventional processes towards
23
the removal of alginate (28.1%) and fucoidan (14.93%) form Saccharina japonica.
25
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6.
Microalgae cultivation strategy for biopolymer enhancement and its scale up
In the modern era of bio-based polymer enrichment process, less land requirement,
3
recyclability, easy growth modulation and the extreme biocompatibility of the product, has
4
made the microalgae-based biopolymers greater than the petrochemical-based. Biopolymers are
5
considered as an ecologically safe bioproducts owing to their biodegradable nature. In this
6
section, the application of cultivation strategies for greater biomass production and scale up
7
opportunities are document.
8
6.1
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of
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Overview of wastewater remediation by microalgae
The integration of wastewater in microalgae cultivation has been deliberated as one of
10
the most potential and attractive substitutes to enhance the sustainability of microalgae assisted
11
biopolymer via biorefinery approach. Enormous release of daily waste and toxic contaminants
12
is one of the global environment issues due to their harsh effect on human health and ecosystem
13
(Viegas et al., 2021). Over a billion of people worldwide are having an inadequate access
14
towards the remedy and reutilization of industrial and other waste discharge (Sharma et al.,
15
2020). Moreover, the frequent lessening of water resources is being treated as over burden by
16
such unprocessed waste management (Falinski et al., 2020; Vasistha et al., 2021). Hence, an
17
innovative circular nutrient recycling approaches for wastewater remediation is needed to be
18
implemented to further maintain an eco-sustainability. The conventional water treatment
19
processes usually emphasize a large and chemically dependent Victorian-age technologies.
20
Most of the available wastewater remediation machineries are designed as energy intensive,
21
which contribute a high economic costs and carbon emissions to the nature (Chan et al., 2022;
22
Ravikumar et al., 2021). Moreover, a massive sludge generation is again enduring with an extra
23
environmental cost (Sharma et al., 2020). On the other hand, few other processes are also
24
involved to generate a plenty of waste hazardous materials which may produce some
25
carcinogenic by-products during disinfection processes, ascribing towards the material
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inadequacy for subsequent and rigorous waste discharge (Mohsenpour et al., 2021). Henceforth,
2
a transition from “use and throw-linear” towards a “use, treat, and reuse-circular” bioeconomy
3
approach is desired to reprocess the wastewater stream by incorporating green microalgae
4
strains. Microalgae commonly operate organic N (amino acids, urea), inorganic N (ammonium,
5
nitrate) and phosphorus (magnesium ammonium phosphate, NH4MgPO4.6H2O) from different
6
wastewater sources for their metabolism (Ross et al., 2018; Sena et al., 2021). It has been stated
7
that the energy requirement by International Energy Agency year 2016 that for the removal of
8
1 kg N & P from wastewater via conventional wastewater treatment technologies would be able
9
to govern 2.8 and 3.4 kg CO2 correspondingly. Additionally, it has also been illustrated that the
10
traditional wastewater treatment process produces approximately 3% of the total anthropogenic
11
GHG emission (Sharma et al., 2020). As we all know that the wastewater is constituted as a
12
large reservoir of nutrients that can be harvested by microalgae cells for not only the production
13
of the well-known applications biofuels, but also administered towards the essential valuables.
14
On behalf of that, we have mentioned an analysis regarding the wastewater treatment by
15
microalgae strains and the biopolymer production from such wastewater grown microalgal
16
biomass, depicted in Table 3.
17
Table 3: Phycoremediation of wastewater and biopolymer production from wastewater
18
grown microalgae strains
Microalgae
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Cultivating
Biopolymer
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media
Botryococcus
Sewage
Dairy
quadricauda
wastewater
Cultivation mode
References
Shake flask
(Kavitha et al.,
removal
(%)
PHB
TN 81.8;
TP 45
braunii
Scenedesmus
Nutrient*
Starch
TN 86;
TP 89
23
2016)
Airlift Photobioreactor
(Daneshvar et
al., 2019)
Journal Pre-proof
Selenastrum
Municipal
PHA
TP 99;
Swine
PLA
TN 97
Chlorella sp.
wastewater
Cyanobacteria
Agricultural
TP 92
PHB
runoff
Chlorella sp.
Cheese whey
TN 95
Photobioreactor
TN 94
2
* TN-Total Nitrogen; TP-Total phosphorous
6.2
(Rueda et al.,
2020)
Shake flask
TP 92
1
(Chen et al.,
2020)
TP 99
PHB
(Gentili, 2014)
repro
of
sp.
Plastic tubes illuminated
with fluorescent lamps at
a PAR
(photosynthetically
active radiation)
Photobioreactor
(Sathya et al.,
2018)
Biosynthetic monom for biopolymer accumulation
The biosynthetic pathway from biological sources usually deals with a scavenging
4
mechanism during adverse growth terms. The synthesis of biopolymers is well-documented in
5
bacteria and cyanobacteria mainly. The knowledge of microalgae-based biopolymer synthesis
6
is in its nascent stage. By overviewing all the metabolic pathways of various microbes, we
7
highlight a plausible pathway to trigger the biopolymers in microalgae. The biopolymer
8
production including PHA, PHB etc. is well-manifested with the housekeeping metabolism
9
including TCA cycle, beta oxidation of fatty acids, amino acids and many more. A variety of
10
intermediates have been involved in these metabolic intricales, most preferably acetyl Co-A
11
treated as the major precursor for all biopolymer assimilation. The nutrient enriched condition
12
like more nitrogen and less organic carbon, triggers the flux of acetyl Co-A which has shifted
13
towards Krebs cycle by obstructing the reactivity of 3ketothiolase enzyme. On another hand,
14
the nitrogen depleted condition (adequacy of organic carbon sources), the channelization occurs
15
towards the assimilation of biopolymers. PHAs and PHBs are usually produced by three
16
pathways (Liu et al., 2021). In view of this, pathway 1 indicates the monomers like 3HB and
17
4HB, which has been widely deliberated short-chain-length polyhydroxyalkanoates (SCL
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PHA); the pathway II describes about numerous precursors which have been provided from
2
fatty acids beta-oxidation with the development of PHA monomers; on the other hand, the
3
pathway III deals with the monomers which of them have been evolved in-situ fatty acids
4
synthesis pathway, which can change structurally the inexpensive carbon sources towards
5
intermediate metabolites 4-hydroxyacyl-CoA in pathway II.
repro
of
1
6
The precise process for the overall biosynthesis of biopolymers, derived from
7
microalgae are depicted below as demonstrated in Fig 3. Step-1 involves the transformation of
8
acetyl Co-A towards acetoacetyl Co-A in presence of 3-ketothiolase enzyme. This pathway is
9
termed as condensation. Step-2 includes the reduction of acetoacetyl Co-A to hydroxybutyryl
10
Co-A by the help of acetoacetyl Co-A reductase. This step is known as reduction. Step 3 is
11
regarded as polymerization whereas, the formation of biopolymer occurs in presence of
12
polymerase enzyme.
15
16
17
18
19
20
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Fig 3 Pictorial representation of metabolic pathway for biopolymer assimilation
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1
6.3
Multi-phasic fed batch cultivation coupled with trophic mode transition for
2
boosting the microalgae biomass and biopolymer assimilation
The challenge of the present comprehensive review is not only to attribute the
4
microalgae derived biopolymer production, while, to program a multiphasic low-cost
5
cultivation strategic development for significant enhancement of biopolymer without affecting
6
the cell growth. Apart from the nutrient (C/N) mediated microalgae cultivation, light irradiation
7
is demonstrated as an essential environmental aspect which is similarly accountable for
8
variation in microalgae cell growth and the production of other bio-based products (Elisabeth
9
et al., 2021). It is known that prolonged light irradiation disperses the enormous photon energy
10
and disturb the photosynthetically activated microalgae cells (Khanra et al., 2021). A huge
11
number of efforts, such as high light source, nutrient deficiency and salinity stress have usually
12
been functioned for value added products’ enhancement; but unfortunately, all these conditions
13
resulted less cell proliferation (Arora et al., 2017; Bharte & Desai, 2019). In this fashion, one
14
of the most commonly used solution to be instructed is the development of bi-phasic cultivation
15
strategy. During the first phase, microalgae cells are nurtured mainly with nutrient abundant
16
stage for boosting the cell growth, whereas, the second phase manifests about the biopolymer
17
synthesis in a great extent, at the end of entire cultivation. Very recently, a progression of such
18
bi-phasic operational strategy, expressed as multi-phasic development has appeared very
19
remarkably, which governs the improvement of essential metabolites without cessation of
20
microalgae biomass. This fact has kindled us to construct a multi-phasic fed batch cultivation
21
coupled with trophic mode transition which probably describe as a requisite solution for
22
bolstering the biopolymer assimilation without hampering the microalgae biomass productivity.
23
Thus, by considering such hitherto light accompanied phase wise nutrient (C/N) feeding
24
cultivation strategic approach, the accumulation of as-synthesized biopolymer substitutes
25
would address a strong potential with increasing the microalgae cell growth. Usually, we have
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selected the different photoperiod as the multiples of 8. In order to understand the mechanistic
2
behaviour of light illuminated feeding strategy under multi-phasic mode, few studies regarding
3
photoperiod regime modulation and light intensity have been carried out to observe the
4
microalgae cell growth as well as the biomolecules’ production (Arora et al., 2017; Khanra et
5
al., 2021). The intermittent operation of light/dark condition (as shown in Fig 4) usually reveals
6
about the photosynthetic unit turnover time that adapts an effectual photosynthetic capacity of
7
microalgae cell. The oxygenic photosynthetic reactions in microalgae can be demonstrated by
8
a redox reaction, governed with light harvested complex (chlorophyll), where, the CO2 and
9
water molecules have been converted to produce carbohydrates and oxygen. This entire
10
phenomenon is divided into two steps namely light and dark reactions. The light dependent
11
biochemical reactions which are bound with photosynthetic membranes, the light energy is
12
converted to chemical energy, providing a biochemical reductant NADPH2 and a high energy
13
compound ATP. On the other hand, the dark reactions, occurred in stroma, the NADPH2 and
14
ATP are used in periodic biochemical reduction of CO2 to carbohydrates. The most essential
15
role of the light reactions during microalgae photosynthesis is to provide the NADPH2 and ATP
16
for the accumulation of inorganic carbon molecules (Cheah et al., 2015).
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The light energy is entrapped in two photosystem arrangements, carried out by two
18
pigment-protein complexes like PS I and PS II, which prompts a photosynthetic electron
19
movement and a proton gradient across the thylakoid membrane of microalgal chloroplast,
20
causing NADPH and ATP, trailed by light dependent and independent stoichiometric
21
characteristics. The photosystems function in series associated with a chain of electron carriers
22
typically observed in a so-called “Z” scheme. In this scheme, redox components are indicated
23
by their equilibrium midpoint potentials and hence, the electron transport reactions progress
24
actively downhill, from a more negative more positive redox potential. These photosynthetic
25
apparatuses have the skill to transit the adequate number of electrons from water molecules to
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lessen the NADP+ ascribing more ATP accumulation. Further, PS I have a superior quantum
2
efficiency to apply the adsorbed photon for the shift of electrons from plastocyanin (PC) to
3
ferridoxin (FD). Hence, it could be projected that enhanced day time possibly allocates extreme
4
photonic excitation energy in the direction of the reaction centre by hitting more chlorophyll to
5
sustain the overall photosynthetic machineries. Basically, microalgae cells are very lucid at low
6
light irradiation to render a moderately large fraction of incident photons that are suitable
7
towards the alteration of photosynthesis into biomass. Microalgae generally arrive a light
8
saturation phase, causing photo-inhibition and henceforth, it is notable that at maximum light
9
intensities, chlorophyll molecules may be damaged, descending of photosynthetic rates. On
10
another hand, the photoperiod regime with fed batch mode is popularly known for the
11
assimilation of value-added products which show numerous advantages in several industrial
12
sectors. In this quest, we believe that the cellular metabolic machineries need to be programmed
13
for functionalization according to C/N ratio with balanced photo-illumination towards
14
biopolymer synthesis.
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Fig 4. Overview of multiphasic intermittent fed batch model with consecutive light
attenuation
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The large scale or industrial level of microalgae cultivation depends on both open and
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20
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6.4
Scale up of biopolymer production using closed photobioreactor system
23
closed systems. However, the contradiction begins to establish a standard one. Table 4 shows
24
the overall advantages and disadvantages of various closed photobioreactor system with its
25
scale up performances.
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Table 4 Scale up performances of microalgae cultivation to boost the biopolymer
2
production
Cultivation
Cost
Scale up
system
rate
high
Tubular PBR
high
high
difficult
high
middle
high
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PBR
plate high
difficult
Advantages
Disadvantages
• Low
power
consumption
• Less
shear
stresses
• Easy
temperature
maintenance
• Better aeration
and agitation
• Good
mass
transfer
• High ratio of
illuminated surface
area: volume
• Less operating
cost
• Large
illuminating surface
area
• Preferrable for
outdoor cultivation
• Relatively less
expensive
•
High capital cost
•
•
•
Fouling
Needs large land area
Gradients of pH,
dissolved
oxygen
and CO2 along the
tubes
• High biomass
yield
• Ease
of
sterilization
• Less
oxygen
build-up
• Better light path
• Huge
illumination surface
area
•
Manufacturing cost
is more
Difficult to balance
the temperature
Occurrence
of
hydrodynamic stress
Some degree of wall
growth
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Flat
Growth
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•
•
•
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Membrane
low
high
middle
Difficult
PBR
high
• Compressed
• Mass
transfer
rate is high
• Less
energy
feeding
• Better mixing
with little shear
stress
• Easy
of
sterilization
• Decreased
photoinhibition and
photo-oxidation
• Easy to handle
as the size is small
• Highly
useful
for
wastewater
remediation
•
•
•
Illumination area is
less
Shear stress is little
bit high
Sophisticated
construction
•
•
•
•
Fouling
Higher operational
cost
Difficult to scale up
Complexity of the
process mainly due
to
membrane
maintenance
and
cleanliness
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High
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Column PBR
In case of open cultivation systems, the manipulation possibilities are extremely limited
3
and there is high risk of microbial contamination, therefore the open system cultivation remains
4
restricted mainly for the hardy species with robust growth rate. The first and foremost choice
5
for large scale microalgae culture towards the production of essential metabolites is considered
6
by implementing PBR owing to its less risk of contamination. Among all the PBR designs, there
7
are three types of PBR have been accomplished for their technoeconomic feasibility are
8
designated as flat panel PBR, horizontal tube PBR and vertical tube PBR (Mal et al., 2022).
9
However, the entire PBR system is also persisting few bottlenecks. It is a well-known fact that
10
the vertical tube PBR affords a flawless agitation and aeration of the microalgae cells. But the
11
bottleneck associated with its extreme rough shear stress owing to its small surface area, which
12
plausible affect the cell damage and henceforth, the cell growth may compromise (Madadi et
13
al., 2021). Furthermore, in case of horizontal tube PBR, the quite greater surface area primes
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to achieve the vast cell density. However, the extreme tube length may generate an imbalance
2
of nutrient concentration and pH estimation along with the tube. Moreover, for flat panel PBR,
3
every panel is dented in itself, which avoids any types of gradient creation, but the difficulty
4
belongs for the cost of discrete compartment. Apart from few limitations of PBR, we firmly
5
believe that the closed systems still deliberate a protected growth environmental condition and
6
better biomass manipulation approaches.
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Microalgae harvesting system is a major problem to maintain an environmental
8
sustainability as cost-effective and suitable cell separation technique are required. Microalgae
9
biomass settled at the bottom through centrifugation by applying the combination of centrifugal
10
as well as gravitational force. The biomass retrieval capacity usually depends on the series of
11
the sedimentation efficacy by maintaining its time and temperature (Mal et al., 2022). However,
12
the expensiveness of huge maintenance and construction makes it inappropriate for large scale
13
culture. Flocculation, another most common and useful technique depend on the lumping of
14
microalgae cell by utilizing various flocculants including physical, chemical and biological one.
15
The pH modulation probably diminishes the entire negative charge of the cellular surface and
16
thus, prompting an auto-flocculation. This technology is well established for freshwater
17
microalgae strains, but not majorly useful for their marine complements. There are numerous
18
polyelectrolytes of inorganic chemicals like aluminium and iron which are commonly treated
19
as sound flocculants, but the incorporation of such inorganic chemicals may be unsuitable for
20
further downstream processing. Moreover, the toxicity of these flocculants limits the usage of
21
this process to some extent (Satpati & Pal, 2018). Filtration is the most common and simplest
22
one for microalgae cell harvest, wherein, the cell suspension is passed through a semi-
23
permeable filter membrane. However, the rheological characteristics of the cultivated
24
microalgae strains do not support always as they can effortlessly ambiguous about the filtration
25
route via the development of a compressible microalgal mat over it. The advanced form of such
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membrane filtration is regarded as ultrafiltration that can be used for the extraction of
2
microalgae biomass as well as metabolites. Further, microalgae biomass are permitted to froth
3
over the culture medium and then stored as scum. There are two major floatation techniques
4
opted like dissolved air floatation (DAF) and froth floatation. In case of DAF, the microalgae
5
cells are explained upon ozonation of the cultivation medium, trailed by the treatment with
6
inorganic salts, which eventually produces well bubbles by passing the relaxation of pressurized
7
fluid. These produced fine bubbles deliver the floccules at ultimate buoyancy via their
8
adherence to it and therefore, triggering their floatation tendency over the surface of a
9
separating vessel. In this manner, the maximum cell froth is thus gained as a semi-liquid
10
mixture. However, the second method depends on the pH modification and aerated through
11
different air columns to generate the microalgal froth (Satpati & Pal, 2018). The complete
12
indication of entire biopolymer production from microalgae by implementing closed system
13
and their intrinsic characterization has been illustrated by Fig 5.
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6.5
20
Microalgae genetic engineering can help to overcome metabolic capacity limitations, allowing
21
for larger accumulation of required biomolecules and, in turn, enhancing the economic
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16
17
18
19
Fig 5 Outline of biopolymer synthesis from microalgae strains and its commercialization
Genetically modified microalgae involved in Biopolymer production
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feasibility of the manufacturing process (Madadi et al., 2021). Few of the microalgae strains
2
has been studied for PHB production such as Synechocystis sp. Synechococcus sp. PCC 6803,
3
Synechococcus elongatus PCC 7942, and Synechococcus sp. PCC 7002 are natural candidates
4
for genetic engineering (Costa et al., 2018). Under balanced growth conditions and using BG11
5
as the algal culture medium, Carpine et al. (2017) studied the overproduction of P(3HB) in
6
Synechocystis sp. PCC6803 by overexpression of phosphoketolase combined with the double
7
deletion of phosphotransacetylase and acetyl-CoA hydrolase (Carpine et al., 2017). Microalgae
8
combine the ability to undertake posttranscriptional and post-translational modification with a
9
fast growth rate and simplicity of cultivation of microorganisms. Low recombinant protein
10
yields, on the other hand, are impeding the development of economically viable microalgal
11
expression systems. PHAs are produced using genetically modified microalgae systems,
12
Chlamydomonas. reinhardtii grows quickly, has a fully sequenced and annotated genome, is
13
easily genetically changed, is susceptible to traditional genetic analysis (Kaparapu, 2018).
14
Chlamydomonas reinhardtii, a green model microalga, has developed into a potent
15
biotechnological production host for a variety of recombinant proteins and metabolites
16
(Perozeni et al., 2020). It has been approved for human consumption, genetically modified
17
strains with no antibiotic or herbicide resistance may find it simpler to acquire acceptance.
18
Although C. reinhardtii CRISPR/Cas-based gene editing has advanced steadily over the past
19
eight years, reported editing frequencies (percentage of transformants with the desired DNA
20
alteration) vary widely (from 0.45% to 95%) depending on the used strain, type of
21
transformation, sgRNA efficiency, Cas enzyme, repair template (donor-DNA), and use of pre-
22
selection (Ghribi et al., 2020).
24
25
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1
7. Microalgae derived biopolymer production-a circular bioeconomy approach
Circular bioeconomy is considered as a sustainable resolution as it accentuates on raw
3
material-product to recycle it for extended time-period in market prior to its disposal as waste
4
material. Hence, this phenomenon lessens promisingly the fresh nutrients feeding in
5
manufacturing and reutilize the nutrients from its end of life. Based on this above fact, the
6
researchers have addressed that microalga-derived biopolymers have gained the most efficient
7
and cost effective alternate approach to accomplish a sustainable circular economy throughout
8
the entire World (Devadas et al., 2021). According to Khoo and Tan 2010, it has been
9
demonstrated that after the use of bioplastic bags, they can alter usually these bags into natural
10
constituents by the help of composting and be executed as a peat replacement to dismiss the
11
complete life cycle of these bioplastic materials (Khoo et al., 2010). Furthermore, study of
12
Karan et al. (2019) portrayed that the land necessity of microalgae culture condition to supply
13
the worldwide plastic production is located around 145000 km2 which absorb only 0.028% of
14
510000000 km2 of Earth’s surface area (Karan et al., 2019). Additionally, the microalgae
15
culture medium can be preserved by using several wastewater resources, compared with the
16
other synthetic media for cost reduction purpose. In order to explore the effectiveness of
17
nutrients for product formation, the microalgae strains have acquired an equal mass alteration
18
towards bioplastics at around 90% conversion efficacy (Karan et al., 2019; López-Hortas et al.,
19
2019). Moreover, the microalgae contain an immense potential to develop in 40% CO2 provided
20
condition with a robust CO2 accumulation process. Total of 1 kg of microalgae dry biomass can
21
synthesize about 1.83 kg of atmospheric CO2 with a significant fixation rate of at least 0.73-
22
2.22 g/L/d (Khan et al., 2018). Thus, microalgae-derived biopolymers or bioplastics are
23
indicating a sustainable path to support the circular bioeconomy as it deals with the beneficial
24
approaches of microalgae cultivation feasibility, less land requirement, media reutilization, CO2
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sequestration etc. Therefore, we envisage that all of these facts possess a potential and positive
2
environmental impact on both land as well as aquatic ecosystem at the end of their life cycle.
3
8. Applications of microalgae-derived biopolymer
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A huge number of applications of microalgae derived biopolymer are illustrated in
5
literatures. On behalf of that, few essential applications have been represented in our current
6
article, as elucidated in Fig 6.
7
8.1 Biosensor
Recent technological advancements have aided the creation of advanced
9
electrochemical biosensor architectures which has a pivotal role in healthcare monitoring
10
(Biswas et al., 2021). Chitosan and carboxymethyl cellulose are two of the most explored
11
polysaccharides in this field. In view of that, due to their marvellous properties include
12
biodegradability, biocompatibility, non-toxicity, adherent thin films forming ability, renewable
13
in nature (Lu et al., 2019). On the other hand, Laccases, a type of biopolymer with copper atom
14
clusters known as glycoproteins at the enzyme's active centre, transport electrons during redox
15
reactions such as the reduction of molecular oxygen to water and the oxidation of phenolic
16
compounds in the reduced form (Eiras et al., 2010). Laccase-based biosensors can be used to
17
determine medicines indirectly by referring to parallel reactions that can occur in enzymatic
18
reactions with substrate or product (Chang et al., 2016).
19
8.2 Wound Healing
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Biopolymer resemblance to extracellular matrix (ECM) biocompatibility properties are
21
widely used in wound and burn dressings (Smith et al., 2016). Living species such as fungus
22
(chitin), algae (alginate), bacteria (bacterial cellulose, exopolysaccharides), plants (starch,
23
cellulose, and natural rubber), and animals (starch, cellulose, and natural rubber) are the primary
24
suppliers of these organic therapeutic elements (collagen, hyaluronic acid, chitosan) (Sahana &
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Rekha, 2018). Collagen is primarily found in connective tissues and is one of the key structural
2
proteins of any organ. The structural and functional similarities between ECM and collagen
3
encouraged the usage of collagen matrices in wound healing applications (Biswas et al., 2021).
4
In wound healing, chitin is the second most commonly employed biopolymer. Invertebrates
5
(crustacean shells or insect cuticles, mushroom envelopes, green algae cell walls, and yeasts)
6
produce chitins, an inert polysaccharide copolymer. Chitosan is made from chitin, which is a
7
partially deacetylated and active form of chitin. They were great candidates for wound healing
8
because of their biocompatibility, biodegradability, non-toxicity, antibacterial, and moisturising
9
qualities. In addition, Cellulose is a polymer synthesized from plants that is commonly utilised
10
in wound healing (Zhou et al., 2018). The cellulose structure is made up of repeated units of -
11
d-glucose joined by -1, 4-glycosidic connections. High purity, good tensile strength, high
12
exudates capacity, biodegradability, and a unique nanofibril shape network structure are all
13
characteristics of cellulose-based films. Cellulose keeps wounds wet and moist wounds heal
14
faster because the repairing tissues get enough growth hormones and other nutrients (Laurienzo,
15
2010).
16
8.3 Food packaging
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Biopolymers are currently frequently employed in food processing as part of specialised
18
structures with attributes due to their ability to interact with other food components to improve
19
their physicochemical properties and stability. Polysaccharides are made up of
20
monosaccharides with the same or different residues (Baranwal et al., 2022). They are a
21
platform for visually appealing food packaging. Food is protected from pathogenic and spoilage
22
bacteria by active packaging, which is made primarily of polysaccharide biopolymers. At
23
various temperatures and pressures, food components undergo phase changes during
24
preparation and storage (liquid-gel or liquid-solid). These changes have an effect on food
25
quality and consistency (Laine et al., 2013). This is because phase transitions in food
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components are linked to changes in the physical properties of meals. The importance of food
2
phase transition has been emphasised in order to improve commodities and processes through
3
process control. There has been a lot of research into the design and quality of fat-replaced food
4
items in recent years. In these food systems, biopolymers, particularly hydrocolloids, are
5
frequently used to mimic the sensory and rheological properties of lipids (Baranwal et al.,
6
2022). Further, biopolymers have been recommended as a suitable medium for silver
7
nanoparticle synthesis and stability (Bankura et al., 2012). The polymer-assisted synthesis
8
method increases nanoparticle dispersion inside a polymer matrix, which affects the
9
nanocomposite film's ultimate structural stiffness and homogeneity, resulting in the
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maintenance of antibacterial properties in nanocomposite films (Kanmani & Lim, 2013).
11
8.4 Removal of pollutants
Photosynthetic mediated fuel cells (PMFC) have been reported to eliminate the
13
Benzene, polycyclic aromatic hydrocarbons (PAHs), azo dyes, and sulphide-containing
14
contaminants found in wastewater (Khan et al., 2022). Biopolymers such as cellulose, starch,
15
alginate, polysaccharides, and chitosans, are naturally occurring biopolymers that adsorb metals
16
and other contaminants, producing a complex hybrid while cleaning wastewater in a PMFC
17
(Zhang et al., 2016). Microalgae at the anode or cathode of a PMFC produces biomass rich in
18
polysaccharides, starch, cellulose, carrageen, and occasionally alginate. Biopolymers are
19
collected or harvested to be utilised externally to treat wastewater at the anode in a PMFC with
20
microalgae at the cathode in a PMFC with microalgae at the anode, however, biopolymers are
21
created and continue to clean wastewater through adsorption while also generating
22
bioelectricity and producing value-added goods such as carotenoids and lipids (Gautam et al.,
23
2016; Khan et al., 2022). Chitosan based adsorbents are essentially used to remove the dye and
24
metal ions from wastewaters. On the other end chitosan and cellulose have together been found
25
most abundant biopolymers in nature. The most suitable example being cellulose nanofibrils
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decorated with chitosans nanocrystals used as nano-adsorbents to clean wastewater removing
2
metals (Gopi et al., 2017; Wang et al., 2017).
3
8.5 Metal corrosion inhibition
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Moreover, the biopolymers have drawn a considerable attraction for their
5
inexpensiveness, inherent stability, adsorption potential, and large size. Keeping these
6
characteristic features in mind, we emphasize that the microalgae derived biopolymers could
7
be used for industrial applications like metal corrosion inhibition. Corrosion is considered as
8
one of the significant interests to the technocrats owing to its devastating impact on the Earth’s
9
economy as well as human safety. Hence, to limit the corrosion rate, the usage of several
10
potential green inhibitors is the paramount focus of research. Therefore, we believe that
11
microalgae derived biopolymer could open a new path for the frontiers of metal corrosion
12
inhibition study.
14
15
16
17
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Fig 6. Potential applications of biopolymers in several fields
9. Market Outlook of Biopolymer Industry
Only 746 billion barrels of the world's remaining oil reserves have not yet been
exploited, according to Colin Campbell, co-founder of the London-based Oil Depletion
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Analysis
Centre
(https://www.marketsandmarkets.com/Market-Reports/biodegradable-
2
plastics). Approximately 944 billion barrels of oil have been produced throughout human
3
history. These figures are concerning because we are rapidly approaching a moment when
4
humans will be totally dependent on a world without oil. As businesses are compelled to
5
innovate, develop, and create new products, the world's biopolymer sector is receiving a crucial
6
boost due to the depletion of oil sources.
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The worldwide polymer market was predicted to be worth $666.6 billion in 2018,
8
according to an industry analytical research and consulting report. With a compound annual
9
growth rate (CAGR) of 5.1%, the market is predicted to increase rapidly (Baranwal et al., 2022).
10
Pharmaceutical, healthcare, food, and beverage industries are the major market of biopolymer.
11
Biodegradable polyester is particularly valuable in the medical field for making surgical
12
implants. Biopolymers are primarily utilised in the food and beverage industry to make
13
cellophane films and are widely employed in food packaging. The global biopolymer market
14
grew significantly in 2018, with an estimated value of $12 billion (Baranwal et al., 2022). The
15
biopolymer market is predicted to increase rapidly at a CAGR of 19% between 2019 and 2025
16
(Baranwal et al., 2022). The European Biomass Industry Association has made various efforts
17
to increase market adoption of biopolymers, as seen by Europe's 55% market share in 2018. In
18
the pharmaceutical sector, biopolymers are commonly utilised to treat wounds of any shape,
19
size, or depth. Hydrogels are made from common biopolymers such chitosan, gelatine, alginate,
20
and pectin provide a moist environment for dry wounds. Additionally, these biopolymers are
21
used in the production of bandages for wounds. The predicted growth driver for the global
22
biopolymer market is a combination of these reasons. In the biopolymer industry, the upfront
23
cost associated with producing a product is crucial. By establishing a joint venture with an
24
agriculture company to create a symbiotic connection for the growth of the biopolymer, key
25
industry participants are striving to resolve this problem. ASF SE, Danimer Scientific,
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Novamont SpA, Galatea Bio Tech, Total Corbion, Plantic Technologies Ltd., FMC BioPolymer
2
AS, NatureWorks LLC, Sigma-Aldrich, and Biome Technologies Ltd. are major industries in
3
the biopolymer market. Sigma-Aldrich is a biotechnology and polymer company established in
4
Missouri.
5
10. Conclusion and Future Prospective
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To the best of our knowledge, microalgae derived biopolymer assimilation is
7
demonstrated as one of the most sustainable and renewable approaches for circular bioeconomy
8
aspect. The recent advancement of multi-phasic feeding strategy with photoperiod modulation
9
is facilitated towards the carbon capture by enhancing the microalgae growth as well as
10
biopolymer production simultaneously. Hence, we envisage that the green microalgae cell can
11
easily substitute the fossil-based polymers rapidly. Therefore, the circular bioeconomy model
12
of microalgae originated biopolymer indicates to become a potential feedstock for several
13
nutraceuticals and pharmaceutical aspects. In future the development and improvement of
14
diverse biopolymeric materials by functionalization or hybridization with different functions is
15
particularly significant for target-oriented applications. Blending biopolymers with other
16
biodegradable polymers, could be one of the most effective ways to create novel polymeric
17
systems with "tailor-made" functional qualities including physical properties and
18
biodegradability. The synthesis of biocomposites could be promising strategy for improving
19
polymeric characteristics while keeping the final polymer biodegradable. Thus, we envisage
20
that this review accentuates a comprehensive insight to ameliorate the concept of microalgae
21
biorefinery towards the assimilation of biopolymers which could build a new avenue in the
22
fields of circular bio-economy development.
23
Acknowledgement
24
This work was also supported by the Kurita Water and Environment Foundation (KWEF)
25
[21Pmy004-21R],
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Fundamental
Research
40
Grant
Scheme,
Malaysia
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[FRGS/1/2019/STG05/UNIM/02/2]
and
MyPAIR-PHC-Hibiscus
Grant
2
[MyPAIR/1/2020/STG05/UNIM/1].
3
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Highlights
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Bioprocessing of microalgae derived biopolymer via circular bioeconomy approach
Multiphasic fed batch for enhancing the microalgae mediated biopolymer yield
Green processing and scale up biopolymer production
Applications of microalgae mediated biopolymers in the industry
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Graphical abstract
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CRedit authorship contribution statement
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CRediT authorship contribution statement Anwesha Khanra: Writing - original draft, Writing review & editing. Shrasti Vasistha: Writing - original draft, Writing - review & editing. Monika
Prakash Rai: Reviewing, Visualization. Wai Yan Cheah: Writing - review & editing. Kuan Siong
Khoo: Writing - original draft, Writing - review & editing. Kit Wayne Chew: Writing - review &
editing. Lai Fatt Chuah: Writing - review & editing. Pau Loke Show: Conceptualization, Project
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administration, Funding acquisition.
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Declaration of interests
☒ The authors declare that they have no known competng fnancial interests or personal relatonships
that could have appeared to influence the work reported in this paper.
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☐ The authors declare the following fnancial interests/personal relatonships which may be considered
as potental competng interests: