Introduction Sargassum belongs to the group brown sea weeds of algae. The brown alga forms the division phaeophyta and constitutes a distinct taxonomic group due to their morphological, anatomical and physiological complexities. This group of alga includes 265 genera and roughly about 1500 species worldwide. With few exceptions the members of phaeophyta are marine and commonly called brown sea weeds. Most of the phaeophycean members grow in the intertidal region and upper littoral region in cold temperate waters particularly in the northern hemisphere. The littoral zone of cold temperate sea is popularly known as sea forest comprising a large number of brown algae. In the tropics, the region where large numbers of phaeophycean are encountered is the Sargasso Sea of Atlantic. The phaeophyta are multicellular and range in size from filamentous strands of sea weeds, such as species of Ectocarpus, to the giant kelps that form under water forests. A number of marine forms penetrate into brackish water where they form an important part of the salt marsh flora. These brackish water plants have totally lost the ability to reproduce sexually and they propagate by vegetative means only (J. N. Butler, et al. 1983). Scientific classification of Sargassum Empire :Eukaryota Kingdom : Chromista Subkingdom : Chromobiota Infrakingdom :Heterokonta Phylum :Heterokontophyta Class :Phaeophyceae Order :Fucales Family :Sargassaceae Genus : Sargassum Sargassum is believed to be the native of Japan and is now believed that it has a wider distribution than previously thought. The transplantation of oyster seed from infected regions of Europe and the transportation of fertile fronds by currents or by boats or ships are believed to be the most likely sources of inoculation to new areas. Sargassum is a genus of generally planktonic (free floating) macro algae (sea weeds). It is named for the Atlantic ocea n's Sargasso Sea which hosts a large amount of free floating mass of sea weeds, particularly S. natans and S. fluitans in the western Atlantic Ocean (J. H. Ryther. 1956). According to Algae base there are 866 species names in the database at present, of which 349 have been flagged as currently accepted taxonomically. Most of the species are generally attached to rocks along coasts in temperate regions. The alga is most conspicuous in tropical and subtropical waters ranging from midlittoral to sublittoral zones. Sargassum is also known as sea holly because of its highly branched thallus with hollow berry-like floats (pneumatocysts) and many leaf like saw-tooth edged (dentate) blades. Most of the species reproduce sexually except S. natans and S. fluitans which reproduce only by fragmentation. Habitat and distribution Sargassum is highly tolerant to environmental parameters such as desiccation, full sunlight and variations in salinity and temperature. This enables it to occupy a broad range of habitats from the upper intertidal (mainly rock pools) to the sub tidal and substrata from exposed rock to Zostera marina (Eelgrass) beds. A free floating meadow of sea weeds almost as large as a continent lies between the United States and west of Africa in the North Atlantic Ocean. This is the famous Sargasso Sea. Christopher Columbus discovered it while he sailed toward the new world in 1492. Its presence suggested that land was near and encouraged him to continue his voyage. (Above Images Courtesy: National Oceanic and Atmospheric Sargassum is commonly found in the beach drift near Sargassum beds where they are also known as Gulf weed, and colloquially as the weed of deceit, a term also used to include all sea weed species washed up on shore. Numerous species of the Sargassum are distributed throughout the temperate and tropical oceans of the world, where they generally inhibit shallow water rocky and coral reefs. However the genus may be best known for its planktonic (free floating species). Any member of the normally benthic species may take on a planktonic, often pelagic existence after being removed from reefs during rough weather. However, two species (S. natans and S. fluitans) have become holopelagicreproducing vegetatively and never attaching to the sea floor during their life cycle. The species of Sargassum are commonly met in tropical seas of Southern Hemisphere. The species are also common along the coast of Australia, Japan, West Indies, Florida and India. The most common centre of occurrence of the species is supposed to be Southern hemisphere (Abbott et al. 1976). In India the species are common along the eastern, western and in southern coasts at Okha, Dwarka and other places. Vegetative Structure of Sargassum The alga has two distinct parts: the perennial, dark brown basal axes, and the lighter coloured annual primary laterals. The latter are shed or torn off in late summer. During the summer months, the number of small round vesicles (air bladders) increases. Receptacles are most abundant in early autumn, just prior to the shedding of annual primary laterals. Because of its strongly seasonal growth pattern. Species of this genus of algae may grow to a length of 10-200cm or more. They are generally brown, olive or dark green in colour. The plant body resembles the seed plants as it is diploid, perennial, erect and bushy with a bilateral or radial symmetry. The main body is differentiated into irregular to solid, discoid, conical holdfast, few simple cylindrical, terete to compressed stem like main axis called stipe and flattened leaf like laterals or frond (leaves). The stipe may be short (a few cm high) or long (upto 100cm or more), cylindrical and branched. The branching is monopodial. The main axis bears primary laterals (branches of unlimited growth) which give rise to secondary laterals (branches of limited growth) which may be further branched (Bilgrami et al. 1992). Some secondary laterals become flat, leaf like and act as photosynthetic organs. These leaf-like laterals possess minute pores on the surface called cryptostomata. The leaves are sometimes replaced by golden brown colored stalked air bladders called as pneumatocysts. These bladders are swollen, berry like, which keep the frond afloat by providing buoyancy. The gas in bladders of Sargassum constitute variable proportions of oxygen, very little amount of carbon dioxide and a light proportion of an inert gas supposed to be nitrogen have been detected. It has been demonstrated that light intensity influences the oxygen content of bladders. Therefore it can be concluded that the oxygen content is correlated with the photosynthetic activity of the plant. In the upper part of the plant body, the axis of the laterals bears fertile and sterile receptacles. Embedded in the fertile, hermaphrodite receptacles are unisexual conceptacles which bear oogonia and antheridia. The hold fast or rhizoidal branches are discoid and it adheres to the substratum with an acid mucopolysacchride. Anatomy of Sargassum Epidermis Anatomically Cortex the thallus is differentiated Medula into a mucilaginous cuticle, epidermis or meristoderm (meristamatic in nature) and peripheral layers of compact, columnar thin-walled parenchymatous cortical cells, which have abundant chromatophores (plastids) and Fucosan vesicles. Inner to cortex is the zone of medulla, which consists of narrow, elongated thick walled cells which serve the function of conduction (Bilgrami et al 1992). The internal structure of leaf resembles the main Cryptostomata Medulla Epidermis Cortex axis. The epidermis is followed by assimilatory cells with numerous plastids. The cryptoblasts open outside by cryptostomata are also found in the epidermis of leaf. These cryptoblasts are flask like bodies with multicellular hairs arising from the base. These hairs are believed to be respiratory and also absorb nutrients from the water Cellular structure The cell wall has an inner cellulosic layer and an outer gelatinous layer of pectic substances which possess mucilage of algenic acid and fucinic acid. The cell wall is distinguished into two fractions: a fibrillar part that gives the wall its strength, and an amorphous part in which the fibrils are embedded. The fibrillar fraction consists of cellulose and this structural component is probably stiffened further by insoluble alginate. The alginate is a salt of algenic acid, a polymer of two sugar acids, D-mannuronic acid and L- guluronic acid which are connected by β 1,4 linkages. Chloroplasts are discoid golden brown coloured and contain chlorophyll a, c1 and c2 as the main photosynthetic pigments and fucoxanthin and βcarotene as accessory pigmants. Individual cells may contain several chloroplasts. The Algae owe its colour to the accessory pigment fucoxanthin. These chloroplasts bear pear-shaped stalked pyrenoids to their inner sides. The chloroplast DNA is organized in a ring-shaped nucleoid. Vesicles containing reserve food material are formed in the cytoplasm, near the pyrenoid. In chloroplasts the thylakoids lie in stack with girdle lamella usually enclosing all the others. The chloroplast is surrounded not only by its own double membrane, but also by a fold of endoplasmic reticulum, so that in all there are four membranes around it. Where a chloroplasts lies against the nucleus, the nuclear envelope, the chloroplast and endoplasmic reticulum are continuous with each other. The most important reserve product of photosynthesis is chrysolaminaran, which is a β-1,3linked glucan and lies in solution in special vacuoles. Manitol is also found which is a low-molecular mass sugar alcohol and has an osmoregulatory role, in addition to its function as a reserve. Its intracellular concentration increases and decreases with increasing and decreasing external salinity. The cells also contain various small highly refractive colourless vesicles called fucosan vesicles, abundant in metabolically active cells. These vesicles contain an acidic fluid (phaeophycean tannins or phlorotannin) that stain red with vanillin and Hydrochloric acid. The phlorotannins are polymers of phloroglucinol. They are readily oxidized in the air leading to formation of brown or black pigment phycopheain (Jarosch R. 1962). These vesicles actually originate from plastids, where tannins are produced than ooze out to cytoplasm. Only reproductive cells are flagellate. The flagellate cells have a typical photoreceptor apparatus, consisting of a swelling on the posterior flagellum. The long anterior flagellum is pleuronematic, bearing tripartite mastigonemes (tubular hairs) which are formed in ER cisternae. The Golgi bodies lie with their forming faces appressed to the nuclear envelope. Reproduction in Sargassum Sargassum and other Fucoid algae have an animal-like life cycle with no alternation of generations. Look upon this as an exception to the general pattern found in the plant kingdom. The plant body of Sargassum is a diploid sporophyte. It does not multiply asexually by means of spores. Instead it reproduces by vegetative method and the formation of sex organs. Vegetative reproduction: Fragmentation is the only known method of reproduction in the free floating species of Sargassum. In Midrib case of S. natans it is very Air bladder prolific. This is Leaf a free floating species. Each Receptacles fragment develops into a Leaf on bladder new plant. Sexual reproduction: Sargassum is characterized by a diplontic life cycle, in which meiosis takes place just before the formation of the gametes (gametic meiosis). Here both the tiny motile male gamete and the large non-motile female gamete (egg) are produced not in plurilocular but in unilocular structures. The sexual reproduction is oogamous and involves the fusion of motile sperm or male gamete with a non-motile ovum or female gamete. They are borne inside antheridia and oogonia respectively. They are also termed as microsporangia and megasporangia. The spores germinate inside the sporangia to produce gametes. The sex organs are produced inside special cavity called conceptacles. The conceptacles are found inside small finger-like branchlets called receptacles. The plants are either monoecious or dioecious. In monoecious forms the antheridia and oogonia can be in the same or different conceptacles. In case of dioecious forms the male receptacles are smooth and the female ones appear spinous. Development of conceptacles: the conceptacles develop from a superficial meristodermal cell which becomes much more prominent. The surrounding cells of this meristodermal cell divide rapidly sinking down this prominent cell deep into a flask-like cavity. By reaching at the base of this flask-like cavity the prominent cell divides transversally into an upper tongue cell and a lower basal cell. The tongue cell degenerate while the basal cell divide anticlinaly and periclinally into two rows of curved cells. These layers represent fertile sheet. The antheridia and oogonia develop on this fertile sheet from the cells of the upper tier. Antheridia: A large number of antheridia develop in each male conceptacle. They are borne on the lower Antheridial initial Conceptacle Fertile sheet Fertile cell C B A Antheridium Stalk cell E F D Antherozoids 64 uninucleate protoplasts Two unequal flagella Nucleus I G H Antheridium Branched paraphyses with antherozoids J K Conceptacle Sargassum: Development of Antheridium; A, V.S.Conceptacle;; B-D Development of antheridium from antheridial initial; E-H, Development of antherozoides; I Antherozoid; J, V.S. conceptacle; K, branched antheridium branches of paraphyses. The first antheridium is borne at the tip of a 3-celled filament which arises from a cell of the fertile layer. The basal cell remains at the level of the fertile layer. The middle one is known as stalk cell. The upper or antheridial cell develops into an antheridium. The antheridium is ovoid in outline. It has a thick wall which becomes mucilaginous towards maturity. Internally the antheridium contains initially a single diploid nucleus and a few chromatophores. The diploid nucleus divides meiotically and then mitotically to form 64 haploid nuclei. The chromatophores also divide and ultimately get placed near the nuclei in one to one ratio. The cytoplasm is cleaved and the contents of the antheridium get divided into 64 haploid protoplasts. Each such protoplast transforms itself into a single sperm or antherozoid. Thus 64 sperms develop inside an antheridium. The wall of the mature antheridium becomes mucilaginous. It breaks its connection from the paraphysis and comes out. Here the wall dissolves and the sperms are released in sea water. A sperm is a pear shaped biflagellate structure of pale brown colour. The flagella are unequal and inserted laterally. The anterior shorter flagellum is of tinsel type while the posterior longer flagellum of whiplash type. The nucleus is larger. Chromatophore is reduced and posses an eye spot. Fig. Sargassum: Development of oogonium, A, V.S. of conceptacle; B, Oogonial initial; C-H, Development of oogonium; I, seven Degenerating nuclei and one functional nuclei Oogonia: Only a few oogonia are borne in a conceptacle. They are formed directly from the cells of the fertile layer, very early in the development of the conceptacle. Some of the cells of the upper tier of the fertile layer protrude out and function as oogonial initial. It divides transversely into a lower stalk cell and an upper oogonial proper cell. The stalk cell gets pressed between the growing oogonial cell or oogonium and the cells of the fertile layer. The young oogonium contains a conspicuous diploid nucleus, dense cytoplasm and a number of oil droplets. The diploid nucleus undergoes meiosis and mitosis to produce 8 haploid nuclei. 7 of them usually degenerate while the functional haploid nucleus enlarges and becomes centrally placed. The protoplast of the oogonium then functions as a single egg or oosphere. The mature oogonium is globular or ellipsoidal in outline. It has thick 3 layered wall and a single ovum or oosphere. The three wall layers are outer exchite, middle mesochite and inner endochite. The middle wall layer or mesochite imbibes water and becomes Egg Antherozoides C Gelatinous stalks D A E Oogonium B F H G Sargassum Sp.: A, Oogonium with geltinous stalk; B, young oogonia; C, fertilizing egg; D, zygote; E-H, Stage of embryo development mucilaginous. It’s swelling breaks the exochite. The oogonium is slowly extruded to the outside of the ostiole. It however remains attached to its original position by means of a mucilage threads formed by the mucilaginous mesochite. Later on the endochite also ruptures and the egg is liberated in the water, still attached with mucilage threads. Fertilization: The ova of Sargassum are not shed in sea. They remain covered over by the gelatinized wall of the oogonia and are held in position just outside the ostiole by means of a mucilaginous stalk made of mesochitin. They attract a large number of sperm freely swimming in water. Several anthrozoides approach the egg. Some times, because of the presence of many antherozoids, the egg appears to be a multiflagellate structure. With the help of these antherozoids the egg may rotate in water. The number of sperm attaches themselves to the gelatinous sheath of the ovum by mean of their anterior flagella. The posterior flagella continue to lash. One of the lucky sperm penetrates the mucilaginous covering and fuses with the ovum. The other sperms swim away. The fertilized ovum has a diploid (2x) nucleus and is called zygote (Panday B.P.1993). Germination of zygote: After fertilization, the zygotes continue to grow on the parent for several days before dropping to ocean floor. The Leaves Receptacle Main axis H A B Oogonia Sperms C G E Egg D F Antheridia Sargassum: stages in life cycle: A thallus; B, Receptacle; C, young oogonia; D, Conceptacle with antheridia; E, Oogonia coming out through ostiole; F Antheridia on branched paraphysis; G, fertilizing egg; H, Young embryo enveloping mucilage protects them from environmental stress, as does their multicellular form. Their large size also allows them to settle rapidly, and the well-developed rhizoids adhere quickly to the substrate. This results in the germlings settling near the parent (within 3 meters), where conditions are likely to be favorable. The zygote germinates directly without any resting period. It begins its germination while surrounded by the gelatinous sheath derived from the oogonial wall and still attached to the interior of the conceptacle by means of mucilage stalk. After some time the gelatinous sheath dissolves and the zygote in its early stage of germination falls down on some solid substratum. The fertilized egg elongates and divides transversely into two cells. The lower cell divides transversely and develops into rhizoids which later become eseptate. The upper cell divides and redivides periclinally and anticlinally. Vertical or longitudinal walls appear in the terminal region which then shows differentiation of outer and inner cells, followed by distinction of meristoderm, cortex and medulla. Simultaneously a three side apical cell appears at the tip of the germling. It produces the typical thallus. Dispersal of gametes Gametes are usually released during or just after the spring tide. The timing may be determined by hydrostatic pressure, or more probably by the light of the moon. (Fletcher 1975.) Gametes are released in cycles of 13 days instead of all simultaneously, increasing the possibility that some of them will encounter favorable tides and conditions. (Fletcher 1975.) Germlings are pear-shaped, enabling the rhizoids to land first. Rhizoids stick within 48 hours. Germlings lose their ability to adhere over time, in connection with declining mucopolysaccaride levels. After 18 days, half can still stick, after 49 days, none can stick. (Deysher and Norton 1982.) Germlings can grow on kelp, but they fall off before they reach a height of 3 cm and can no longer re-adhere. (Deysher and Norton 1982.) Individuals that become free-floating can survive in the water column indefinitely. Alternative Dispersal Mechanism Fertile branches break off from the holdfast and float away. When the germlings are released a long distance from the parent, they do not have to compete with their own relatives and can settle in new territory. The combination of these two dispersal mechanisms proves to be an effective system for global spread. Developmental morphology or Morphogenesis of Sargassum The thallus structure (stem, branch and leaves) develop by the activities of a three-sided apical cell. The thallus consists of three compact tissues, called for convenience the epidermal, cortical, and conducting tissues. The latter consists of only thinwalled cells in the leaves, but in mature stems contains both thick and thin-walled elements. Both the conceptacles and cryptostomata originate in a single flask-shaped initial cell which develops the entire structure. The first division of the initial cell results in two unlike segments: a large lower cell which develops the walls of the conceptacle and cryptostoma; and an upper cell, the tongue cell, which remains inactive, divide to form a short filament, or degenerate. The "initial cell" is apparently the tongue cell, a product of the true initial cell. The conceptacle and cryptostoma are undoubtedly homologous structures. Every stage of development in both structures is the same, from the appearance of the similar initial cells to the development of paraphyses in the cryptostomata and sexual organs in the conceptacle. The paraphyses are developed basipetally by the division of the lowermost cell in each structure. Spermatocysts or their degenerate representatives occur in some cryptostomata. Such conditions indicate that the cryptostomata have been derived from conceptacles whose sexual organs have become sterile. The spermatocysts develop as in other Fucales, each finally producing sixty-four sperms which are discharged from a partly terminal and partly lateral vent. The sister cell of an oocyst does not become a stalk and consequently the oocyst is an embedded structure. The oocyst normally gives rise to but one egg. The nucleus of the oocyst accordingly becomes the nucleus of the egg. The oocysts were found containing eight eggs each. These must be considered a rare reversion to the Fucus type. The entire oocyst of Sargassum, unlike other genera of the Fucales which have been studied, is discharged with its enclosed egg. The oocyst wall may break, partially freeing the egg, or it may persist even enveloping a many-celled sporeling. Segmentation of the egg takes place while it is attached to the surface of the plant by the mucilaginous wall which surrounds it. This segmentation results first in a many-celled undifferentiated ellipsoidal sporeling. Rhizoids develop late at one end of the multicellular sporeling, with no apparent relation to gravity or other stimulus. Economic uses of Sargassum Human uses: Sargassum seaweeds are eaten by people, and used fish bait in basket traps, animal feed, fertilizer, insect repellent. Various species are used as medicine for ailments ranging from children's fever, cholesterol problems, cleansing the blood and skin ailments (Chevallier 1996). Alginates are cell-wall forming materials found in Sargassum. Their principal organic constituents are chain-forming hetero-polysaccharides, guluronic acid and mannuronic acid. Sodium alginate finds its applications as a stabilizer and as an emulsifier. It was once used in textile printing industry. But, things have changed since the upheaval of textile industry as well as the changes in textile fashions. Till today, sodium alginate is used in ice cream and dairy products to enhance flavour and taste. Sodium alginate molecules are non-polar in nature. So they are insoluble in polar solvents, like water. This property of sodium alginate has led to the development of high quality fibers which can be used in the textile industry. Sodium alginate fibers also are used to weave bandages for larger wounds and burns. Also, they can be removed much more easily as compared to cellulose-based bandages. The hard part of sodium alginate fibers are used in the manufacturing of fire-fighter’s fire-proof clothing. In some parts of the world, it is used as a fertilizer. Sargassum seaweeds are a preferred additive used to create a specific favour in many Japanese dishes like soy sauce. Sargassum seaweed is a shelter for many marine animals as it is an ideal hiding place from predators. It is also an abundant food source for them. People living alongside the seashores of Japan, often eat fresh Sargassum seaweeds as vegetables. The Journal of the American Medical Informatics Association (JAMIA) claimed that Sargassum seaweeds promote weight loss by discharging more water thought urination. Sargassum has been used as a topical medicine to treat goiters (the bump in the neck due to the enlargement of thyroid gland) in some provinces of China. It is salty and bitter in taste. Iodine deficiency leads to the swelling of thyroid gland because without iodine thyroid gland cannot produce enough thyroxin-a critical hormone secreted by thyroid gland-in order to regulate body metabolism. Sargassum seaweeds guarantee a continuous supply of dietary iodine. In addition, Sargassum seaweed promotes the excretion of urine and reduces edema. It is also prescribed to alleviate pain from swollen testes and hernia (AAOM).
