Sargassum

Sargassum is a genus of brown macroalgae, commonly known as seaweed or marine algae. It comprises over 150 different species, and its members are primarily found in shallow oceanic waters, especially in the Sargasso Sea, a region of the North Atlantic Ocean. 

1.0Classification of Sargassum 

2.0Common Sargassum Species

• Sargassum muticum: Also known as Japanese wireweed
• Sargassum natans: Also known as the Gulfweed, this species is a pelagic (free-floating) seaweed that forms floating masses in the Atlantic Ocean, particularly in the Sargasso Sea.
• Sargassum vulgare: This species is commonly found in the Mediterranean Sea 

3.0Habitat of Sargassum  

sargassum found on the open waters of the ocean in regions with slow-moving currents. Floating on the water's surface due to unique structures called pneumatocysts, Sargassum seaweed forms expansive, buoyant mats, notably in the Sargasso Sea within the North Atlantic Ocean. This pelagic habitat allows Sargassum to drift with ocean currents, and its preference for areas with high salinity is characteristic of tropical and subtropical waters. Some species of sargassum are found attached to the substratum. 

4.0Structure of Sargassum

Sargassum plant is diploid in nature; it consists of stipe and a holdfast, the stipe is made up of main axis and lateral branches.  Sargassum attaches to floating debris or other Sargassum plants through a structure called the holdfast. 

The main axis bears primary and secondary laterals. Each primary laterals is of unlimited growth. It is also known as long shoot. These long shoot or primary lateral bears numerous secondary lateral which are limited in growth. 

Some lateral branches in the leaf axils are repeatedly branched and are called fertile branches or receptacles. These receptacles bear flask-shaped cavities called conceptacles, these conceptacles have sex organs.

In addition to the fertile conceptacles, certain species of Sargassum feature sterile flask-shaped structures known as cryptoblasts, situated on the primary branches.

These structures open externally through openings called cryptoblamata but do not contain any reproductive organs. 

Similar to conceptacles, cryptoblasts undergo the same developmental process and possess a similar structure. Many multicellular hairs arise from the base of cryptoblast known as paraphyses These hairs extend in tufts through the cryptoblamata and are shed as they age, leaving only the persistent basal portions within the mature cryptoblasts.

One of the most notable features of Sargassum is the presence of pneumatocysts, which are gas-filled, bladder-like structures. These pneumatocysts provide buoyancy, allowing Sargassum to float on the water's surface. 

5.0Sargassum Diagram

Internal Structure of Thallus

Transverse section of thallus shows the following layers : 

• Meristoderm : This layer consists of a palisade layer, a peripheral zone of columnar cells, lacking distinct air spaces between them. Abundant plastids and fucosan granules are present within the cells of this layer, collectively constituting the meristoderm. The primary function of the palisade layer is photosynthesis. Externally, the palisade cells are protected by a mucilaginous cuticle. 
• Cortex :  Within the hypodermal layer is the cortex consisting of thick-walled polygonal cells. The cortex may be several layers thick. It probably functions as the storage region and plays a mechanical role.
• Medulla : The thallus's central region, known as the medulla, consists of loosely arranged cells, narrow and elongated. Outer cells have thick walls, while inner cells have thinner walls, and there's no gelatinized material. In free-floating species, medullary regions are minimal. The medulla likely serves as a conduit for nutrient transport, with interconnected cells facilitated by pores in the cross walls.

6.0Reproduction in Sargassum 

Sargassum reproduces by two methods : vegetative reproduction and sexual reproduction. 

Vegetative reproduction : Sargassum reproduces vegetatively by fragmentation. The older parts die and decay separating the younger parts. The latter continue to grow and finally develop into a full-fledged Sargassum plant. In sargassum natans, vegetative reproduction is the only mode of multiplication. 

Sexual Reproduction : The typical reproductive method in Sargassum is oogamous, with sex organs emerging in flask-shaped structures called conceptacles on the sporophyte plant. A single receptacle holds numerous conceptacles, each exclusively unisexual. In monoecious plants, male and female sex organs develop on separate conceptacles, while in dioecious species, male and female conceptacles are found on distinct plants.

Oogonium : Oogonia are generated within the female conceptacles. The diploid nucleus of the young oogonium undergoes meiotic and mitotic divisions, resulting in eight haploid daughter nuclei. Among these, seven daughter nuclei degenerate, while one enlarges to become the ovum or egg. 

As the oogonium matures, its walls thicken and differentiate into three layers: the outer exochite, the middle mesochite, and the innermost endochite. The mature oogonia discharged through the ostiole.

Antheridium : Antheridia are produced in the male conceptacles on branched paraphyses. Each branch of the paraphyses may bear two to three antheridia. The mature antheridium is small, ovoid, and contains 64 biflagellate male sperm. The protoplast of the antheridium first divides meiotically and then mitotically to produce 64 biflagellate male gametes.When sperms reach maturity antheridia separate from their respective stalk cells by the action of water currents. The liberated sperm is a pear-shaped structure and has two unequal lateral flagella. The body of gamete contains a haploid nucleus, an eye-shot and vestigial chromatophore. 

Fertilization : The liberated sperm swims towards the female conceptacles. The sperm is attached to the oogonial wall by the anterior flagella. One of the sperms penetrates the oogonial wall and fuses with the egg to form diploid zygote. 

Germination of zygote : The germination of the zygote in Sargassum initiates while enclosed within the oogonium. Within this protective structure, the zygote undergoes transverse division, giving rise to two distinct cells: the upper cell and the lower cell. The upper cell contributes to the formation of the vegetative part of the thallus, which constitutes the main body of the Sargassum algae. Simultaneously, the lower cell undergoes further divisions to develop into the holdfast, a structure that anchors the algae to a substrate.

7.0Life Cycle of Sargassum

8.0Ecological Importance of Sargassum

Sargassum forms extensive floating mats, providing a unique and essential habitat for a diverse range of marine species. These floating ecosystems serve as a refuge, feeding ground, and nursery for various organisms, including fish, invertebrates, and juvenile sea turtles.

Sargassum supports high biodiversity, fostering a variety of species adapted to this specialized floating environment. The presence of numerous organisms within and around Sargassum contributes to the overall diversity of marine ecosystems.

Sargassum is a source of primary production in the open ocean, forming the base of the food web. It provides a food source for herbivores and, in turn, supports predatory species higher up the food chain.