Sargassum: Classification, Thallus, Reproduction & Life Cycle
Sargasum is a seaweed found in tropical, subtropical, and temperate regions. It is distributed in the midlittoral to sublittoral zones. They grow luxuriantly on boulders, rocks, and hard substrata in open surf zones, forming large beds. Due to its dense population in the Atlantic Ocean near Africa, the sea here is called, Sargasso Sea.
Here is the detailed structure and life cycle of Sargassum.
Systematic Position of Sargassum
Division: Phaeophyta
Class: Cyclosporae
Order: Fucales
Family: Sargassaceae
Genus: Sargassum
Since this genus contains brown chromatophores and mannitol and laminarin as the reserve food material, it is classified under Phaeophyta.
The complex morphological structure and anatomical differentiation, along with the absence of zoospores, categorise it under the order Fucales.
Moreover, they have specialized sex organs called conceptacles.
The presence of distinct foliar organs and air bladders is are main feature that brings Sargassum under the family Sargassaceae.
Sargassum Morphological Characteristics
The plant body is diploid, and the thallus is erect and branched. The main axis can grow up to 0 to 50 cm long.
Sargassum Thallus Structure
The thallus is differentiated into a basal holdfast, a cylindrical main axis, and expanded leafy branches from this axis. .
The discoid holdfast helps with anchorage to the substratum.
The main axis bears spirally arranged structures called primary laterals arranged spirally.
Primary laterals bear the flat, expanded secondary laterals.
Secondary laterals grow conceptacles that can be fertile or sterile.
The sterile conceptacles are called cryptoblasts or cryptostomata.
Primary and Secondary Laterals in Sargassum
The diploid thallus of Sargassum has a central axis. From this axis arise branches which are called primary laterals. These primary laterals have unlimited growth and are also called long shoots.
From these long shoots arise the secondary laterals, which have limited growth. The basal secondary lateral is more conspicuous and appears as a leaf. These leaves could be narrow with serrated edges. In S.enerve, these leaves have a midrib. Moreover, the leaves are flattened vertically.
Axillary Branches
The lower part of long shoots bears axillary rudiments that have limited growth.
In the upper part, a more condensed axillary branch system arises.
The basal member of this axillary branch swells to form air bladders.
The other members of the axillary branch are flattened or cylindrical in shape and are called receptacles.
From the receptacles arise flask-shaped fertile conceptacles.
Moreover, the receptacles are formed only when the plant becomes fertile.
In some species, the leaf-like structures are modified into air bladders, which are homologous to the lateral leaves. These air bladders are filled with air and help the plant float.
Anatomical Characteristics
The thallus anatomy of Sargassum shows a high degree of differentiation and division of labor.
A cross-section of the Sargassum thallus shows that it consists of a continuous layer of columnar cells.
The peripheral layer lacks air spaces and contains plastids and fucosan granules.
This peripheral layer is photosynthetic.
The palisade layer is protected on the exterior by the thick mucilagenous cuticle layer.
Another columnar layer on the interior of the palisade layer forms the hypodermal layer.
Cells of the hypodermis have pits in their inner walls.
The multilayered cortex is made of thick-walled polygonal cells that function as storage and mechanical support.
The innermost layer of the cortex has gelatinous cells.
The central zone or medulla has loosely arranged, narrow, and elongated cells.
The outer cells have thick walls, while cells in the inner layer have thin walls.
There is no gelatin in the cells of the medulla.
Medullary hyphae are also absent in the Sargassum thallus.
The thin-walled leaves make the medullary rays, which help with food conduction.
The attaching organs contain hyphae in their medullary regions.
Reproduction in Sargassum
Sargassum exhibits vegetative and sexual reproduction. There is no asexual reproduction in Sargassum.
Vegetative Reproduction
Fragmentation is the vegetative mode of reproduction in Sargassum. When its older parts die and decay, the younger parts will separate from the main thallus. This younger part continues to grow into a new plant.
In Sargassum natans, fragmentation is the only mode of reproduction.
Sexual Reproduction
Sexual reproduction in Sargassum is by the oogamous method.
The flask-shaped structures on concetacles produce sex organs from their receptacles.
Sex organs may be formed on the same conceptacles or separate plants.
The male receptacles are smooth, while the female receptacles are spinous.
The male and female sex organs are called antheridium and oogonium.
Male And Female Conceptacles Of Sargassum
Fertilization
The mature oogonia will be discharged but stay attached to the receptacle with the help of a stalk.
Liberated male gametes swim towards the female conceptacle and get attached to the oogonial wall with the help of flagella.
Later, fertilization occurs to form a zygote.
This zygote germinated to form the diploid thallus.
Post-Fertilization Changes in Sargassum
The fertilization of male and female gametes of Sargassum results in a diploid zygote. Fertilization happens while the oogonium is still attached to the conceptacle.
The germination of the zygote is initiated while still enclosed in the oogonium, and the oogonium is attached to the conceptacle.
Gelatinization of the oogonial wall eventually liberates the zygote.
This liberated zygote moves on and finds a solid object, resting on it.
Here, the zygote undergoes a transverse division to form an upper larger and a lower smaller cell.
The upper cell divides and develops into the diploid plant body.
The lower cell produces fibrous rhizoid-like outgrowths.
Alternation of Generation in Sargassum
Sargassum exhibits a cyclosporian life cycle. In the life cycle of Sargassum, there is only a single morphological individual, the diploid zygote.
The sporophyte bears sex organs, antheridia and oogonia, in flask-shaped depressions called conceptacles. Sargassum plant can be monoecious or dioecious, but the conceptacles are unisexual.
These sex organs undergo meiotic divisions to produce haploid male and female gametes. These haploid gametes fuse to form the diploid zygote, then the diploid zygote divides mitotically to develop into the sporophyte plant body.
Thus, there is no clear alternation of generation in the life cycle of Sargassum in a morphological sense. However, there is a cytological alternation of generations.