Reproduction in general helps in increasing the number of individuals, dispersal and survival under adverse conditions, and enhancement of species fitness through genetic recombination.
Algae reproduces by three methods namely, vegetative, asexual, and sexual reproductions. Of these, vegetative reproduction does not involve spore production of any kind. In addition, vegetative reproduction in algae occurs under favorable conditions.
Types of Vegetative Reproduction in Algae
Here, we will discuss the various methods of vegetative reproduction in algae.
Vegetative Reproduction in Algae by Cell Division
The simplest of all, cell division happens in unicellular algae such as Chlorococcus and Microcystis. Cell division or binary fission is the only mechanism of reproduction in many unicellular species.
Division of protoplast and formation of cell wall by each daughter protoplast is termed eleutheroschisis. True cell division in which the parental cell wall is retained is called desmoschisis.
Some algae have evolved specialized cytokinetic processes to maintain distinctive morphologies in their offspring cells. In diatoms, two halves move apart in cytokinesis and new valves form in the cleavage furrow. The epi valves in the offspring are formed from the epitheca and hypotheca of their parents.
Thus in actively growing populations, there is a progressive diminution of algal cell size. Return to maximum size is brought about via sexual reproduction and auxospore formation.
Binary fission is the most common vegetative reproduction method in planktonic flagellates.
In Dinophyceae (Peridinium, Glenodinium) the theca may be shed before division, necessitating its total reformation in the offspring. In Ceratium sp, and Peridinium bahamense, the parent theca is divided between the daughter cells and only the missing plates are re-formed.
Placoderm or true desmids have semi-cells. Here, cytokinesis occurs in the isthmus. Later, each cell develops into a new semi-cell.
Here, a part of the algal thallus gets detached and grows independently into another individual. The genetic constitution of the parent and the daughter cells thus formed will be the same. Moreover, any part of the thallus can produce a new individual.
Vegetative Reproduction in Algae by Budding
The thallus produces bud-like structures due to the proliferation of some vesicles. These buds will later develop a septum. This septum will lead to the separation of the bud and it grows into a new individual. Budding is common in Protosiphon.
Fragmentation
The process of fragmentation is associated with filamentous algae such as Ulothrix and Spirogyra. Under unfavorable conditions such as mechanical pressure, the formation of a separation disc, or by accident, the filament breaks into smaller fragments. Each of these fragments will then grow into new filaments.
Hormogonia
It is a characteristic of blue-green algae. In this method, the trichomes or filaments of Nostoc, Oscillaltoria, etc., break into two, due to form heterocyst. These broken filaments are called hormogones.
Adventitious branches
Algae such as Chara and Fucus develop adventitious branches from their nodal cells or at the storage cells. The thallus breaks at these points gets separated and grows into a new plant.
Amylum star
In Chara, the nodal cells multiply and produce star-shaped bodies that are filled with starch. These bodies get detached and grow into a new plant. Such star-shaped parts are called amylum stars, due to the presence of amylum starch.
Protonema
Algae that produce a protonema, will have a secondary protonema growing from the rhizoidal node of its primary structure or the basal node of the primary rhizoid. This secondary protonema can develop into a new plant. Example: Chara.
Tuber
Tubers are characteristic features of algae such as Chara. Due to the food storage, some rhizoids and nodes produce tuber-like structures that are capable of growing into new plants.
