Chlamydomonas

Chlamydomonas is a diverse genus of green algae comprising approximately 150 species of single-celled biflagellates. These organisms thrive in various environments, ranging from stagnant water and damp soil to freshwater, seawater, and even in unique habitats like snow, where they are commonly referred to as "snow algae."

1.0General Characteristics of Chlamydomonas : 

Chlamydomonas is one of the most primitive, unicellular, motile, eukaryotic organisms. The main character of this algae is the cellulose cell wall, which resembles the green plants, with chloroplast and autotrophic nutrition. Photosynthesis in Chlamydomonas resembles the higher plants. The cell varies in shape; it can be oval, oblong, cylindrical, pear-shaped, or spherical.  

The protoplast of Chlamydomonas is differentiated into a plasma membrane, cytoplasm, a single nucleus, a chloroplast containing one or more pyrenoids, two contractile vacuoles, an spot and typical organelles of the eukaryotic cells (Mitochondria, Golgi apparatus, endoplasmic reticulum, and ribosomes).

The cell is provided with two flagella, which are whiplash types with a typical 9+2 arrangement. Flagella in Chlamydomonas arise from basal granules that control the movement of their respective flagellum; they are equal in length and longer than the length of the cell. 

A tiny spot of orange colour present in at the anterior end of the cell which is an integrated part of chloroplast, it functions as a photoreceptive organ which is known as stigma.

2.0Classification of Chlamydomonas 

3.0Diagram of Chlamydomonas

4.0Reproduction In Chlamydomonas

Chlamydomonas Reproduces are following types : 

Asexual Reproduction In Chlamydomonas

Zoospore

Chlamydomonas multiplies by zoospore in favourable conditions. The parent cell comes to rest, and flagella are resorbed, and the contractile vacuole disappears. The cytoplasm, chloroplast, and the nucleus divide mitotically in a longitudinal plane into two daughter protoplasts.   

The second division is at the right angle to the first; in this way, four daughter cells are usually formed, and the chloroplast is halved along with the pyrenoid at each successive division. Occasionally, when conditions are exceptionally favourable, mitotic cell division in Chlamydomonas may continue longer than usual, forming eight or more daughter cells through successive bipartitioning. The parental cell then ruptures, and daughter cells are released. The liberated daughter cells soon grow to full size and repeat the process. 

Aplanospore  

During unfavourable conditions, Chlamydomonas forms aplanospores. The parental cell comes to rest, and the flagella is resorbed. The protoplast separates from the cell wall and rounds up; the protoplast secretes a thin layer around it and divides mitotically. When favourable conditions return, each spore germinates into a new cell. 

Hypnospore

Under specific conditions, aplanospores develop protective measures by secreting thick walls around themselves and accumulating significant food reserves. Hypnospore are produced to overcome the unfavourable conditions in which each spore secretes a thick wall around it. 

Palmella stage 

In some conditions, the parental cell divides to form four to eight daughter cells; these cells fail to develop flagella. Each daughter cell gelatinised itself. Numerous cells are embedded in the common mucilaginous, this stage is known as palmella stage (this name is given by a scientist on the name of palmella which is an algae). When the conditions favour all the cells become motile and grow into mature cells. This stage is characterised by immobile reproductive phase. 

Sexual Reproduction In Chlamydomonas 

Isogamy

In this type of sexual reproduction Chlamydomonas produce gametes which are morphologically identical and physiologically different to each other; these gametes are called isogametes. These gametes are motile, cell wall may or may not be present. After the fertilisation the zygote which has four flagella at this stage is converted into the zygospore which is non-motile. Some species of Chlamydomonas are homothallic, while some are heterothallic. In homothallic, both the gametes are produced by the same parent. In heterothallic, both gametes are produced by different parents; one is called the (+) strain, and the other is the (-) strain. 

Homothallic Species: An Illustration of Isogamous Sexual Reproduction

• Homothallic species exemplify a form of sexual reproduction known as isogamy, where sexual fusion occurs between two motile, biflagellate gametes originating from the same parent cell. These gametes are identical in form, size, structure, and behaviour—termed isogametes. They are pear-shaped, lack cell walls, and are equipped with two flagella for mobility.
• During gamete formation, a Chlamydomonas cell retracts its flagella and enters a resting state. The cell's protoplast undergoes successive bipartitions to produce 16, 32, or 64 daughter protoplasts. 
• Each protoplast develops two flagella, transforming into a motile gamete. These gametes, smaller than zoospores, are released into the surrounding water when the parent cell wall ruptures. They swim freely for a period before pairing off to fuse, either end-to-end or side-by-side, resulting in a mingling of cytoplasm and the fusion of nuclei. The resulting zygote, initially a naked, motile, quadriflagellate entity, eventually secretes a thick wall around itself to form a zygospore.

Heterothallic Species: Isogamous Sexual Reproduction with Mating Types

• Heterothallic species, on the other hand, demonstrate isogamous sexual reproduction involving gametes from two distinct parent cells. These cells are classified into two mating types: the plus strain (+) and the minus strain (-). Despite being morphologically similar, the gametes are chemically distinct, facilitating mutual attraction and fusion between + and - strains.
• In these species, the adult individuals or young vegetative cells derived from the zygospore act as calyptogametes. Before fusion, these calyptogametes exhibit a strong tendency to clump together, a process facilitated by the entanglement of flagellar tips from opposite mating types. This attraction is attributed to complementary glycoproteins known as isoagglutinins.
• Calyptogametes emerge from these clumps in pairs, united at their flagellar tips, and proceed to swim using a single pair of flagella from the plus strain. The protoplasts eventually exit their cell walls and fuse entirely in the water, combining both cytoplasm and nuclei to complete the sexual reproduction process.

Anisogamy

Certain species of Chlamydomonas exhibit a unique reproductive strategy wherein gametes differ in size and are enveloped by a cell wall, termed calypto gametes. The female gametes, also referred to as macrogametes, are larger and are generated within female gametangia. Conversely, the male gametes, known as microgametes, are smaller and are produced within male gametangia.

During fertilisation these gametes comes in contact with each other and the protoplast of microgamete slips out from its cell wall and enters into the macrogamete and fuses with it to form zygote.

Oogamy 

In this condition, the female parent cell resorbed its flagella and converted into the large spherical macrogamete. The male parent cell divides mitotically to form microgametes which are mobile, both kinds of gametes possess a cell wall. Microgametes swim towards the macrogametes and fertilisation takes place resulting in formation of zygote.

Zygospore 

The zygote secretes a thick wall around it to become a resting zygospore. During this period it is red orange coloured and contains fat and reserve food material other than starch. 

During germination  of zygospore this red orange colour changes into the green colour and the diploid nucleus undergoes meiosis (reductional division) to form four to eight haploid nuclei. These are also known as meiospore, these spores released and grow to an adult Chlamydomonas cell.   

5.0Life Cycle of Chlamydomonas

6.0Some Common Species of Chlamydomonas

• Chlamydomonas reinhardtii: this species serves in various scientific experiments such as genetics and cell biology. 
• Chlamydomonas nivalis: this species is also known as "watermelon snow algae," and found on polar regions, snow appears pink or red due to this algae. 
• Chlamydomonas debaryana: This extremophilic species is found in harsh environments.