Chlamydomonas — A Unicellular, Flagellated, Green Algae

Chlamydomonas are single-celled (unicellular) algae. The most significant characteristics of Chlamydomonas are it is pyriform in shape, possesses a cup-shaped chloroplast, and has a photoreceptive eyespot at the chloroplast.

It is a small (<10 µm) unicellular, motile organism. It is roughly spherical in shape with two anteriorly located flagella used to ‘swim’ freely in the water. Unlike many green algae, the cell wall of Chlamydomonas is not made of cellulose (as it is in land plants) but instead, the glycoprotein is present.

Classification

On the basis of pigments, green algae (Chlorophyta) are a group of eukaryotes that have some characteristics in common with plants (they are photosynthetic), possess both chlorophyll a and b, generally store carbohydrates (food) as starch, and have cellulosic cell walls.

But they also differ from plants in several ways; most of them are not multicellular, being either unicellular, siphonaceous, or filamentous; they do not retain embryos inside the previous generation as all plants do; few grow on land as almost all plants do.

Ecology and habitat

Species of Chlamydomonas are widely distributed in stagnant water, freshwater, seawater, damp soil, and in snow. It is generally found in a habitat rich in ammonium salt. It possesses red eye spots for photosensitivity and reproduces both asexually and sexually.

Morphology

• Chlamydomonas can be spherical, oval, or sometimes oblong in shape.
• It is unicellular, motile, green algae.
• Generally found in ditches, ponds, stagnant pools, damp soil, and in the snow on mountaintops.
• Pear-shaped and ellipsoidal forms are also very common.
• The cell wall comprises a glycoprotein and non-cellulosic polysaccharides instead of cellulose which may be surrounded by a gelatinous sheath.
• The protoplasm at the anterior end contains two contractile vacuoles which are present near the bases of flagella. They undergo contraction and relaxation alternatively and are considered organs of excretion.
• Two fine hair-like protoplasmic flagella are also present at its anterior end. Each flagellum originates from a basal granule in the anterior papillate or non-papillate region of the cytoplasm. Each flagellum shows a typical arrangement of 9+2 in the component fibrils. They function as organs of locomotion and with their lashing movements the alga swim freely in the water.
• Chloroplast is present in the shape of a cup or bowl. Which contains bands composed of a variable number of photosynthetic thylakoids which are not organized into grana-like structures.
• A reddish pigmented area, the eye spot is present on the lateral side on the anterior end. It consists of two or three, more or less parallel rows of linearly arranged fat droplets. It is considered to be a photo-receptive organ.
• The alga moves away from bright light and moves towards diffused light.
• A large spherical centrally located nucleus is present in the cytoplasm. The nucleus is enclosed in a cup-shaped chloroplast with a single large pyrenoid (a microcompartment within the chloroplasts of algae and hornworts where protein is present besides starch helps in storage as well as to promote photosynthetic CO2 fixation by the enzyme Rubisco) where starch is formed from photosynthetic end products. Pyrenoid with starch sheath is present in the posterior end of the chloroplast.
• The Chlamydomonas under the electron microscope reveal the Golgi apparatus, endoplasmic reticulum, mitochondria, and ribosomes.

Nutrition

Most species are obligate phototrophs but Chlamydomonas reinhardtii and Chlamydomonas dysostosis are facultative heterotrophs that can grow in dark places in the presence of acetate as a carbon source.

Chlamydomonas prepare their food in the same way as green plants do, but without the elaborate system of roots, stems, and leaves of the higher plants.

It is surrounded by water containing dissolved carbon dioxide and salts (ammonium salts) because of that in the light, with the help of its chloroplast, it can build up starch as its food by the process of photosynthesis.

Reproduction & life cycle

Reproduction in Chlamydomonas takes place by asexual as well as sexual methods. In Chlamydomonas asexual reproduction takes place by the formation of zoospores, aplanospores, hypnospores, and palmella stage, while its sexual reproduction is through isogamy, anisogamy, and oogamy.

Asexual reproduction

Asexual reproduction in Chlamydomonas takes place by four means that is by the formation of zoospores, aplanospore, hypnospores, and a stage called palmella stage.

Zoospores

This type of reproduction is very common and takes place through the formation of zoospores under favorable conditions. The haploid algal body loses its motion by withdrawing its flagella.

The contractile vacuoles disappear and the protoplasm divides longitudinally and 2, 4, 8, or, 16 daughter protoplasts may be formed.

The daughter cells grow their own cell wall and flagella. The zoosporangium or the parent cell ruptures to release the daughter cells that grow into separate individual cells.

Each of the daughter protoplasts develops two flagella within the cell wall and forms its own cell wall. They are known as Zoospores.

Each one of them may swim away and grow to full size. Under unfavorable conditions that are, in the damp soil, the daughter protoplasts don’t develop flagella and they do not escape from the parent body.

Aplanospores

Aplanospores are formed in Chlamydomonas under unfavorable conditions. It is a nonmotile asexual spore formed by rejuvenescence.

The parent cell loses flagella, and the protoplast separates from the cell wall. The protoplast divides into 2-16 daughter protoplasts that do not have flagella.

They secrete a thin layer around the protoplast and are known as aplanospores. On arrival of favorable conditions, either the aplanospore may germinate or convert into zoospores.

Hypnospores

Hypnospores are formed under unfavorable conditions by secreting thick walls around the protoplast and are non-motile. The spores germinate into separate individuals on the arrival of favorable conditions.

Palmella stage

Under unfavorable conditions, the daughter cells also undergo repeated divisions and develop mucilage around them but do not develop flagella. The protoplast divides slowly to form an indefinite number of spores this is called the palmella stage.

On the setting of favorable circumstances that is; in flooding of water, etc each one of these develops the flagella and swims independently till it acquires full size.

Sexual reproduction

Sexual reproduction occurs when special cells (gametes) are produced that are capable of attaching to one another, first by their flagella, and later by their anterior ends, thereby achieving protoplast fusion and forming a zygote. This develops into a zygospore (a dormant or resistant cell stage) in which meiosis takes place. Eventually, zygospore germination occurs, releasing haploid motile cells.

Sexual reproduction in Chlamydomonas occurs by three means that is isogamy, anisogamy, and oogamy.

Isogamy

In isogamy, Chlamydomonas produces gametes that are morphologically similar in size but physiologically different. Sexual reproduction takes place by the fusion of two isogametes which are formed in the same way as the zoospores are formed. But these gametes are smaller in size.

The vegetative thallus of the organism, referred to as gametangium, loses its flagella, and the protoplast is separated from the cell wall. The separated protoplast divides into 6 to 32 daughter protoplast. Each daughter protoplast grows flagella and turns into a gamete. The gametes are released from the gametangium.

The flagella of each gamete are covered by agglutinins that secrete a hormone called gamone (a chemical substance secreted by a gamete that attracts another gamete during sexual reproduction). This hormone helps in the recognition of gametes of the opposite strain.

Gametes of different parents conjugate. Thus gametes can be distinguished as male or (+) gametes and female or (-) gametes. Only (+) gametes can fuse with (-) gametes and they do not fuse with (+) gametes.

Similarly, (-) gametes do not fuse with (-) gametes. Their bodies completely fuse to form a zygote that continues swimming for a while and has four flagella which are withdrawn soon and come to rest.

Anisogamy

In anisogamy, gametes of two different sizes are produced, these dissimilar gametes are known as anisogametes. The male gametes, also known as microgametes, are smaller in size and are produced in the male gametangium. The female gametes, also known as macrogamete, are larger in size and are produced in the female gametangium. These two gametes come together to form a diploid cell called a zygote which is later converted into zygospores.

Oogamy

In oogamy, the vegetative thallus of the Chlamydomonas becomes non-motile and acts as a female gamete or macrogamete, or egg. The male gamete is formed by the division of protoplasts and is known as microgametes. The microgamete and macrogamete come together, and a zygote is formed.

Zygote or zygospores

This zygote has a diploid nucleus and is known as a zygospore. It has a thick wall around it and rests for some period. Now, this undergoes meiotic divisions to form four haploid zoospores. They are set free by the breaking of the wall. They now develop their own pairs of flagella.

The asexual reproduction which is haploid dominates and the sporophytic stage which is zygote is very short-lived. The zygospores tolerate a long stretch of unfavorable conditions and start to germinate as soon as favorable conditions are restored. It divides by meiosis to form four haploid nuclei that develop into separate individuals.

Species

Till now more than 500 different species of Chlamydomonas have been described, but most scientists work with only a few.

• Chlamydomonas acidophila
• Chlamydomonas braunii
• Chlamydomonas caudata
• Chlamydomonas debaryanum
• Chlamydomonas ehrenbergii
• Chlamydomonas elegans
• Chlamydomonas moewusii
• Chlamydomonas nivalis
• Chlamydomonas ovoidae
• Chlamydomonas reinhardtii

Facts

It is mobile and phototactic, with the help of a pigment similar to the rhodopsin of human eyes to direct its movement. As phytoplankton, Chlamydomonas is eaten by small heterotrophs in the sea and oceans, e.g. Daphnia.

Uses and importance

• Chlamydomonas has been an important organism for establishing the role of carbonic anhydrase enzyme (organic catalyst) in the eukaryotic carbon dioxide concentrating mechanism (CCM).
• Chlamydomonas reinhardtii is a potential food supplement with the ability to outperform Chlorella and Spirulina.
• Chlamydomonas is used as a model organism for molecular studies in biology, more especially in the studies of flagellar motility and chloroplast dynamics, biogenesis, and, biotechnology, and genetics.

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