Green Algae are put under the division Chlorophyta comprising of more than 7,000 species.
Green Algae occur in both aquatic and semi-aquatic habitats. Only about 10% of species are marine. The remaining are freshwater aquatics or semi-aquatics. Several species grow in moist terrestrial habitats like moist soils, rocks, tree trunks etc. Chlorella can tolerate moderately warm waters. Some species are found in cold waters and even snow. Snow dwelling forms are called cryophytes. Example- Scotiella, Chlamydomonas nivalis, Ankistrodesmus.
There is wide variation in the plant body or thallus. Some are unicellular and microscopic. Chlamydomonas is flagellated and Chlorella is non-flagellated. Some are colonial like Volvox and Pandorina, and some are filamentous and unbranched like Ulothrix and Spirogyra. Some algae are two-layered sheets of cells with a holdfast, like the edible sea-lettuce, Ulva. Some algae such as Schizomeris has a three-dimensional body composed of several layers of cells. Acetabularia is the largest unicellular plant measuring up to 6cm in length and is umbrella-shaped.
The chloroplasts may be discoid (Chara), plate-like (Mougeoutia), reticulate (Oedogonium), cup-shaped (Chlamydomonas), spiral (Spirogyra) or ribbon-shaped in different species.
Photosynthetic pigments are chlorophyll a, chlorophyll b, carotenes and xanthophylls. The green colour is due to the excess of chlorophyll pigments.
They have storage bodies called pyrenoids containing protein besides starch.
Eye Spots occur in motile or flagellate forms. An eye spot is a photosensitive area associated with the chloroplast.
They have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose. Many forms like Spirogyra and Zygnema secrete mucilage around the cell wall.
Asexual reproduction takes place by motile zoospores, and rarely by non-motile spores called aplanospores, hypnospores, akinetes or autospores.
Sexual reproduction occurs by the formation of sex cells and it may be isogamous (union between two morphologically and physiologically similar gametes), anisogamous (union between structurally similar gametes which differ in size and behaviour) or oogamous (union between a large food-laden nonmotile ovum and a small male gamete without any reserve food).
Three types of life cycles occur in green algae. These are haplontic, diplontic and diplohaplontic. In haplontic life cycle, there is a single somatic phase which is haploid. Diploid stage is represented by a single cell or zygote. It undergoes meiosis and produces haploid spores or meiospores that regenerate the haploid somatic phase. Example- Ulothrix, Spirogyra. In a diplontic life cycle, the dominant phase is diploid. It gives rise to haploid gametes through meiosis. It is called gametic meiosis and occurs in Caulerpa. On the fusion of gametes, a zygote is formed which regenerates the diploid phase. In diplohaplontic life cycle, both diploid and haploid somatic phases are present. Meiosis is sporic. It is of two types, isomorphic (example- Ulva) and heteromorphic (example- Stigeoclonium Subspinosum).
Economic Importance of Green Algae:
Green Algae are the primary producers of energy-rich compounds that form the basis of the food cycle of all aquatic consumers.
Green Algae can be harvested from water bodies and used as manure. Calcium containing green algae (example- Chara) is useful in acidic soils while others are useful in alkaline soils.
Chlorella and Caulpera are used for extracting antibiotics.
Cephleuros causes red rust disease in tea and coffee plants and reduces their yield.
Chlorella and Chlamydomonas help in oxidation in the sewage ponds.
Chlorella can be used for prolonged space flight to dispose off exhaled CO2 and excretions, generate O2 and food.
They increase the level of dissolved oxygen in the immediate environment during photosynthesizing.
A large number of marine algae are used as food in the orient.