Chlorophyta: Green Algae

Examples: Chlamydomonas, Spirogyra, Ulva.

Characteristics: Green colour from Chlorophyll a and b in the same proportions as the 'higher' plants; ß-carotene; and various characteristic xanthophylls. Food reserves starch, some fats or oils like higher plants. Thought to be the origin of the higher green plants but there is currently some doubt on this point.

Green algae may be unicellular, multicellular, colonial or coenocytic (composed of one large cell without cross-walls; the cell may be uninucleate or multinucleate). They have membrane-bound chloroplasts and nuclei. Most species are aquatic and are found commonly in freshwater and marine habitats; some are terrestrial, growing on soil, trees, or rocks. Some are symbiotic with fungi giving lichens. Others are symbiotic with animals, e.g. the freshwater coelentrate Hydra has a symbiotic species of Chlorella as does Paramecium bursaria, a protozoan. Chlorella is now grown and sold as a health supplement. A number of freshwater green algae (charophytes, desmids and Spirogyra) are now included in the Charophyta, a phylum of mostly freshwater and terrestrial algae, which are more closely related to the higher plants than the marine green algae.

Asexual reproduction may be by fission (splitting), fragmentation or by zoospores (motile spores). Sexual reproduction is very common and may be isogamous (gametes both motile and same size); anisogamous (both motile and different sizes - female bigger) or oogamous (female non-motile and egg-like; male motile). May have an alternation of haploid and diploid phases. The haploid phases form gametangia (sexual reproductive organs) and the diploid phases form zoospores by reduction division (meiosis). Some do not have an alternation of generations, meiosis occurring in the zygote. There are about 8,000 species of green algae, about 800 of which are marine.

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