Algae classified as green are numerous (17,000 species) and widely distributed. Most live in either marine or freshwater habitats. Some are terrestrial. Others form symbiotic relationships with fungi (e.g., Trebouxia in lichens) or animals. Organisms within this group can either be unicellular, colonial, filamentous, or three dimensional, the latter usually being tubular or membranous. A few species of green algae are heterotrophic. Most photosynthesize using chlorophyll’s a and b. Most also have cellulose cell walls and store true starch as a food reserve.

The life cycle of a green alga appears to be correlated with whether it lives in a freshwater or marine habitat. Marine species have either a well-defined alteration of haploid and diploid generations (e.g., Ulva Fig.1) or are vegetatively diploid (e.g., Acetabularia, Fig.2). In freshwater species like Spirogyra (Fig.3), the haploid, gamete-producing generation is the vegetative plant. Dormant zygotes are the only 2n cells. They form in response to conditions unfavorable for vegetative growth. Gametes produced during sexual reproduction in either freshwater or marine species have two or more whiplash-type flagella. Vegetative cells in motile unicellular or colonial species (e.g., Volvox Fig.4), also produce flagella, as do asexual spores in some species. Flagellated cells in the Charophyceae differ from those of the other green algal classes in having an asymmetrical flagella root system of microtubules, similar to that found in sperm of some green land plant roots. Members of the Charophyceae also produce a unique microtubular system for cell division similar for that found in cells of green land plants.

The green algae are of considerable ecological importance and also have good research potential. Planktonic forms make strong contributions to global productivity. Some can be especially important in the recycling of waste and sewage. The calciferous greens contribute to coral reef formation. Green algae and lichens invade very dry or cold regions otherwise unsuitable for growth. Cultured planktonic species are widely used in biochemical research focusing on cellular metabolism. Giant green algal cells provide opportunities for biological research that would be otherwise be difficult using typically sized plant cells.

The fossil record suggests that green algae are an ancient group. Those that produce calcium carbonate walls fossilize especially well. Members of this subgroup extend back 600 million years to Cambrium times. Fossils from other green algal classes extend back to the Hadrynian period 400 million years earlier. Along with blue-green algae, they apparently co-dominate all ecosystems through that age, and remain prevalent through to the present time.