Chlorophyta is a division of green algae, informally called chlorophytes. It refers to a highly paraphyletic group of all the green algae within the green plants (Viridiplantae) and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. Like the land plants (bryophytes and tracheophytes), green algae contain chlorophylls a and b and store food as starch in their plastids.
In newer classifications (which will not be used on this Wiki), it refers to one of the two clades making up the Viridiplantae, which are the chlorophytes and the streptophytes or charophytes. In this sense it includes only about 4,300 species.
The division contains both unicellular and multicellular species. While most species live in freshwater habitats and a large number in marine habitats, other species are adapted to a wide range of environments. Watermelon snow, or Chlamydomonas nivalis, of the class Chlorophyceae, lives on summer alpine snowfields. Others live attached to rocks or woody parts of trees. Some lichens are symbiotic relationships between fungi and green algae.
Members of the Chlorophyta also form symbiotic relationships with protozoa, sponges, and cnidarians. All are flagellated, and these have an advantage of motility. Some conduct sexual reproduction, which is oogamous or isogamous.
Species of Chlorophyta are common inhabitants of marine, freshwater and terrestrial environments. Several species have adapted to specialised and extreme environments, such as deserts, arctic environments, hypersaline habitats, marine deep waters and deep-sea hydrothermal vents. Some groups, such as the Trentepohliales are exclusively found on land. Several species of Chlorophyta live in symbiosis with a diverse range of eukaryotes, including fungi to form lichens, ciliates, Foraminifera, Cnidaria and molluscs. Some species of Chlorophyta are heterotrophic, either free-living or parasitic. The heterotrophic green alga Prototheca can cause infections in humans and animals known as protothecosis.