Green algae

Green algae are a diverse group of photosynthetic, autotrophic organisms that contain chlorophyll a and b, giving them a characteristic bright green color, along with accessory pigments such as beta carotene and xanthophylls. They store carbohydrates as starch and typically have cell walls composed of cellulose. Their mitochondria possess flat cristae, and when flagella are present for motility, they are anchored by a cross-shaped system of microtubules. The group encompasses a wide range of forms, from unicellular and colonial flagellates (most with two flagella per cell) to coccoid, filamentous, and macroscopic multicellular seaweeds.

Approximately 22,000 species are known, many living as single cells, while others form colonies, long filaments, or highly differentiated seaweeds. A few lineages, such as Polytomella and Prototheca, have secondarily lost their chlorophyll. Green algae are also crucial as photosynthetic partners in symbiotic relationships: they provide chloroplasts to certain dinoflagellates, euglenids, and chlorarachniophytes through endosymbiosis, and live symbiotically within ciliates, Hydra viridissima, and flatworms. Species of Trebouxia and Trentepohlia commonly form lichens with fungi, with the alga often capable of living independently.

The terrestrial alga Prasiola crispa can form extensive carpets on Antarctic soil. In terms of cell division, members of the class Chlorophyceae typically undergo closed mitosis via a phycoplast, while charophyte green algae and land plants use open mitosis with a phragmoplast to form a cell plate. The group originated from a primary endosymbiotic event in which a heterotrophic eukaryote engulfed a cyanobacterium-like prokaryote, giving rise to primary plastids.

Evolution and classification

Green algae are classified within the clade Viridiplantae, which also includes land plants (Embryophyta), as the latter emerged deep within the charophytes as a sister group to the Zygnematophyceae. The Viridiplantae diverged into two main clades: Chlorophyta (including prasinophytes and core Chlorophyta) and Streptophyta (including charophytes and land plants). The ancestral green alga was a unicellular flagellate. Reproduction varies, with species exhibiting haplobiontic life cycles (only the haploid gametophyte is multicellular) or diplobiontic cycles involving alternation of generations, as seen in Ulva.