Green alga

The green algae are the large group of algae from which the higher plants emerged. As such they form a paraphyletic group, variously included among the Plantae or with the Protista. The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales, the closest relatives of higher plants, full differentiation of tissues occurs.

Almost all forms have chloroplasts. These contain chlorophylls a and b, giving them a bright green colour, and have stacked thylakoids. They are bound by a double membrane, so presumably were acquired by direct endosymbiosis of cyanobacteria. A number of cyanobacteria show similar pigmentation, but this appears to have arisen more than once, and the chloroplasts of green algae are no longer considered closely related to such forms. The only other groups with primary chloroplasts are the red algae and glaucophytes, and it may be that the green algae share a common origin with them. The euglenids and chlorarchniophytes also have green chloroplasts, which were presumably acquired from ingested green algae.

All green algae have mitochondria with flat cristae. When present flagella are typically anchored by a cross-shaped system of microtubules, but these are absent among the higher plants and charophytes. They usually have cell walls containing cellulose, and undergo open mitosis without centrioles. Sexual reproduction varies from fusion of identical cells (isogamy) to fertilization of a large non-motile cell by a smaller motile one (oogamy). However, these traits show some variation, most notably among the basal green algae, called prasinophytes. The remaining forms are usually classified as follows:

• Chlorophyta
  • Chlorophyceae
  • Ulvophyceae
  • Trebouxiophyceae
• Charophyta - paraphyletic to higher plants, together comprising the Streptophyta
  • Chlorokybales
  • Klebsormidiales
  • Zygnematales
  • Desmidales
  • Coleochaetales
  • Charales (stoneworts)

Sometimes the Zygnematales and Desmidales are given their own division, the Gamophyta, but in that case the Charophyta become paraphyletic to two groups instead of becoming monophyletic. The exact circumscription of the Chlorophyta varies, and sometimes all green algae are included there, but as listed above they probably comprise a monophyletic group.