Green algae

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The Green Algae (singular: Green Alga) are the large group of algae from which the embryophytes (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 (as well as the accessory pigments beta carotene and xanthophylls), 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. Instead, the green algae probably share a common origin with the red algae; the two are grouped as the Archaeplastida or Plantae sensu lato.

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
• Chlorokybales
• Klebsormidiales
• Zygnematales
• Desmidiales
• Coleochaetales
• Charales (stoneworts)

The orders outside the Chlorophyta are often grouped as the division Charophyta, which is paraphyletic to higher plants, together comprising the Streptophyta. Sometimes the Charophyta is restricted to the Charales, and a division Gamophyta is introduced for the Zygnematales and Desmidiales. In older systems the Chlorophyta may be taken to include all the green algae, but taken as above they appear to form a monophyletic group.

A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were presumably acquired from ingested green algae, and in the latter retain a vestigial nucleus (nucleomorph). Some species of green algae, particularly of genera Trebouxia or Pseudotrebouxia (Trebouxiophyceae), can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus.

The first land plants probably evolved from green algae.

See also:

Red algae

Brown algae

References

Lewis, L.A, McCourt, R.M. (2004) Green algae and the origin of land plants. AMERICAN JOURNAL OF BOTANY 91 (10): 1535-1556 OCT.

 

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