Stigeoclonium, a chlorophyte green alga genus
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, although the group including both green algae and embryophytes is monophyletic (and often just known as kingdom Plantae). The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic seaweeds. In the Charales, the closest relatives of higher plants, full differentiation of tissues occurs. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form colonies, coenobia, long filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a vestigial nucleus (nucleomorph). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridis and flatworms. Some species of green algae, particularly of genera Trebouxia and 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. Trentepohlia is a green filamentous alga that can live freely on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae.
Almost all forms have chloroplasts. These contain chlorophylls a and b, giving them a bright green color (as well as the accessory pigments beta carotene and xanthophylls), and have stacked thylakoids.
All green algae have mitochondria with flat cristae. When present, flagella are typically anchored by a cross-shaped system of microtubules and fibrous strands, but these are absent among the higher plants and charophytes, which instead have a 'raft' of microtubules, the spline. Flagella are used to move the organism. Green algae usually have cell walls containing cellulose, and undergo open mitosis without centrioles.
Photosynthetic eukaryotes originated following a primary endosymbiotic event, where a heterotrophic eukaryotic cell engulfed a photosynthetic cyanobacterium-like prokaryote that became stably integrated and eventually evolved into a membrane-bound organelle, the plastid. This primary endosymbiosis event gave rise to three autotrophic clades with primary plastids: the green plants, the red algae and the glaucophytes.
Evolution and classification
Green algae are often classified with their embryophyte descendants in the green plant clade Viridiplantae (or Chlorobionta). Viridiplantae, together with red algae and glaucophyte algae, form the supergroup Primoplantae, also known as Archaeplastida or Plantae sensu lato.
The Viridiplantae diverged into two clades. The Chlorophyta includes the early diverging prasinophyte lineages and the core Chlorophyta, which contain the majority of described species of green algae. The Streptophyta includes the charophytes, a paraphyletic assemblage of freshwater algae from which the land plants have evolved. Below is a consensus reconstruction of green algal relationships, mainly based on molecular data. 
Green algae are eukaryotic organisms that follow a reproduction cycle called alternation of generations.
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.
Haploid algae cells (containing only one copy of their DNA) can fuse with other haploid cells to form diploid zygotes. When filamentous algae do this, they form bridges between cells, and leave empty cell walls behind that can be easily distinguished under the light microscope. This process is called conjugation.
The species of Ulva are reproductively isomorphic, the diploid vegetative phase is the site of meiosis and releases haploid zoospores, which germinate and grow producing a haploid phase alternating with the vegetative phase.
The green algae span a wide range of δ13C values, with different groups having different typical ranges.
|Algal group||δ13C range|
|HCO3-using red algae||−22.5‰ to −9.6‰|
|CO2-using red algae||−34.5‰ to −29.9‰|
|Brown algae||−20.8‰ to −10.5‰|
|Green algae||−20.3‰ to −8.8‰|
The green algae, including the characean algae, have served as model experimental organisms to understand the mechanisms of the ionic and water permeability of membranes, osmoregulation, turgor regulation, salt tolerance, cytoplasmic streaming, and the generation of action potentials.
- Jeffrey D. Palmer, Douglas E. Soltis and Mark W. Chase (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany 91 (10): 1437–1445. doi:10.3732/ajb.91.10.1437. PMID 21652302.
- Guiry, M.D. (2012). "How many species of algae are there?". Journal of Phycology 48: 1057–1063. doi:10.1111/j.1529-8817.2012.01222.x.
- Burrows 1991. Seaweeds of the British Isles. Volume 2 Natural History Museum, London. ISBN 0-565-00981-8
- Hoek, C. van den, Mann, D.G. and Jahns, H.M. 1995. Algae An introduction to phycology. Cambridge University Press, Cambridge. ISBN 0-521-30419-9
- Keeling, PJ (2010). "The endosymbiotic origin, diversification and fate of plastids". Philosophical Transactions of the Royal Society B: Biological Sciences 365 (1541): 729–748. doi:10.1098/rstb.2009.0103. ISSN 0962-8436.
- De Clerck, O; Bogaert, KA; Leliaert, F (2012). "Diversity and Evolution of Algae". Advances in Botanical Research 64: 55–86. doi:10.1016/B978-0-12-391499-6.00002-5. ISSN 00652296.
- Lewis, L. A & R. M. McCourt (2004). "Green algae and the origin of land plants". American Journal of Botany 91 (10): 1535–1556. doi:10.3732/ajb.91.10.1535. PMID 21652308.
- Leliaert, Frederik; Smith, David R.; Moreau, Hervé; Herron, Matthew D.; Verbruggen, Heroen; Delwiche, Charles F.; De Clerck, Olivier (2012). "Phylogeny and Molecular Evolution of the Green Algae" (PDF). Critical Reviews in Plant Sciences 31: 1–46. doi:10.1080/07352689.2011.615705.
- Marin, Birger (2012). "Nested in the Chlorellales or Independent Class? Phylogeny and Classification of the Pedinophyceae (Viridiplantae) Revealed by Molecular Phylogenetic Analyses of Complete Nuclear and Plastid-encoded rRNA Operons". Protist 163: 778–805. doi:10.1016/j.protis.2011.11.004.
- Laurin-Lemay, Simon; Brinkmann, Henner; Philippe, Hervé (2012). "Origin of land plants revisited in the light of sequence contamination and missing data". Current Biology 22: R593–R594. doi:10.1016/j.cub.2012.06.013.
- Maberly, S. C.; Raven, J. A.; Johnston, A. M. (1992). "Discrimination between 12C and 13C by marine plants". Oecologia 91 (4): 481. doi:10.1007/BF00650320. JSTOR 4220100.
- Tazawa, Masashi (2010). "Sixty Years Research with Characean Cells: Fascinating Material for Plant Cell Biology". Progress in Botany 72: 5–34. Retrieved 7-10-2012.