The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, as well as the potentially more basal , Chlorokybus and Spirotaenia.
The land plants, or , are thought to have emerged from the . Therefore, Cladistics, embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. The clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial , most with two flagellum per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular . There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conduct photosynthesis for them. The in and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and in . Some species of green algae, particularly of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in symbiotic associations with fungi to form . 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 filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is terrestrial, and Prasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.
All green algae have mitochondrion with flat cristae. When present, paired flagellum are used to move the cell. They are anchored by a cross-shaped system of and fibrous strands. Flagella are only present in the motile male gametes of charophytes bryophytes, pteridophytes, cycads and Ginkgo, but are absent from the gametes of Pinophyta and Angiosperm.
Members of the class Chlorophyceae undergo closed mitosis in the most common form of cell division among the green algae, which occurs via a phycoplast. By contrast, charophyte green algae and land plants (embryophytes) undergo open mitosis without . Instead, a 'raft' of microtubules, the phragmoplast, is formed from the mitotic spindle and cell division involves the use of this phragmoplast in the production of a cell plate.P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York,
The Viridiplantae diverged into two clades. The Chlorophyta include the early diverging Prasinophyceae lineages and the core Chlorophyta, which contain the majority of described species of green algae. The Streptophyta include Charophyta and land plants. Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.
The basal character of the Mesostigmatophyceae, Chlorokybophyceae and spirotaenia are only more conventially basal Streptophytes.
The algae of this paraphyletic group "Charophyta" were previously included in Chlorophyta, so green algae and Chlorophyta in this definition were synonyms. As the green algae clades get further resolved, the embryophytes, which are a deep charophyte branch, are included in "algae", "green algae" and "Charophyta", or these terms are replaced by cladistic terminology such as Archaeplastida, , Viridiplantae or Streptophyta, respectively.
Reproduction varies from fusion of identical cells (isogamy) to fertilisation 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 .
Haploid algal 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 and occurs for example in Spirogyra.
The species of Sea lettuce are reproductively isomorphic, the diploid vegetative phase is the site of meiosis and releases haploid , which germinate and grow producing a haploid phase alternating with the vegetative phase.
The Closterium peracerosum-strigosum-littorale (C. psl) complex is a unicellular, isogamous Charophyceae alga group that is the closest unicellular relative to land plants. Heterothallic strains of different mating type can conjugate to form . Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.