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   » » Wiki: Green Algae
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The green algae (singular: green alga) are a large, informal grouping of consisting of the and /, which are now placed in separate divisions, as well as the potentially more basal , and .

The land plants, or , are thought to have emerged from the . Therefore, , embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. The that includes both green algae and embryophytes is and is referred to as the clade and as the kingdom . The green algae include unicellular and colonial , most with two 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 for them. The in and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a (vestigial nucleus). Green algae are also found symbiotically in the ciliate , and in Hydra viridissima and in . Some species of green algae, particularly of genera of the class and Trentepohlia (class ), can be found in symbiotic associations with 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 . 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.


Cellular structure
Green algae have chloroplasts that contain and , giving them a bright green color, as well as the accessory pigments and ,Burrows 1991. Seaweeds of the British Isles. Volume 2 Natural History Museum, London. in stacked .
(1995). 9780521304191, Cambridge University Press. .
The of green algae usually contain , and they store carbohydrate in the form of .
(2019). 9780878934034, Sinauer Associates Inc..

All green algae have with flat . When present, paired 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 bryophytes, pteridophytes, cycads and , but are absent from the gametes of and .

Members of the class undergo closed mitosis in the most common form of cell division among the green algae, which occurs via a . By contrast, green algae and land plants (embryophytes) undergo open without . Instead, a 'raft' of microtubules, the , is formed from the and cell division involves the use of this in the production of a .P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York,


Origins
Photosynthetic eukaryotes originated following a primary endosymbiotic event, where a heterotrophic eukaryotic cell engulfed a photosynthetic -like prokaryote that became stably integrated and eventually evolved into a membrane-bound : the . This primary endosymbiosis event gave rise to three clades with primary plastids: the , the and the .
(2019). 9780123914996


Evolution and classification
Green algae are often classified with their embryophyte descendants in the green plant (or ). Viridiplantae, together with red algae and algae, form the supergroup Primoplantae, also known as or Plantae sensu lato. The ancestral green alga was a unicellular flagellate.

The Viridiplantae diverged into two clades. The include the early diverging lineages and the core Chlorophyta, which contain the majority of described species of green algae. The include and land plants. Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.

(2019). 9780674021839, Harvard University Press. .

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 "", "green algae" and "", or these terms are replaced by cladistic terminology such as , , or , respectively.

(2019). 9783319281476, Springer International Publishing.


Reproduction
Green algae are eukaryotic organisms that follow a reproduction cycle called alternation of generations.

Reproduction varies from fusion of identical cells () to of a large non-motile cell by a smaller motile one (). 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 .

The species of are reproductively isomorphic, the vegetative phase is the site of and releases haploid , which germinate and grow producing a haploid phase alternating with the vegetative phase.


Sex pheromone
production is likely a common feature of green algae, although only studied in detail in a few model organisms. is a genus of . Different species form spherical colonies of up to 50,000 cells. One well-studied species, (2,000 – 6,000 cells) occupies temporary pools of water that tend to dry out in the heat of late summer. As their environment dries out, asexual V. carteri quickly die. However, they are able to escape death by switching, shortly before drying is complete, to the sexual phase of their life cycle that leads to production of dormant desiccation-resistant . Sexual development is initiated by a pheromone (Hallmann et al., 1998). This pheromone is one of the most potent known biological effector molecules. It can trigger sexual development at concentrations as low as 10−16M. Kirk and Kirk showed that sex-inducing pheromone production can be triggered experimentally in somatic cells by . Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.

The Closterium peracerosum-strigosum-littorale (C. psl) complex is a unicellular, isogamous alga group that is the closest unicellular relative to land plants. strains of different can conjugate to form . Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.


Physiology
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, , regulation, , cytoplasmic streaming, and the generation of action potentials.
(2019). 9783642131455


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