Green algae

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Green algae
Stigeoclonium, a chlorophyte green alga genus
Stigeoclonium, a chlorophyte green alga genus
Scientific classificationEdit this classification
(unranked): Archaeplastida
Kingdom: Plantae
Groups included
Cladistically included but traditionally excluded taxa
  • Embryophyta

The green algae (sg.: green alga) are a group of

coenobia
(colonies), long filaments, or highly differentiated macroscopic seaweeds.

A few other organisms rely on green algae to conduct

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 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,[10]
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.[11]

Cellular structure

Green algae have chloroplasts that contain

beta carotene (red-orange) and xanthophylls (yellow) in stacked thylakoids.[12][13] The cell walls of green algae usually contain cellulose, and they store carbohydrate in the form of starch.[14]

All green algae have

flowering plants
.

Members of the class

mitotic spindle and cell division involves the use of this phragmoplast in the production of a cell plate.[17]

Origins

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.[18] This primary endosymbiosis event gave rise to three autotrophic clades with primary plastids: the (green) plants (with chloroplasts) the red algae (with rhodoplasts) and the glaucophytes (with muroplasts).[19]

Evolution and classification

land runoff, and thus can be indicators of nutrient pollution
.

Green algae are often classified with their embryophyte descendants in the green plant

Chlorobionta). Viridiplantae, together with red algae and glaucophyte algae, form the supergroup Primoplantae, also known as Archaeplastida or Plantae sensu lato. The ancestral green alga was a unicellular flagellate.[20]

The Viridiplantae diverged into two clades. The

prasinophyte lineages and the core Chlorophyta, which contain the majority of described species of green algae. The Streptophyta include charophytes and land plants. Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.[21][20][22][23][5][24][25][26][27][28][29][30][1][excessive citations
]

Viridiplantae/
Prasinodermophyta

Palmophyllophyceae (prasinophyte clade VI)

Prasinodermophyceae

Chlorophyta
core Chlorophyta
Chlorophytina

Ulvophyceae

Chlorophyceae

Trebouxiophyceae

Chlorodendrophyceae

Pedinophyceae

Prasinophytes
Clade VIIA

Prasinophytes
Clade VIIC

Pycnococcaceae

Nephroselmidophyceae

Mamiellophyceae

Pyramimonadales

Streptophyta/

Mesostigmatophyceae

Spirotaenia

Chlorokybophyceae

Streptofilum

Klebsormidiophyceae

Phragmoplastophyta

Charophyceae

Coleochaetophyceae

Zygnematophyceae

Mesotaeniaceae s.s.

Embryophyta
(land plants)

Charophyta
green algae

The basal character of the Mesostigmatophyceae, Chlorokybophyceae and spirotaenia are only more conventionally 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 "Charophytes", or these terms are replaced by cladistic terminology such as Archaeplastida, Plantae/Viridiplantae, and streptophytes, respectively.[31]

Reproduction

Green algae conjugating

Green algae are a group of photosynthetic, eukaryotic organisms that include species with haplobiontic and diplobiontic life cycles. The diplobiontic species, such as Ulva, follow a reproductive cycle called alternation of generations in which two multicellular forms, haploid and diploid, alternate, and these may or may not be isomorphic (having the same morphology). In haplobiontic species only the haploid generation, the gametophyte is multicellular. The fertilized egg cell, the diploid zygote, undergoes meiosis, giving rise to haploid cells which will become new gametophytes. The diplobiontic forms, which evolved from haplobiontic ancestors, have both a multicellular haploid generation and a multicellular diploid generation. Here the zygote divides repeatedly by mitosis and grows into a multicellular diploid sporophyte. The sporophyte produces haploid spores by meiosis that germinate to produce a multicellular gametophyte. All land plants have a diplobiontic common ancestor, and diplobiontic forms have also evolved independently within Ulvophyceae more than once (as has also occurred in the red and brown algae).[32]

Diplobiontic green algae include isomorphic and heteromorphic forms. In isomorphic algae, the morphology is identical in the haploid and diploid generations. In heteromorphic algae, the morphology and size are different in the gametophyte and sporophyte.[33]

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 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.

Sex pheromone

heat shock. Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.[34]

The Closterium peracerosum-strigosum-littorale (C. psl) complex is a unicellular, isogamous

Heterothallic strains of different mating type can conjugate to form zygospores. Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.[36]

Physiology

The green algae, including the characean algae, have served as

References

External links