| Green algae | |
|---|---|
 | |
| Green algal diversity. From top left corner:Picocystis (Picocystophyceae),Acetabularia (Ulvophyceae),Botryococcus (Trebouxiophyceae),Volvox (Chlorophyceae),Klebsormidium (Klebsormidiophyceae),Chara (Charophyceae),Spirogyra andMicrasterias (Zygnematophyceae) | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Archaeplastida |
| Clade: | Viridiplantae |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
Thegreen algae (sg.:green alga) are a group ofchlorophyll-containingautotrophicalgae consisting of the phylumPrasinodermophyta and its unnamedsister group that contains theChlorophyta andCharophyta/Streptophyta. Theland plants (Embryophyta) have emerged deep within the charophytes as a sister of theZygnematophyceae.[1][2][3] Since the realization that the Embryophyta emerged within the green algae, some authors are starting to include them.[2][4][5][6][7][excessive citations] The completedclade that includes both green algae and embryophytes ismonophyletic and is referred to as the cladeViridiplantae and as the kingdomPlantae. The green algae include unicellular and colonialflagellates, most with twoflagella per cell, as well as various colonial, coccoid (spherical), and filamentous forms, and macroscopic, multicellularseaweeds. There are about 22,000 species of green algae,[8] many of which live most of their lives as single cells, while other species formcoenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conductphotosynthesis for them. Thechloroplasts indinoflagellates of the genusLepidodinium,euglenids andchlorarachniophytes were acquired fromingestedendosymbiont green algae,[9] and in the latter retain anucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliateParamecium, and inHydra viridissima and inflatworms. Some species of green algae, particularly of generaTrebouxia of the classTrebouxiophyceae andTrentepohlia (classUlvophyceae), can be found in symbiotic associations withfungi to formlichens. 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 familyGraphidaceae. Also the macroalgaPrasiola calophylla (Trebouxiophyceae) is terrestrial,[10] andPrasiola crispa, which live in thesupralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.[11]
Green algae have chloroplasts that containchlorophylla andb, giving them a bright green color, as well as the accessory pigmentsbeta carotene (red-orange) andxanthophylls (yellow) in stackedthylakoids.[12][13] Thecell walls of green algae usually containcellulose, and they store carbohydrate in the form ofstarch.[14]
All green algae havemitochondria with flatcristae. When present, pairedflagella are used to move the cell. They are anchored by a cross-shaped system ofmicrotubules and fibrous strands. Flagella are only present in the motile male gametes ofcharophytes,[15] bryophytes, pteridophytes, cycads andGinkgo, but are absent from the gametes ofPinophyta andflowering plants.
Members of the classChlorophyceae undergo closed mitosis in the most common form of cell division among the green algae, which occurs via aphycoplast.[16] By contrast,charophyte green algae and land plants (embryophytes) undergo openmitosis withoutcentrioles. Instead, a 'raft' of microtubules, thephragmoplast, is formed from themitotic spindle and cell division involves the use of thisphragmoplast in the production of acell plate.[17]
Photosyntheticeukaryotes originated following a primaryendosymbiotic event, where a heterotrophic eukaryotic cell engulfed a photosyntheticcyanobacterium-like prokaryote that became stably integrated and eventually evolved into a membrane-boundorganelle: theplastid.[18] This primary endosymbiosis event gave rise to threeautotrophic clades with primary plastids: the(green) plants (withchloroplasts), thered algae (with rhodoplasts), and theglaucophytes (with muroplasts).[19]
Green algae are often classified with their embryophyte descendants in the green plantcladeViridiplantae (orChlorobionta). Viridiplantae, together with red algae andglaucophyte algae, form the supergroup Primoplantae, also known asArchaeplastida or Plantaesensu lato. The ancestral green alga was a unicellular flagellate.[20]
The Viridiplantae diverged into two clades. TheChlorophyta include the early divergingprasinophyte lineages and the core Chlorophyta, which contain the majority of described species of green algae. TheStreptophyta includecharophytes 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]
| Green algae |
The basal character of the Mesostigmatophyceae, Chlorokybophyceae andspirotaenia 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 asArchaeplastida,Plantae/Viridiplantae, andstreptophytes, respectively.[31]
Green algae are a group of photosynthetic, eukaryotic organisms that include species with haplobiontic and diplobiontic life cycles. The diplobiontic species, such asUlva, follow a reproductive cycle calledalternation 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, thegametophyte is multicellular. The fertilized egg cell, the diploidzygote, undergoesmeiosis, 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 bymitosis and grows into a multicellular diploidsporophyte. The sporophyte produces haploid spores by meiosis that germinate to produce a multicellular gametophyte. Allland plants have a diplobiontic common ancestor, and diplobiontic forms have also evolved independently withinUlvophyceae 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) tofertilization 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 calledprasinophytes.[citation needed]
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 calledconjugation and occurs for example inSpirogyra.[citation needed]
Sex pheromone production is likely a common feature of green algae, although only studied in detail in a few model organisms.Volvox is a genus ofchlorophytes. Different species form spherical colonies of up to 50,000 cells. One well-studied species,Volvox carteri (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, asexualV. 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-resistantzygotes. Sexual development is initiated by aglycoprotein 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.[34] Kirk and Kirk[35] showed that sex-inducing pheromone production can be triggered experimentally insomatic cells byheat shock. Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.[34]
TheClosterium peracerosum-strigosum-littorale (C. psl) complex is a unicellular, isogamouscharophycean alga group that is the closest unicellular relative to land plants.Heterothallic strains of differentmating type can conjugate to formzygospores. Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.[36]
The green algae, including the characean algae, have served asmodel experimental organisms to understand the mechanisms of the ionic and water permeability of membranes,osmoregulation,turgor regulation,salt tolerance,cytoplasmic streaming, and the generation ofaction potentials.[37]
{{citation}}: CS1 maint: work parameter with ISBN (link)Classification ofArchaeplastida orPlantaes.l. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| International | |
|---|---|
| National | |
| Other | |
