| Glaucophyta | |
|---|---|
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| Glaucocystis sp. | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Archaeplastida |
| Division: | Glaucophyta Skuja 1948 |
| Class | |
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| Synonyms | |
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Theglaucophytes, also known asglaucocystophytes orglaucocystids, are a small group of unicellularalgae found in freshwater and moist terrestrial environments,[1][2] less common today than they were during theProterozoic.[3] The stated number of species in the group varies from about 14 to 26.[4][5][6] Together with thered algae (Rhodophyta) and thegreen algae plus land plants (Viridiplantae or Chloroplastida), they form theArchaeplastida.
The glaucophytes are of interest to biologists studying the evolution ofchloroplasts as they may be similar to the original algal type that led to the red algae and green plants, i.e. glaucophytes may bebasal Archaeplastida.[1][7][4]
Unlike red and green algae, glaucophytes only haveasexual reproduction.[8]
Unlike red and green algae, glaucophytes reproduce exclusively through asexual means. They undergo open mitosis without centrioles, a trait shared with other basal eukaryotes. Reproductive modes include binary fission, zoospore formation, and autosporulation. For example,Cyanophora paradoxa divides longitudinally, producing two daughter cells, each inheriting a single cyanelle. Species ofGlaucocystis reproduce via non-motile autospores. To date, there is no evidence of sexual reproduction in glaucophytes.[9]
Theplastids of glaucophytes are known as 'muroplasts',[10] 'cyanoplasts', or 'cyanelles'. Unlike the plastids in other organisms, they have apeptidoglycan layer, believed to be a relic of theendosymbiotic origin of plastids fromcyanobacteria.[1][11]This peptidoglycan layer plays a functional role in plastid division and is considered molecular evidence of their cyanobacterial ancestry.[12] Glaucophytes contain the photosynthetic pigmentchlorophyll a.[1] Along withred algae[1] andcyanobacteria, they harvest light viaphycobilisomes, structures consisting largely ofphycobiliproteins. Thegreen algae andland plants have lost that pigment.[13] Like red algae, and in contrast to green algae and plants, glaucophytes storefixed carbon in thecytosol.[14]
This cytosolic carbon fixation, rather than fixation within plastids, is considered a retained ancestral trait. Glaucophyte phycobilisomes are composed primarily of phycocyanin and allophycocyanin, two key pigments also present in cyanobacteria. These pigments allow absorption of light at wavelengths that chlorophyll cannot, enhancing light harvesting in low-light aquatic environments.[15]Studies of endosymbiotic gene transfer (EGT) suggest that several genes originally encoded in cyanobacterial ancestors have been relocated to the nuclear genome in glaucophytes, reflecting early stages of plastid-host genomic integration.[16]The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis.[citation needed]
The most early-diverging genus isCyanophora, which only has one or two plastids. When there are two, they are semi-connected.[17]
Glaucophytes havemitochondria with flatcristae, and undergo openmitosis withoutcentrioles. Motile forms have two unequalflagella, which may have fine hairs and are anchored by a multilayered system ofmicrotubules, both of which are similar to forms found in some green algae.[13]
Together withred algae andViridiplantae (green algae andland plants), glaucophytes form theArchaeplastida – a group ofplastid-containing organisms that may share a unique common ancestor that established anendosymbiotic association with acyanobacterium. The relationship among the three groups remains uncertain, although it is most likely that glaucophytes diverged first:[4]
| Archaeplastida |
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The alternative, that glaucophytes and red algae form a clade, has been shown to be less plausible, but cannot be ruled out.[4]
The internal phylogeny of the glaucophytes and the number of genera and species varies considerably among taxonomic sources. A phylogeny of the Glaucophyta published in 2017 divided the group into three families, and includes five genera:[18]
| Glaucophyta |
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A 2019 list of the described glaucophyte species has the same three subdivisions, treated as orders, but includes a further five unplaced possible species, producing a total of between 14 and 19 possible species.[4]
A recent study by Takahashi et al. (2023) used both morphological and molecular data to distinguish five distinct species within the genusCyanophora, confirming species boundaries and supporting the utility of combined phylogenetic analysis.[19]
As of March 2022[update],AlgaeBase divided glaucophytes into only two groups, placingCyanophora in Glaucocystales rather than Cyanophorales (however the entry was dated 2011).[20] AlgaeBase included a total of 26 species in nine genera:[21]
None of the species of Glaucophyta is particularly common in nature.[1]
The glaucophytes were considered before as part of familyOocystaceae, in the orderChlorococcales.[22]
Classification ofArchaeplastida orPlantaes.l. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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