The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments,[ Genomic Insights Into the Biology of Algae] less common today than they were during the Proterozoic.[ The monoplastidic bottleneck in algae and plant evolution | Journal of Cell Science]
The glaucophytes are of interest to biologists studying the evolution of as they may be similar to the original algal type that led to the red algae and green plants, i.e. glaucophytes may be basal Archaeplastida.[
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Unlike red and green algae, glaucophytes only have asexual reproduction.
Reproduction
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 of Glaucocystis reproduce via non-motile autospores. To date, there is no evidence of sexual reproduction in glaucophytes.[Jackson, C. (2015). The Glaucophyta: The blue-green plants in a nutshell. Acta Societatis Botanicorum Poloniae
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Characteristics
The of glaucophytes are known as '',[Skuja, A. (1948). Taxonomie des Phytoplanktons einiger Seen in Uppland, Schweden. Symbolae Botanicae Upsalienses 9(3): 1-399.] Like red algae, and in contrast to green algae and plants, glaucophytes store carbon fixation in the cytosol.
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.[Nowack, E. C. M., et al. (2008). Gene transfers from diverse bacteria compensate for reductive genome evolution in the chromatophore of Paulinella chromatophora. PNAS
/ref>The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis.
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The most early-diverging genus is Cyanophora, which only has one or two plastids. When there are two, they are semi-connected.
Glaucophytes have mitochondrion with flat cristae, and undergo open mitosis without . Motile forms have two unequal flagella, which may have fine hairs and are anchored by a multilayered system of , both of which are similar to forms found in some green algae.[
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Phylogeny
External
Together with red algae and Viridiplantae (green algae and Embryophyte), glaucophytes form the Archaeplastida – a group of plastid-containing organisms that may share a unique common ancestor that established an Endosymbiont association with a cyanobacteria. The relationship among the three groups remains uncertain, although it is most likely that glaucophytes diverged first:[
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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.[
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Internal
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:
Taxonomy
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.[Takahashi, T., Hirano, Y., Ichinomiya, M., & Nozaki, H. (2023). Five *Cyanophora* (Cyanophorales, Glaucophyta) species delineated based on morphological and molecular data. Journal of Phycology
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Order Glaucocystales
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Order Gloeochaetales
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Other possible species
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? Archaeopsis monococca Skuja
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? Chalarodora azurea Pascher
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? Glaucocystopsis africana Bourrelly
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? Peliaina cyanea Pascher
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? Strobilomonas cyaneus Schiller
, AlgaeBase divided glaucophytes into only two groups, placing Cyanophora in Glaucocystales rather than Cyanophorales (however the entry was dated 2011).
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Glaucocystales
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Chalarodora Pascher – 1 species
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Corynoplastis Yokoyama, J.L.Scott, G.C.Zuccarello, M.Kajikawa, Y.Hara & J.A.West – 1 species
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Cyanophora Korshikov – 6 species
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Glaucocystis Itzigsohn – 13 species
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Glaucocystopsis Bourrelly – 1 species
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Peliaina Pascher – 1 species
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Strobilomonas Schiller – 1 species
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Gloeochaetales
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Cyanoptyche Pascher – 1 species
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Gloeochaete Lagerheim – 1 species
None of the species of Glaucophyta is particularly common in nature.
The glaucophytes were considered before as part of family Oocystaceae, in the order Chlorococcales.
