The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of species in the group varies from about 14 to 26. Together with the much larger sister taxa Rhodophyta (red algae) and Viridiplantae/Chloroplastida (green algae and land plants), they form the primary algae clade Archaeplastida.
The glaucophytes are of interest to biologists studying the evolution of chloroplasts as they may be similar to the ancestral algal type that led to the red algae and green plants, i.e. glaucophytes may be basal archaeplastids.
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.
Characteristics
The plastids of glaucophytes are known as 'muroplasts', 'cyanoplasts', or 'cyanelles'. Unlike the plastids in other organisms, they have a peptidoglycan layer, believed to be a relic of the endosymbiotic origin of plastids from cyanobacteria. This peptidoglycan layer plays a functional role in plastid division and is considered molecular evidence of their cyanobacterial ancestry. Glaucophytes contain the photosynthetic pigment chlorophyll a. Like red algae, and in contrast to green algae and plants, glaucophytes store fixed carbon 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. 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. The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis.
The most early-diverging genus is Cyanophora, which only has one or two plastids. When there are two, they are semi-connected.
Glaucophytes have mitochondria with flat cristae, and undergo open mitosis without centrioles. Motile forms have two unequal flagella, which may have fine hairs and are anchored by a multilayered system of microtubules, both of which are similar to forms found in some green algae.
Taxonomy
thumb|[[Cyanophora paradoxa]]
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.
- Order Cyanophorales
- Genus Cyanophora – 5–6 species
- Order Glaucocystales
- Genus Glaucocystis – 7–8 species
- Order Gloeochaetales
- Cyanoptyche – 1 species
- Gloeochaete – 1 species
- Other possible species
- ?Archaeopsis monococca <small>Skuja</small>
- ?Chalarodora azurea <small>Pascher</small>
- ?Glaucocystopsis africana <small>Bourrelly</small>
- ?Peliaina cyanea <small>Pascher</small>
- ?Strobilomonas cyaneus <small>Schiller</small>
, AlgaeBase divided glaucophytes into only two groups, placing Cyanophora in Glaucocystales rather than Cyanophorales (however the entry was dated 2011). AlgaeBase included a total of 25 species in eight genera:
- Glaucocystales
- Chalarodora <small>Pascher</small> – 1 species
- Cyanophora <small>Korshikov</small> – 6 species
- Glaucocystis <small>Itzigsohn</small> – 13 species
- Glaucocystopsis <small>Bourrelly</small> – 1 species
- Peliaina <small>Pascher</small> – 1 species
- Strobilomonas <small>Schiller</small> – 1 species
- Gloeochaetales
- Cyanoptyche <small>Pascher</small> – 1 species
- Gloeochaete <small>Lagerheim</small> – 1 species
None of the species of Glaucophyta is particularly common in nature.