Glaucophyte

The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments,^[1][2] less common today than they were during the Proterozoic.^[3] The stated number of species in the group varies from about 14 to 26.^[4][5][6] Together with the red algae (Rhodophyta) and the green algae plus land plants (Viridiplantae or Chloroplastida), they form the Archaeplastida.

Glaucophyta
Glaucocystis sp.
Scientific classification
Domain:	Eukaryota
Clade:	Archaeplastida
Division:	Glaucophyta Skuja 1948
Class
Glaucocystophyceae Schaffner 1922
Synonyms
Glaucocystophyta Kies & Kremer, 1986

The glaucophytes are of interest to biologists studying the evolution of chloroplasts 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.^[1][7][4]

Unlike red and green algae, glaucophytes only have asexual reproduction.^[8]

Reproduction

Unlike red and green algae, glaucophytes only have asexual reproduction. 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.^[9]

Characteristics

The plastids of glaucophytes are known as 'muroplasts',^[10] '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.^[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 pigment chlorophyll a.^[1] Along with red algae^[1] and cyanobacteria, they harvest light via phycobilisomes, structures consisting largely of phycobiliproteins. The green algae and land plants have lost that pigment.^[13] Like red algae, and in contrast to green algae and plants, glaucophytes store fixed carbon in the cytosol.^[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.

The most early-diverging genus is Cyanophora, which only has one or two plastids. When there are two, they are semi-connected.^[17]

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.^[13]

Representation of a glaucophyte

1. Anterior flagellum (with hairs)
2. Mucocyst, discharges a mucous mass sometimes used in cyst formation
3. Plate
4. Plate vesicle
5. Starch granule
6. Furrow
7. Anterior folds
8. Basal body
9. Contractile vacuole, regulates the quantity of water inside a cell
10. Golgi apparatus; modifies proteins and sends them out of the cell
11. Plastid membranes (2, primary)
12. Peptidoglycan, a polysaccharide layer surrounding the cytoplasmic membrane
13. Central body
14. Thylakoids, site of the light-dependent reactions of photosynthesis
15. Phycobilisome
16. Nucleolus
17. Nucleus
18. Endoplasmic reticulum, the transport network for molecules going to specific parts of the cell
19. Mitochondrion, creates ATP (energy) for the cell, (flat cristae)
20. Posterior flagellum

Phylogeny

External

Together with red algae and Viridiplantae (green algae and land plants), glaucophytes form the Archaeplastida – a group of plastid-containing organisms that may share a unique common ancestor that established an endosymbiotic association with a cyanobacterium. The relationship among the three groups remains uncertain, although it is most likely that glaucophytes diverged first:^[4]

Archaeplastida	Glaucophyta Red algae Viridiplantae

The alternative, that glaucophytes and red algae form a clade, has been shown to be less plausible, but cannot be ruled out.^[4]

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:^[18]

Glaucophyta	Cyanophoraceae Cyanophora Gloeochaetaceae Cyanoptyche Gloeochaete Glaucocystidaceae Glaucocystopsis Glaucocystis

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.^[4]

• Order Cyanophorales
  • Genus Cyanophora – 5–6 species

A recent study by Takahashi et al. (2023) used both morphological and molecular data to distinguish five distinct species within the genus Cyanophora, confirming species boundaries and supporting the utility of combined phylogenetic analysis.^[19]

• Order Glaucocystales
  • Genus Glaucocystis – 7–8 species
• Order Gloeochaetales
  • Cyanoptyche – 1 species
  • Gloeochaete – 1 species
• Other possible species
  • ?Archaeopsis monococca Skuja
  • ?Chalarodora azurea Pascher
  • ?Glaucocystopsis africana Bourrelly
  • ?Peliaina cyanea Pascher
  • ?Strobilomonas cyaneus Schiller

As of March 2022^[update], AlgaeBase divided glaucophytes into only two groups, placing Cyanophora in Glaucocystales rather than Cyanophorales (however the entry was dated 2011).^[20] AlgaeBase included a total of 26 species in nine genera:^[21]

• Glaucocystales
  • Chalarodora Pascher – 1 species
  • Corynoplastis Yokoyama, J.L.Scott, G.C.Zuccarello, M.Kajikawa, Y.Hara & J.A.West – 1 species
  • Cyanophora Korshikov – 6 species
  • Glaucocystis Itzigsohn – 13 species
  • Glaucocystopsis Bourrelly – 1 species
  • Peliaina Pascher – 1 species
  • Strobilomonas Schiller – 1 species
• Gloeochaetales
  • Cyanoptyche Pascher – 1 species
  • Gloeochaete Lagerheim – 1 species

None of the species of Glaucophyta is particularly common in nature.^[1]

The glaucophytes were considered before as part of family Oocystaceae, in the order Chlorococcales.^[22]