The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater unicellular algae,[1] less common today than they were during the Proterozoic.[2] The stated number of species in the group varies from about 14 to 26.[3][4][5] Together with the red algae (Rhodophyta) and the green algae plus land plants (Viridiplantae or Chloroplastida), they form the Archaeplastida. However, the relationships among the red algae, green algae and glaucophytes are unclear,[6] in large part due to limited study of the glaucophytes.[7]
The glaucophytes are of interest to biologists studying the development of chloroplasts because some studies suggest they may be similar to the original algal type that led to green plants and red algae in that glaucophytes may be basal Archaeplastida.[1][8][3]
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] 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.[12] Like red algae, and in contrast to green algae and plants, glaucophytes store fixed carbon in the cytosol.[13]
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.[12]
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 remained uncertain as of March 2022[update], although studies suggest it is most likely that glaucophytes diverged first:[3]
The alternative that glaucophytes and red algae form a clade has been shown to be less plausible, but cannot be ruled out.[3]