Glaucophyte

The glaucophytes, also called glaucocystophytes or glaucocystids, are a minor group of single-celled algae that live in freshwater and damp places on land. They are far less abundant now than during the Proterozoic era. Estimates of how many species exist range from about 14 to 26. Along with their much larger relatives, the red algae (Rhodophyta) and the green algae plus land plants (Viridiplantae or Chloroplastida), they belong to the major algal lineage called Archaeplastida. Researchers studying how chloroplasts evolved find glaucophytes particularly useful because these algae may resemble the ancient ancestor that gave rise to red algae and green plants—in other words, glaucophytes could be the earliest-branching archaeplastids.

Unlike red and green algae, glaucophytes reproduce only asexually. They undergo open mitosis without centrioles, a trait shared with other early eukaryotes. Their reproductive methods include binary fission, forming zoospores, and producing autospores. For instance, *Cyanophora paradoxa* splits lengthwise, creating two daughter cells that each get one cyanelle. Species of *Glaucocystis* reproduce by releasing non-motile autospores. So far, no one has found any evidence of sexual reproduction in glaucophytes.

The plastids of glaucophytes are called muroplasts, cyanoplasts, or cyanelles. Unlike plastids in other organisms, they have a peptidoglycan layer, which is thought to be a leftover from the endosymbiotic origin of plastids from cyanobacteria. This layer helps with plastid division and serves as molecular proof of their cyanobacterial ancestry. Glaucophytes contain the photosynthetic pigment chlorophyll a. Like red algae and cyanobacteria, they capture light using phycobilisomes—structures made mostly of phycobiliproteins. (Green algae and land plants have lost this pigment.) Also like red algae, but unlike green algae and plants, glaucophytes store fixed carbon in the cytosol rather than inside plastids; this is considered an ancestral trait they have retained. Their phycobilisomes are mainly composed of phycocyanin and allophycocyanin, two key pigments also found in cyanobacteria. These pigments let glaucophytes absorb light at wavelengths chlorophyll cannot, improving light capture in dim aquatic settings. Studies of endosymbiotic gene transfer show that several genes originally from cyanobacterial ancestors have moved to the nuclear genome in glaucophytes, reflecting early stages of plastid–host genomic integration. The evolution of glycogen and starch metabolism in eukaryotes offers molecular clues about how plastid endosymbiosis became established.

The earliest-diverging genus is *Cyanophora*, which has only one or two plastids; when two are present, they are partly 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—features similar to those seen in some green algae.

Phylogenetically, glaucophytes, red algae, and Viridiplantae together make up Archaeplastida—a group of plastid-bearing organisms that likely share a unique common ancestor that took in a cyanobacterium as an endosymbiont. The exact relationships among these three groups are still uncertain, though it is most probable that glaucophytes branched off first. The alternative idea—that glaucophytes and red algae form a single clade—has been shown to be less likely, but it cannot be ruled out entirely.

Internally, the classification of glaucophytes varies widely among sources. A 2017 phylogeny split the group into three families with five genera. A 2019 list of described species uses the same three subdivisions, treated as orders, and adds five possible but unplaced species, for a total of 14 to 19 possible species. As of May 2026, AlgaeBase divided glaucophytes into only two groups, placing *Cyanophora* within Glaucocystales rather than its own order, and listed 25 species across eight genera. None of the glaucophyte species is particularly common in nature. Historically, glaucophytes were once placed in the family Oocystaceae, within the order Chlorococcales.