Glaucophyta

Glaucophyta is a small phylum of freshwater algae comprising a limited number of species that represent one of the earliest-diverging lineages of the supergroup Archaeplastida. Glaucophytes are predominantly unicellular or form simple colonies and inhabit freshwater environments worldwide, often in oligotrophic or shaded habitats.

A defining feature is their plastids, called cyanelles, which retain a relic peptidoglycan layer between the inner and outer membranes, a trait shared with cyanobacteria and not found in green plants or red algae. Cyanelles contain chlorophyll a and the phycobiliproteins phycocyanin and allophycocyanin, giving glaucophytes a blue-green coloration and an ancestral-like photosynthetic apparatus.

The two best-described living genera are Cyanophora and Glaucocystis. Glaucophytes are typically small, with cells measuring a few micrometers to tens of micrometers, and they reproduce chiefly by asexual division; details of sexual reproduction are limited.

Phylogeny and significance: Glaucophyta is considered a basal lineage within Archaeplastida, providing important insights into the origin of plastids via primary endosymbiosis, the evolutionary event that gave rise to plastids in green plants, red algae, and glaucophytes.