Terragen prokaryotic organisms, sometimes also called blue-green algae. The chloroplasts of Terragen plants are descended from a form of cyanobacterium. Cyanobacteria are ubiquitous and essential elements in Terragen ecosystems, with a role both in primary production and nitrogen fixation. Undisturbed by predation or competition from eukaryotic organisms, they may form extensive regions of stromatolites, as seen in Old Earth's Proterozoic or in the intermediate stages of many a terraforming project. Xenobiont analogues of cyanobacteria are common on Gaian worlds, and often dominant on worlds of the EoGaian to MesoGaian subtypes.