A phylum of oxygenic photosynthetic bacteria comprised of unicellular to multicellular bacteria possessing chlorophyll a and carrying out oxygenic photosynthesis. cyanobacteria are the only known organisms capable of fixing both carbon dioxide (in the presence of light) and nitrogen. Cell morphology can include nitrogen-fixing heterocysts and/or resting cells called akinetes. Formerly called blue-green algae, cyanobacteria were traditionally treated as ALGAE.