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A subgroup of the oxygenic photosynthetic bacteria comprised of unicellular to multicellular photosynthetic 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. Formerly called blue-green algae, cyanobacteria were traditionally treated as algae. By the late 19th century, however, it was realized that the blue-green algae were unique and lacked the traditional nucleus and chloroplasts of the green and other algae. The comparison of nucleotide base sequence data from 16S and 5S rRNA indicates that cyanobacteria represent a moderately deep phylogenetic unit within the gram-negative bacteria.