Studies of metabolic similarities and ribosomal RNA sequence suggest that cyanobacteria form a good, monophyletic taxon. Because motile species of cyanobacteria utilize the same mysterious gliding locomotion as the gram-negative gliding bacteria, some microbiologists suggest that cyanobacteria should be classified together as a subgroup of gliding bacteria. Although they are truly prokaryotic, cyanobacteria have an elaborate and highly organized system of internal membranes which function in photosynthesis. Cyanobacteria may be single-celled or colonial. Depending upon the species and environmental conditions, colonies may form filaments, sheets or even hollow balls.
Some filamentous colonies show the ability to differentiate into three different cell types.
Vegetative cells are the normal, photosynthetic cells formed under favorable growing conditions. Climate-resistant spores may form when environmental conditions become harsh. A third type of cell, a thick-walled heterocyst , contains the enzyme nitrogenase , vital for nitrogen fixation. The greenish slime on the side of your damp flower pot, the wall of your house or the trunk of that big tree is more likely to be cyanobacteria than anything else. Cyanobacteria may be single-celled or colonial.