The autotrophic (auto = "self" tropho = "nourishment", Greek) cyanobacteria were once classified as "blue green algae" because of their superficial resemblance to eukaryotic green algae. Although both groups are photosynthetic, they are only distantly related: cyanobacteria lack internal organelles, a discrete nucleus and the histone proteins associated with eukaryotic chromosomes. Like all eubacteria, their cell walls contain peptidoglycan.
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. Chlorophyll a and several accessory pigments (phycoerythrin and phycocyanin) are embedded in these photosynthetic lamellae, the analogs of the eukaryotic thylakoid membranes. The photosynthetic pigments impart a rainbow of possible colors: yellow, red, violet, green, deep blue and blue-green cyanobacteria are known.

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.

Heterocyst-forming species are able to "fix" nitrogen gas, which cannot be absorbed by plants, into ammonia (NH₃), nitrites(NO₂) or nitrates (NO₃), which can be absorbed by plants and converted to protein and nucleic acids. The rice paddies of Asia, which feed about 75% of the world's human population, could not do so were it not for healthy populations of nitrogen-fixing cyanobacteria in the rice paddy waters.

Found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil, cyanobacteria produce the compounds responsible for "earthy" odors we detect in soil and some bodies of water (such as those being cyanobacterially cleaned at water treatment plants). 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 have even been found on the fur of polar bears, to which they impart a greenish tinge!

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. They come in a variety of forms, both singular and colonial...

Heterocyst-forming species are able to "fix" nitrogen gas, which cannot be absorbed by plants, into ammonia (NH₃), nitrites(NO₂) or nitrates (NO₃), which can be absorbed by plants and converted to protein and nucleic acids. The rice paddies of Asia, which feed about 75% of the world's human population, could not do so were it not for healthy populations of nitrogen-fixing cyanobacteria in the rice paddy waters.

Found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil, cyanobacteria produce the compounds responsible for "earthy" odors we detect in soil and some bodies of water (such as those being cyanobacterially cleaned at water treatment plants). 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 have even been found on the fur of polar bears, to which they impart a greenish tinge!