Even though the Cyanobacteria are classified as bacteria (lacking a membrane-bounded nucleus) they are photosynthetic and are included among our algal collections. Cyanobacteria played a decisive role in elevating the level of free oxygen in the atmosphere of the early Earth. Cyanobacteria can change remarkably in appearance, depending on the environmental conditions. Blue-green algae are common in soil, in both salt and fresh water, and can grow over a wide range of temperatures. They have been found to form mats in Antarctic lakes under several meters of ice and are responsible for the beautiful colors of the hot springs at Yellowstone and elsewhere. Cyanobacteria can also occur as symbionts of protozoans, diatoms and lichen-forming fungi, and vascular plants. Some blue-greens can fix nitrogen as well as photosynthesize, allowing them to grow with only light, water, a few minerals, and the nitrogen and carbon dioxide in the atmosphere.
Cyanobacteria are different in many important ways from other photosynthetic prokaryotes. Instead of the bacteriochlorophylls found in purple and green bacteria, blue-greens contain chlorophyll a, as in eucaryotic phototrophs, and, produce free oxygen as a byproduct of photosynthesis. Cyanobacteria, however, lack the organized chloroplasts of eukaryotes and have their photosynthetic apparatus distributed peripherally in the cytoplasm.
The variety of striking colors exhibited by Cyanobacteria are a result of their major light-gathering pigments, the phycobilins, that are bound to protein granules, (phycobilisomes), that are attached to the photosynthetic membranes.
Large blooms of freshwater Cyanobacteria may produce toxins that can kill livestock. Other forms (Spirulina) are grown commercially and marketed as a high-protein dietary supplement.
Additional introductory information about this algal group may be found at the University of California - Museum of Paleontology.