In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a byproduct, though some may also use hydrogen sulfide a process which occurs among other photosynthetic bacteria such as the purple sulfur bacteria.
Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. They are often found as symbionts with a number of other groups of organisms such as fungi (lichens), corals, pteridophytes (Azolla), angiosperms (Gunnera), etc. The carbon metabolism of cyanobacteria include the incomplete Krebs cycle, the pentose phosphate pathway, and glycolysis.
There are some groups capable of heterotrophic growth, while others are parasitic, causing diseases in invertebrates or algae (e.g., the black band disease).