A large and varied group of bacteria which possess chlorophyll a (carried on specialized membranes (thylakoids) within the cells) and carry out photosynthesis in the presence of light and air with concomitant production of oxygen. They do not have chloroplasts. Cyanobacteria were formerly regarded as algae (division Cyanophyta) and were called ‘blue-green algae’. Fossil cyanobacteria have been found in rocks almost 3000 Ma old and they are common as stromatolite colonies in rocks 2300 Ma old. They are believed to have been the first oxygen-producing organisms and to have been responsible for generating the oxygen in the atmosphere, thus profoundly influencing the subsequent course of evolution. The organisms may be single-celled or filamentous, and may or may not be colonial. Some are capable of a gliding motility when in contact with a solid surface. Many species can carry out the fixation of atmospheric nitrogen.