Cyanobacteria are the photoautotrophs which are capable of utilizing the sunlight energy so as to produce oxygen. These are the relatives of bacteria. These organisms also have chloroplast like green plants which is an organelle responsible for absorption of sunlight. On primitive earth, the Cynobacteria were the only living species which was responsible for releasing oxygen by conducting photosynthesis. Thus these were responsible for the increase in abundance of oxygen in the atmosphere.
Both cyanobacteria and plants use water as an electron donor for the light phase of photosynthesis and make ATP and NADPH. The ATP and NADPH are used in the Calvin cycle to fix the atmospheric CO2 into glucose. The use of water as an electron donor releases oxygen gas as a by-product. This photosynthesis wherein the use of water as an electron donor releases oxygen gas as a by-product is also called oxygenic photosynthesis.
