Oxygen photosynthesis is the mode of photosynthesis in which water is the primary electron donor and therefore releases oxygen (O2) as a by-product.
This metabolic modality is characteristic of cyanobacteria and their offspring by endosymbiosis, the various types of cyanelas and plastids observed in eukaryotic (algae) and plants.
Other photosynthetic bacteria and archaebacteria resort to other electron donors, such as sulfur or hydrogen. Since oxygen photosynthesis appeared in cyanobacteria and became the main form of autotrophic metabolism, the earth's atmosphere began to become enriched in oxygen, hitherto almost absent.
In simple words, the process of combining carbon dioxide with water to form sugar results in an excess of oxygen, which is released.
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