Oxygen cycle

While there are many abiotic sources and sinks for O2, the presence of the profuse concentration of free oxygen in modern Earth's atmosphere and ocean is attributed to O2 production in the biological process of oxygenic photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics. Biology is the main driver of O2 flux on modern Earth, and the evolution of oxygenic photosynthesis by bacteria, which is discussed as part of the Great Oxygenation Event, is thought to be directly responsible for the conditions permitting the development and existence of all complex eukaryotic metabolism. Biological production The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water: :\mathrm{6 \ CO_2 + 6H_2O + energy \longrightarrow C_6H_{12}O_6 + 6 \ O_2} Photosynthesizing organisms include the plant life of the land areas, as well as the phytoplankton of the oceans. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for up to half of the photosynthesis of the open oceans. Abiotic production An additional source of atmospheric free oxygen comes from photolysis, whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free hydrogen and nitrogen atoms escape into space, leaving O2 in the atmosphere: :\mathrm{2 \ H_2O + energy \longrightarrow 4 \ H + O_2} :\mathrm{2 \ N_2O + energy \longrightarrow 4 \ N + O_2} Biological consumption The main way free oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide. == Capacities and fluxes ==