Oxygen Cycle

The oxygen cycle describes the movement of oxygen through Earth's atmosphere, biosphere, hydrosphere, and lithosphere as oxygen atoms shift between oxidation states in redox reactions across the planet's systems. Its main source is photosynthesis, in which plants, marine phytoplankton, and cyanobacteria such as the tiny Prochlorococcus produce sugars and free oxygen from carbon dioxide and water; oxygen is lost mainly through respiration and decay, as animal life and bacteria consume it and release carbon dioxide in turn. A smaller, abiotic source comes from photolysis, in which ultraviolet radiation splits water and nitrous oxide molecules high in the atmosphere, releasing oxygen while hydrogen and nitrogen escape to space. The evolution of oxygen-producing photosynthesis transformed the composition of Earth's atmosphere and made complex life possible, and biological activity continues to drive the modern oxygen flux.