Oxygen cycle

Sources and sinks

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:

${\displaystyle 6\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}+\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}⟶{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2}$

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:

${\displaystyle 2{\mathrm{H}}_2\mathrm{O}+\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}⟶4\mathrm{H}+{\mathrm{O}}_2}$

${\displaystyle 2{\mathrm{N}}_2\mathrm{O}+\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}⟶4\mathrm{N}+{\mathrm{O}}_2}$

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

The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from (Walker, J. C. G.): More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.

Ozone

The presence of atmospheric oxygen has led to the formation of ozone (O₃) and the ozone layer within the stratosphere:

${\displaystyle {\mathrm{O}}_2+\mathrm{u}\mathrm{v}\mathrm{l}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}⟶2\mathrm{O}(\lambda \lesssim 200\text{nm})}$

${\displaystyle \mathrm{O}+{\mathrm{O}}_2⟶{\mathrm{O}}_3}$

The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation:

${\displaystyle {\mathrm{O}}_3+\mathrm{u}\mathrm{v}\mathrm{l}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}⟶{\mathrm{O}}_2+\mathrm{O}(\lambda \lesssim 300\text{nm})}$