Oxygen Cycle

The oxygen cycle allows for the regeneration of freely available diatomic oxygen (O2) in the atmosphere. Oxygen accounts by volume for approximately 21 percent of the atmosphere, is reactive with myriad inorganic and organic substances, and is vital to living organisms for aerobic respiration and energy production. The cycle involves any source of oxygen within the world, and is not limited to the oxygen animals must breathe to sustain life; any compound containing an atom of oxygen is considered part of the oxygen cycle. Furthermore, the cycle is composed of many distinct biological and geological chemical reactions that together allow oxygen initially consumed and lost from the atmosphere to be released back into the atmosphere.

These reactions take place among the three different primary reservoirs, or storage areas, of all of Earth’s oxygen. These storage areas are varied and differ in physical and chemical form. The lithosphere, which contains the vast majority of the Earth’s total oxygen, comprises the entirety of the Earth’s crust and the uppermost portion of the mantle (tectonic plates can be viewed as lithospheric plates); in this reservoir, oxygen is bound in the form of rocks and minerals, primarily in silica (SiO2) and alumina (Al2O3). The second reservoir is the biosphere, in which all living matter resides, including bacteria, plant life, animals, and human beings. The oxygen bound in this reservoir is found in the macromolecules of life, including nucleic acids, carbohydrates, proteins, and water. The last oxygen reservoir is the atmosphere, which is composed of approximately 20.95 percent oxygen gas, 0.038 percent carbon dioxide (CO2), and water vapor (H2O).

All of the reactions that drive the oxygen cycle occur between any two different reservoirs of oxygen. However, two chief reactions account for most of the activity in the use and regeneration of oxygen on Earth; these are cellular respiration and photosynthesis, both of which occur between the atmospheric reservoir and the biospheric reservoir. Other notable reactions contributing to the cycle include the commonplace reaction between the atmosphere’s free oxygen and the lithosphere in the form of the oxidation of minerals and carbon dioxide emissions (for example, from volcanic eruptions). The biosphere and lithosphere interact in the oxygen cycle through weathering and absorbed soil nutrients for organisms and deposition of organism shells and bones into the lithosphere.