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 macromol-ecules 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, .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.
The primary action of the oxygen cycle occurs through photosynthesis and cellular respiration. Together, they represent the ability of diatomic oxygen gas to be used and produced by living organisms from the oxygen present in the air. Cellular respiration is the process by which an organism consuming food generates energy to sustain life. Most organisms use oxygen to maximize the amount of energy the food can yield, using oxygen as an electron acceptor in the electron transport chain, and using oxidative phosphorylation. Photosynthesis involves combining atmospheric carbon dioxide with water to generate carbohydrate sources and oxygen gas. The reactions are complementary:
There are other reactions involving oxygen that are vital for life that occur within a single reservoir. The ozone-oxygen reactions allow for stratospheric ozone to absorb ultraviolet light radiated from the sun, in addition to the visible light photosynthesis requires, which can cause damage to DNA, and break other important chemical bonds. The net reaction for the generation and breakdown of ozone by the absorption of ultraviolet light is:
O3 (ozone) + UV Light Energy -> O2 + O (reactive radical)
O (reactive radical) + O2 (atmospheric oxygen gas) -> O3 (ozone)
Other important reactions include the dissolving of oxygen in bodies of water, which allows for aquatic life to undergo aerobic cellular respiration and the evaporation of different bodies of water.
The oxygen cycle is not an isolated system; rather, it has intimate links with many other geological and biological cycles, such as the carbon cycle and the hydrologic cycle. Additionally, other smaller cycling systems, such as the nitrogen and sulfur cycles, require oxygen from the atmosphere, living organisms, or the ground to make their vital nutrients available for uptake and binding to useful substrates. For example, the oxidation of sulfur creates sulfur dioxide, which may then react with water vapor to form sulfuric acid, which can delivered to, and taken up by, plant roots; the sulfur can be used to make cysteine, an important structural amino acid.
Current scientific theories and opinions indicate that the oxygen cycle itself has not always been native to the Earth. In the far geological past, when the Earth was first cooling and forming, the atmosphere contained virtually no free oxygen and was primarily composed of hydrogen and helium gas. As conditions changed, a second type of atmosphere formed, composed of volcanic emissions, still largely devoid of diatomic oxygen gas. While life first developed on Earth, oxygen was released as a waste product from the most ancient forms of life, cyanobacteria. The oxygen cycle, thus, could not have been native to the Earth and must have developed as the Earth matured, because oxygen seemingly was not present. An interesting side note is that if the regeneration of free atmospheric oxygen stopped by cessation of photosynthesis and other oxygen cycle reactions, current estimates indicate that at current rates of oxygen consumption, it would take approximately 5,000 years for the Earth to deplete the atmosphere.
