The oxygen cycle describes the continuous movement of oxygen atoms through Earth’s major systems—the atmosphere, the water bodies, the rock layers, and all living organisms. This biogeochemical process governs the availability of free oxygen (O2), which is fundamental for sustaining aerobic life on the planet. The cycle operates across vastly different timescales, from rapid biological exchanges to slow geological processes. Maintaining this circulation is necessary because oxygen atoms are constantly being incorporated into various compounds and removed from the accessible environment.
Where Oxygen Is Stored (The Major Reservoirs)
Oxygen is the most abundant element in Earth’s crust and the second-most abundant gas in the atmosphere, stored in three primary reservoirs. The largest storage area is the lithosphere (crust and mantle), holding approximately 99.5% of the planet’s total oxygen supply. Here, oxygen is chemically bound within silicate and oxide minerals, such as silica (SiO2) and iron oxides, making it relatively inaccessible for biological use.
The second major reservoir is the hydrosphere, which includes all the planet’s water bodies. Oxygen atoms are primarily locked up in water molecules (H2O), but a smaller fraction exists as dissolved O2 gas, used by aquatic organisms for respiration.
The atmosphere constitutes the third reservoir, where oxygen makes up about 21% of the total gas volume. This atmospheric oxygen is the most readily available form for terrestrial life, existing mostly as O2 and a small amount of ozone (O3).
The Biotic Exchange (Life’s Role)
The biotic exchange represents the rapid circulation of oxygen driven by living organisms, cycling O2 between the atmosphere and the biosphere. The primary input of free oxygen comes from photosynthesis, conducted by terrestrial plants and marine phytoplankton. These organisms use light energy to convert carbon dioxide (CO2) and water (H2O) into glucose, releasing O2 as a byproduct.
Marine cyanobacteria, such as Prochlorococcus, are significant contributors, collectively producing a substantial portion of atmospheric oxygen. This O2 is then consumed by nearly all other forms of life. The primary output of the biotic cycle is aerobic respiration, where organisms (including animals and plants) use O2 to metabolize sugars for energy, releasing CO2 and H2O back into the environment.
A related consumption process is the decomposition of dead organic matter by aerobic bacteria and fungi. These microbes break down complex carbon compounds, consuming dissolved or atmospheric O2 and returning CO2 to the atmosphere or water. The balance between global photosynthesis and respiration maintains the current concentration of atmospheric O2.
The Geologic Exchange (Earth’s Processes)
The geologic exchange controls the long-term, slow movement of oxygen, impacting atmospheric levels over millions of years. This cycle primarily involves the lithosphere and the atmosphere through two major processes: oxidative weathering and the burial of organic carbon. Oxidative weathering acts as a long-term sink, where atmospheric oxygen reacts with exposed minerals in rocks, especially those containing reduced iron and sulfur compounds.
This chemical reaction, often seen as the formation of rust (iron oxides), binds the free oxygen atoms into the solid earth, removing them from the atmospheric gas supply. Conversely, the burial of organic carbon acts as the long-term source that allows free oxygen to accumulate in the atmosphere. When organic matter is rapidly covered by sediment before it can be fully decomposed, the O2 that would have been used for its decay remains in the atmosphere.
This geological sequestration of carbon ensures that the corresponding oxygen remains as a surplus in the air. Historically, the rate of organic carbon burial relative to oxidative weathering has been the primary mechanism governing the rise of atmospheric O2. These processes occur over vast timescales, maintaining a redox balance between the surface environment and the planet’s interior.
How Human Activity Affects the Cycle
Human activities primarily influence the oxygen cycle through changes in consumption and production rates, though the impact on total atmospheric O2 is minor. The burning of fossil fuels (such as coal and oil) is a large-scale consumption process, involving rapid combustion that uses atmospheric O2. For every molecule of carbon released as CO2, a corresponding amount of O2 is consumed, reversing the chemical process of organic carbon burial that created the fossil fuel.
Deforestation further impacts the cycle by reducing the global photosynthetic capacity, the main mechanism for O2 production. Removing large tracts of forests diminishes the planet’s ability to convert CO2 into O2, shifting the biotic balance. A more immediate impact is seen in the hydrosphere due to nutrient runoff from agriculture, which causes eutrophication in coastal waters.
The resulting massive algal blooms eventually die and decompose, and microbial decay consumes dissolved O2. This leads to localized areas of hypoxia, or “dead zones,” where oxygen levels are too low to support most aquatic life. While total atmospheric O2 concentration remains relatively stable due to its sheer volume, these localized deoxygenation events pose a significant threat to marine ecosystems.