The oxygen cycle is the biogeochemical cycle that moves oxygen among Earth’s atmosphere, oceans and other waters, living organisms, rocks, and soil. Photosynthesis supplies most of the free molecular oxygen (O2) in the biosphere, while respiration, decomposition, combustion, oxidation, and other chemical reactions consume it.
Unlike a simple circular pathway, the oxygen cycle consists of many interconnected processes operating over very different timescales. Oxygen atoms move through water (H2O), carbon dioxide (CO2), organic molecules, minerals, molecular oxygen (O2), ozone (O3), and many other compounds. Some parts of the cycle occur within seconds or years, while geological processes store and exchange oxygen over millions or billions of years.
Key Takeaways: Oxygen Cycle
- The oxygen cycle moves oxygen among the atmosphere, hydrosphere, biosphere, and geosphere.
- Photosynthesis is the main biological process that produces free oxygen (O2), while respiration and decomposition consume it.
- Most of Earth’s oxygen atoms are not present as atmospheric O2. They are chemically bound in rocks, minerals, and water.
- The oceans play a major role through photosynthesis, respiration, gas exchange, and storage of dissolved oxygen.
- Burial of organic carbon allows some photosynthetically produced oxygen to accumulate over geological time.
- Combustion and oxidation are important nonbiological sinks for free oxygen.
- Human activities alter the oxygen cycle through fossil fuel combustion, deforestation, nutrient pollution, land-use change, and climate change.
Why Is the Oxygen Cycle Important?
The oxygen cycle helps maintain conditions that support most complex life on Earth. Aerobic organisms use molecular oxygen during cellular respiration, a process that releases usable energy from organic compounds.
Oxygen also influences many other Earth systems. Atmospheric oxygen participates in combustion and chemical weathering. Dissolved oxygen determines which organisms can survive in aquatic environments. In the upper atmosphere, oxygen chemistry produces ozone, which absorbs much of the Sun’s biologically harmful ultraviolet radiation.
The oxygen cycle is closely linked to the carbon cycle, water cycle, nitrogen cycle, and other biogeochemical cycles. For example, photosynthesis removes carbon dioxide while ultimately releasing oxygen, whereas aerobic respiration consumes oxygen while returning carbon dioxide to the environment.
Over geological time, the oxygen cycle has also transformed Earth’s surface. Rising oxygen levels altered ocean chemistry, oxidized minerals, enabled formation of an ozone layer, and eventually supported organisms with high-energy aerobic metabolisms.
What Is the Oxygen Cycle?
The oxygen cycle describes the movement and chemical transformation of oxygen within and between four major Earth systems:
- Atmosphere: Contains molecular oxygen (O2), ozone (O3), water vapor, carbon dioxide, and other oxygen-containing gases.
- Hydrosphere: Contains enormous amounts of oxygen chemically bound in water (H2O), plus smaller amounts of dissolved O2 and other oxygen-containing compounds.
- Biosphere: Stores oxygen in water and organic molecules and continually exchanges oxygen through photosynthesis, respiration, decomposition, and other metabolic processes.
- Geosphere: Contains by far the largest reservoir of oxygen atoms, mostly bound in silicate minerals, oxides, carbonates, and other compounds in Earth’s crust and mantle.
An important distinction is the difference between oxygen as an element and free molecular oxygen (O2). Oxygen is extremely abundant on Earth, but most of it is chemically bonded to other elements. The oxygen that animals breathe represents only one comparatively small reservoir within the much larger elemental oxygen cycle.
Where Is Oxygen Found on Earth?
Rocks and Minerals
The largest reservoir of oxygen atoms is the solid Earth. Oxygen makes up about 46% of Earth’s crust by mass and occurs in minerals such as silicates, oxides, carbonates, sulfates, and phosphates.
Common minerals including quartz (SiO2) and feldspars contain substantial amounts of oxygen. This oxygen is chemically bound and is not directly available for respiration.
Oceans and Other Water
Water contains oxygen as H2O, making the hydrosphere another enormous oxygen reservoir. Again, this oxygen is chemically bound rather than present as breathable O2.
Water also contains dissolved oxygen, which aquatic organisms use for aerobic respiration. Dissolved oxygen concentrations vary with temperature, salinity, circulation, photosynthesis, respiration, and decomposition.
Atmosphere
Molecular oxygen makes up about 20.95% of dry air by volume. This atmospheric reservoir is particularly important because it supplies oxygen for terrestrial respiration and exchanges O2 continuously with surface waters.
The atmosphere also contains oxygen in carbon dioxide, water vapor, ozone, and trace gases.
Living Organisms
Living things contain abundant oxygen, particularly because cells contain large amounts of water. Oxygen also occurs in carbohydrates, proteins, nucleic acids, lipids, and many other biological molecules.
Although the biosphere is a relatively small oxygen reservoir compared with rocks or oceans, organisms drive some of the fastest and most important oxygen exchanges.
How the Oxygen Cycle Works
There is no single starting point for the oxygen cycle. However, photosynthesis provides a convenient place to begin because it is the major source of free oxygen in the modern biosphere.
1. Photosynthesis Produces Oxygen
Plants, algae, and cyanobacteria perform oxygenic photosynthesis. Using light energy, they convert carbon dioxide and water into organic compounds and release molecular oxygen.
A simplified overall equation is:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
The released O2 ultimately comes from the splitting of water molecules, not from the carbon dioxide.
Photosynthesis occurs both on land and in aquatic environments. Marine phytoplankton, including photosynthetic algae and cyanobacteria, account for a large fraction of global oxygen production.
2. Oxygen Enters the Atmosphere and Water
Oxygen produced by terrestrial photosynthesis enters the atmosphere. In aquatic environments, photosynthetic organisms release O2 into surrounding water.
Oxygen continuously moves between surface waters and the atmosphere by gas exchange. Depending on local conditions, a body of water may either absorb atmospheric oxygen or release dissolved oxygen into the air.
3. Respiration Consumes Oxygen
Plants, animals, fungi, protists, and many microorganisms use oxygen during aerobic cellular respiration.
A simplified equation is:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
Respiration therefore returns carbon dioxide and water to the environment while consuming molecular oxygen.
Plants both produce and consume oxygen. They release O2 during photosynthesis but also use O2 for cellular respiration.
4. Decomposition Consumes Oxygen
When organisms die or release organic waste, decomposers such as bacteria and fungi break down the organic material.
Under aerobic conditions, decomposition consumes oxygen and releases carbon dioxide. Rapid decomposition can therefore dramatically lower dissolved oxygen in water.
If oxygen becomes scarce or absent, anaerobic metabolic processes become more important.
5. Combustion Uses Oxygen
Combustion consumes molecular oxygen as carbon-containing fuels react with oxygen.
A simplified reaction for complete combustion of a hydrocarbon is:
hydrocarbon + O2 → CO2 + H2O + energy
Wildfires are part of the natural oxygen cycle. Human combustion of coal, petroleum, natural gas, and biomass adds an additional oxygen sink.
6. Chemical Oxidation Consumes Oxygen
Oxygen reacts with many substances without biological respiration or combustion. For example, iron-bearing minerals react with oxygen during weathering to form oxidized iron compounds.
A familiar example is rust formation.
Oxidation of reduced minerals, volcanic gases, and other substances consumes free oxygen. These reactions have been particularly important over geological time.
7. Geological Processes Store and Release Oxygen
Weathering, erosion, sedimentation, burial, metamorphism, volcanism, and tectonic processes connect the surface oxygen cycle with the long-term geological cycle.
Organic carbon burial is particularly important. Photosynthesis produces organic matter and oxygen, but respiration and decomposition normally reverse much of that reaction. If some organic carbon is buried in sediment before it is completely oxidized, its corresponding oxygen is not immediately consumed.
Over geological timescales, the burial of reduced carbon and other reduced substances allows free oxygen to accumulate in the atmosphere and oceans.
A Simplified Oxygen Cycle
A useful simplified pathway is:
CO2 + H2O → photosynthesis → organic matter + O2 → respiration/decomposition → CO2 + H2O
However, the complete cycle also includes:
Atmospheric O2 ↔ dissolved O2 in water
O2 → combustion and oxidation → oxygen-containing compounds
Organic matter → burial → sediments and rocks
Rocks → weathering, tectonics, and volcanism → surface reservoirs
These pathways explain why the oxygen cycle is both a biological cycle and a geochemical cycle.
Sources and Sinks of Oxygen
A source adds free molecular oxygen to a reservoir, while a sink removes it.
Major Oxygen Sources
The dominant immediate source of O2 is oxygenic photosynthesis by:
- Land plants
- Algae
- Cyanobacteria
- Marine phytoplankton
But gross oxygen production is not the same as a permanent increase in atmospheric oxygen. Much of the oxygen generated during photosynthesis is soon consumed by respiration and decomposition.
For atmospheric oxygen to accumulate over long geological periods, some reduced material must escape reoxidation. Burial of organic carbon is an important mechanism. Burial of reduced sulfur compounds also contributes to long-term oxygen balance.
Major Oxygen Sinks
Important sinks for molecular oxygen include:
- Aerobic cellular respiration
- Decomposition of organic matter
- Wildfires and other combustion
- Fossil fuel combustion
- Oxidation of minerals during weathering
- Oxidation of reduced gases and chemicals
- Reactions involving reduced materials from Earth’s interior
At the global scale, oxygen concentration reflects the balance between sources and sinks rather than simply the total amount of photosynthesis occurring at any given moment.
The Oxygen Cycle in the Ocean
The ocean deserves special attention because it is both biologically productive and extremely important to global oxygen cycling.
Phytoplankton and other marine photosynthetic organisms produce a large fraction of the world’s oxygen. However, marine organisms and microorganisms also consume enormous amounts of oxygen through respiration and decomposition.
Most oxygen produced in the ocean does not simply accumulate in the atmosphere. Much of it is used again within the marine ecosystem.
Dissolved oxygen enters seawater through:
- Photosynthesis
- Exchange with the atmosphere
- Mixing and circulation of oxygen-rich surface water
Dissolved oxygen leaves seawater through:
- Respiration
- Decomposition
- Chemical oxidation
- Gas exchange with the atmosphere when water is supersaturated
Deep ocean oxygen depends heavily on ocean circulation. Because sunlight does not reach the deep sea, photosynthesis cannot replenish oxygen there directly. Instead, oxygen-rich waters formed near the surface sink and circulate through the ocean.
Oxygen, Ozone, and the Atmosphere
Atmospheric oxygen also participates in a smaller but important cycle involving ozone (O3).
High in the stratosphere, ultraviolet radiation splits O2 molecules:
O2 + UV light → 2 O
Individual oxygen atoms can then react with O2 to form ozone:
O + O2 → O3
Ozone absorbs ultraviolet radiation and undergoes additional reactions that regenerate O2. This oxygen-ozone cycle creates the ozone layer, which shields Earth’s surface from much of the Sun’s harmful ultraviolet radiation.
The ozone cycle is therefore a specialized part of atmospheric oxygen chemistry.
Oxygen Cycle and Carbon Cycle
The oxygen and carbon cycles are strongly coupled.
During photosynthesis:
CO2 is consumed → organic carbon is produced → O2 is released.
During respiration, decomposition, and combustion:
O2 is consumed → organic carbon is oxidized → CO2 is released.
However, the two cycles are not perfect mirror images. Oxygen also participates in reactions that do not involve carbon, including oxidation of iron, sulfur, and other elements. Carbon likewise participates in processes that do not directly exchange molecular oxygen.
Over geological time, burial and weathering processes further complicate the relationship.
History of Oxygen in Earth’s Atmosphere
Earth’s early atmosphere contained little free molecular oxygen compared with today’s atmosphere.
Oxygenic photosynthesis evolved in microorganisms long before the atmosphere became oxygen-rich. Early oxygen reacted readily with reduced substances, particularly dissolved iron and other materials in the oceans and crust.
Beginning roughly 2.4 billion years ago, atmospheric oxygen increased dramatically during an interval known as the Great Oxidation Event.
This change permanently altered Earth’s chemistry. Iron oxidation produced extensive ancient mineral deposits, aerobic metabolism became increasingly important, and atmospheric oxygen eventually enabled development of a protective ozone layer.
Oxygen levels continued to change throughout geological history rather than rising smoothly to their modern value. Interactions among biological evolution, organic carbon burial, weathering, tectonics, and climate have all affected atmospheric oxygen.
Human Effects on the Oxygen Cycle
Human activity changes several oxygen-cycle processes. The effects are especially important locally and regionally and in aquatic ecosystems.
Fossil Fuel Combustion
Burning coal, oil, and natural gas consumes molecular oxygen and releases carbon dioxide.
This process causes a measurable decline in atmospheric O2, but the atmosphere contains such an enormous oxygen reservoir that fossil fuel combustion does not pose a realistic threat of humans running out of breathable atmospheric oxygen.
The much more immediate global concern is the increase in atmospheric CO2 and its effects on climate.
Deforestation and Land-Use Change
Removing vegetation changes photosynthesis, respiration, decomposition, carbon storage, and fire patterns.
It is therefore reasonable to say that deforestation alters the oxygen cycle. However, statements that forests simply “provide our oxygen” can be misleading. Mature ecosystems both produce oxygen through photosynthesis and consume it through respiration and decomposition.
Their importance extends far beyond net atmospheric oxygen production. Forests store carbon, regulate water, provide habitat, affect climate, stabilize soil, and support biodiversity.
Nutrient Pollution and Eutrophication
Fertilizer runoff and wastewater add nutrients such as nitrogen and phosphorus to aquatic ecosystems. These nutrients can stimulate large algal blooms.
Although algae produce oxygen while photosynthesizing, the eventual death and decomposition of large amounts of organic matter consumes dissolved oxygen. This process can cause hypoxia, or abnormally low oxygen levels.
Severe oxygen depletion creates aquatic dead zones, where many animals cannot survive.
Combustion and Fire
Human-caused fires and biomass burning consume oxygen and release carbon dioxide and other gases. Changes in fire frequency and intensity also affect vegetation, carbon storage, soil processes, and ecosystem productivity.
Climate Change and the Oxygen Cycle
Climate change affects oxygen cycling most clearly in the oceans.
Warmer Water Holds Less Oxygen
The solubility of gases decreases as water warms. As ocean temperature rises, seawater generally holds less dissolved oxygen.
This is a direct physical effect of warming.
Ocean Stratification Reduces Oxygen Supply
Warming can increase the density difference between warm surface water and cooler deeper water, strengthening stratification.
Stronger stratification reduces vertical mixing in many regions. As a result, less oxygen-rich surface water reaches deeper layers, while respiration continues to consume oxygen below.
Metabolism Changes
Warmer temperatures can increase biological metabolic and decomposition rates, increasing oxygen demand in some ecosystems.
The combined effects of reduced oxygen solubility, altered circulation, and biological oxygen consumption contribute to ocean deoxygenation.
Changes in Photosynthesis
Climate change also affects photosynthesis by changing temperature, rainfall, nutrient availability, ocean circulation, sea ice, wildfire patterns, and ecosystem distributions.
The net response varies by ecosystem and cannot be described simply as “warming causes more” or “warming causes less” photosynthesis.
Oxygen Cycle vs Water Cycle
The oxygen and water cycles overlap but describe different things.
The water cycle tracks water as it evaporates, condenses, precipitates, freezes, melts, flows, and moves through organisms.
The oxygen cycle tracks oxygen atoms through many different chemical forms.
An oxygen atom in a water molecule can eventually become part of atmospheric O2 through photosynthesis. Later, that same oxygen atom might enter an organism during respiration and become part of water again.
So, the cycles are interconnected rather than independent.
Fast and Slow Oxygen Cycles
It is useful to think of oxygen cycling on two broad timescales.
The fast oxygen cycle involves photosynthesis, respiration, decomposition, air-sea exchange, and combustion. These processes operate over timescales ranging from seconds to centuries.
The slow oxygen cycle involves weathering, sediment burial, rock formation, tectonics, and interactions between Earth’s surface and interior. These processes operate over thousands to billions of years.
The modern oxygen-rich atmosphere exists because these fast and slow processes have produced a long-term balance that permits O2 to accumulate.
Common Misconceptions About the Oxygen Cycle
Misconception: Most of Earth’s oxygen is in the atmosphere.
Most oxygen atoms are chemically bound in rocks, minerals, and water. Atmospheric O2 represents only a small portion of Earth’s total oxygen.
Misconception: The oxygen in O2 produced by photosynthesis comes from carbon dioxide.
The released molecular oxygen comes from water, which is split during the light-dependent reactions of photosynthesis.
Misconception: Plants only produce oxygen.
Plants produce oxygen during photosynthesis, but they also consume oxygen continuously through cellular respiration.
Misconception: The Amazon rainforest produces 20% of the oxygen humans breathe.
Figures of this type usually confuse gross oxygen production with net oxygen production. Forest ecosystems consume much of the oxygen they produce through respiration and decomposition.
The Amazon is extraordinarily important for biodiversity, carbon storage, regional rainfall, and climate, but its importance should not be justified using exaggerated claims about the global atmospheric oxygen supply.
Misconception: Half of the oxygen we breathe comes from the ocean.
Marine photosynthetic organisms are responsible for a very large fraction of Earth’s gross annual oxygen production, with estimates often around half. But most of this oxygen is also consumed by marine respiration and decomposition.
So, “the ocean produces about half of Earth’s oxygen” refers to gross production, not necessarily the origin of half of the atmospheric oxygen reservoir people breathe.
Misconception: Algal blooms add oxygen to lakes and oceans.
Algae produce oxygen during daylight photosynthesis, so dissolved oxygen may initially rise. However, dense blooms also respire, and when large quantities of algae die, microbial decomposition consumes oxygen. The overall result can be severe oxygen depletion.
Misconception: Humans are in danger of using up atmospheric oxygen.
Human activity measurably affects atmospheric oxygen, but the atmospheric O2 reservoir is enormous. Rising carbon dioxide, climate change, ocean deoxygenation, habitat loss, and local hypoxia are far more immediate concerns than depletion of breathable atmospheric oxygen.
Frequently Asked Questions
What is the oxygen cycle in simple terms?
The oxygen cycle is the movement of oxygen among the atmosphere, water, living organisms, rocks, and soil. Photosynthesis, respiration, decomposition, combustion, oxidation, and geological processes continually move and transform oxygen.
What are the main steps of the oxygen cycle?
The major biological steps are photosynthesis, which releases O2, and respiration and decomposition, which consume it. The complete cycle also includes combustion, chemical oxidation, air-water gas exchange, weathering, sedimentation, burial, and geological processes.
What is the main source of oxygen in the oxygen cycle?
Oxygenic photosynthesis is the main immediate source of free molecular oxygen. Plants, algae, and cyanobacteria perform this process.
What is the largest reservoir of oxygen on Earth?
The largest reservoir of oxygen atoms is Earth’s rocks and minerals, particularly oxygen-containing silicate minerals in the crust and mantle.
Where does the oxygen released during photosynthesis come from?
It comes from water. During the light-dependent reactions of photosynthesis, water molecules are oxidized, ultimately producing molecular oxygen.
Do plants use oxygen?
Yes. Plants use oxygen for aerobic cellular respiration just as animals and many other organisms do. Plants also produce oxygen when they perform photosynthesis.
Do oceans produce oxygen?
Yes. Phytoplankton, algae, cyanobacteria, and other photosynthetic organisms in the ocean produce enormous amounts of oxygen. Much of this oxygen is subsequently consumed within the ocean through respiration and decomposition.
How does decomposition affect the oxygen cycle?
Aerobic decomposers consume oxygen while breaking down dead organisms and wastes. In aquatic environments, intense decomposition can reduce dissolved oxygen enough to cause hypoxia or anoxia.
How do humans affect the oxygen cycle?
Humans alter oxygen cycling through fossil fuel combustion, deforestation, land-use change, biomass burning, nutrient pollution, and climate change. These activities influence photosynthesis, respiration, decomposition, combustion, and dissolved oxygen concentrations.
How does global warming affect oxygen in the ocean?
Warmer water holds less dissolved oxygen. Warming can also increase ocean stratification, reducing mixing between surface and deeper waters. Along with biological changes, these effects contribute to ocean deoxygenation.
Is the oxygen cycle connected to the carbon cycle?
Yes. Photosynthesis consumes carbon dioxide and produces organic matter and oxygen, while respiration, decomposition, and combustion consume oxygen and return carbon dioxide to the environment. The two cycles are closely linked but are not exact opposites.
Oxygen Cycle Glossary
Aerobic respiration: Cellular respiration that uses molecular oxygen to release energy from organic compounds.
Anoxia: A condition in which oxygen is essentially absent from an environment.
Biogeochemical cycle: Movement and transformation of an element or compound through living organisms and Earth’s physical environment.
Decomposition: Breakdown of dead organisms and organic wastes by microorganisms and other decomposers.
Dissolved oxygen (DO): Molecular oxygen dissolved in water and available to aquatic organisms.
Eutrophication: Excessive enrichment of water with nutrients, often resulting in algal growth and subsequent oxygen depletion.
Hypoxia: Abnormally low oxygen concentration, particularly in aquatic environments.
Ocean deoxygenation: A long-term decline in the oxygen content of ocean water.
Oxidation: A chemical process involving loss of electrons. Many oxidation reactions in the environment consume molecular oxygen.
Oxygenic photosynthesis: Photosynthesis that uses water as an electron donor and releases molecular oxygen.
Oxygen sink: A process or reservoir that removes free oxygen from another reservoir.
Oxygen source: A process that adds free oxygen to a reservoir.
Photosynthesis: The process by which plants, algae, cyanobacteria, and some other organisms use light energy to produce organic compounds.
Stratification: Formation of layers in a body of water that resist vertical mixing.
Oxygen Cycle Summary
The oxygen cycle connects life with Earth’s atmosphere, oceans, rocks, and climate. Photosynthesis produces molecular oxygen, while respiration, decomposition, combustion, and chemical oxidation consume it. Gas exchange moves oxygen between the atmosphere and surface waters, while ocean circulation transports it into deeper water.
On longer timescales, weathering, organic carbon burial, sedimentation, tectonics, and other geological processes control the balance between oxygen sources and sinks. This long-term balance transformed an early Earth with little free oxygen into the oxygen-rich planet that supports complex aerobic life today.
Human activities do not threaten to exhaust Earth’s atmospheric oxygen supply, but they do alter the oxygen cycle. Particularly important consequences include changes in ecosystems and the continuing loss of dissolved oxygen from parts of the ocean. Understanding the oxygen cycle therefore connects fundamental biology and chemistry with Earth’s geological history, modern ecosystems, and climate change.
References and Further Reading
- Cloud, P.; Gibor, A. (1970). “The oxygen cycle”. Scientific American. 223 (3): 110–123. doi:10.1038/scientificamerican0970-110
- Keeling, R.F.; Shertz, S.R. (1992). “Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle”. Nature. 358 (6389): 723–727. doi:10.1038/358723a0
- Lyons, T.W.; Reinhard, C.T.; Planavsky, N.J. (2014). “The rise of oxygen in Earth’s early ocean and atmosphere”. Nature. 506 (7488): 307–15. doi:10.1038/nature13068
- Petsch, S.T. (2014). “The Global Oxygen Cycle”. Treatise on Geochemistry. Elsevier. pp. 437–473. doi:10.1016/b978-0-08-095975-7.00811-1. ISBN 978-0-08-098300-4.