Oxygen Cycle

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The continual interchange of oxygen between the atmosphere and the water, the plants and animals and mineral matter is called the oxygen cycle.

During the early evolution of earth, oxygen was released from H2O vapor by UV radiation. This accumulated in the atmosphere as the hydrogen escaped into the earth's atmosphere. With the advent of plant life, photosynthesis also became a source of oxygen. Oxygen was also released as organic carbon in CHO, and got buried in sediments.


Why should I be aware of this?

Oxygen cycle is critical to both our health and the health of our environment. We need oxygen for respiration. The oxygen that we breath oxidises the sugars in the food to generate energy. During this process carbon dioxide is released in the atmosphere.

  • Humans need oxygen to breathe
  • Oxygen is needed for decomposition of organic waste.
  • Water can dissolve oxygen and it is this dissolved oxygen that supports aquatic life.

Oxygen cycle, greenhouse effect and global warming

With recent developments it is now feasible to measure variations in the oxygen content of the atmosphere at the parts per million (ppm) level. Regular measurements of changes in atmospheric oxygen (O2) are currently being made at a number of locations around the world using two independent techniques, one based on interferometry and one based on stable isotope mass spectroscopy. Oxygen measurements can inform us about fundamental aspects of the global carbon cycle. Oxygen is generated by green plants in photosynthesis and converted to carbon dioxide (CO2) in animal and human respiration. Carbon dioxide is the greenhouse gas of most concern due to its abundance in the atmosphere (~ 360 ppm) and anthropogenic sources. Variations in atmospheric O2 are controlled largely by fluxes of carbon (e.g., photosynthesis and respiration CO2 + H2O <=> CH2O + O2).

All about the Oxygen Cycle?

The oxygen cycle is a complex series of processes in which all of the oxygen atoms present in the earth circulate. Oxygen is one of the more than 100 known elements, and one of six main constituents of living organisms. The earth does not receive matter from outside except from occasional meteorites (and this is negligible). Thus, the oxygen atoms (along with other elements) are part of a closed system, that is, they cannot be lost or replenished. These fixed number of oxygen atoms are recycled throughout all of earth’s processes that use chemicals that contain oxygen. These processes form a cycle that involves the living sphere called the biosphere, and the nonliving spheres – lithosphere, atmosphere and hydrosphere.

What is oxygen cycle?

The oxygen cycle is one of the biogeochemical cycles; there are also other biogeochemical cycles like the carbon cycle, the nitrogen cycle and the water cycle.


Plants mark the beginning of the oxygen cycle. Plants are able to use the energy of sunlight to convert carbon dioxide and water into carbohydrates and oxygen in a process called photosynthesis. During the day, plants hold onto a bit of the oxygen which they produced in photosynthesis and use that oxygen to break down carbohydrates. But in order to maintain their metabolism and continue respiration at night, the plants must absorb oxygen from the air and give off carbon dioxide just as animals do. Even though plants produce approximately ten times as much oxygen during the day as they consume at night, the night-time consumption of oxygen by plants can create low oxygen conditions in some water habitats.


Oxygen in water is known as dissolved oxygen or DO. In nature, oxygen enters water when water runs over rocks and creates tremendous amounts of surface area. The high surface area allows oxygen to transfer from the air into the water very quickly.

When the water in a stream enters a pond, microorganisms in the pond begin to metabolize (break down) organic matter, consuming oxygen in the process. This is another form of oxygen cycle - oxygen enters water in rapids and leaves water in pools.

Oxygen uptake rate (O.U.R.) is the rate at which oxygen is consumed by living organisms in the water. Since organisms are constantly using up oxygen in the water and oxygen is constantly reentering the water from the air, the amount of oxygen in water remains relatively constant. In a healthy ecosystem, the rates of oxygen transfer (being used up) and oxygen uptake are balanced in the water.


Last are the organisms of the world. They use oxygen in many forms. Their role in the cycle begins with carbon dioxide in the atmosphere. Plants take in that carbon dioxide and combine it with water to create sugars and oxygen molecules. Animals breathe that oxygen and both plants and animals use the sugars for energy. Through the process of metabolism, the sugars are broken down into water and carbon dioxide. Then the cycle begins again.


Oxygen is one of the major compounds found in the atmosphere of the Earth. It is never found floating around as individual atoms, but is always with other elements. The oxygen molecule has two oxygen atoms. Some molecules have three oxygen atoms called ozone. It is also found in water molecules and carbon dioxide. That oxygen floats through the atmosphere until it comes down to Earth and starts one of many cycles

It is highly reactive. A colorless, odorless gas at ordinary temperatures, it turns to a bluish liquid at -183° C. During the early evolution of earth, ultraviolet radiation on water vapour is believed to have released Oxygen which then accumulated in the atmosphere. Oxygen is essential for life, as we know it on earth. Living organisms require oxygen to breathe and to produce energy. It is vital in many other ways too. It is present in the atmosphere in the form of ozone and provides protection to life by filtering out the sun’s ultraviolet rays. Burning or combustion is essentially an oxidation reaction.

Reservoirs of oxygen

By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle (99.5%). Only a small fraction is present as free oxygen in the biosphere (0.01%) and atmosphere (0.49%).

Biosphere and Atmosphere

Within the biosphere and atmosphere, plants mark the beginning of the oxygen cycle. Plants are able to use the energy from sunlight to convert carbon dioxide and water into carbohydrates and oxygen in a process called photosynthesis. This means that plants "breathe" in carbon dioxide and "breathe" out oxygen. Animals form the other half of the oxygen cycle. They breathe in oxygen which they use to break carbohydrates down into energy in a process called respiration. Carbon dioxide produced during respiration is breathed out by animals into the air. But the oxygen cycle is not actually quite that simple. Plants must also break carbohydrates down into energy just as animals do. But plants produce approximately ten times as much oxygen as they consume.

An additional supply of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component atoms. The free Hydrogen and Nitrogen atoms escape into space leaving Oxygen in the atmosphere.

Most of the Oxygen present in the atmosphere is used during respiration and decay mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide. Because lithospheric minerals are oxidised using oxygen, chemical weathering of exposed rocks also consumes oxygen.

Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create calcium carbonate shell material (CaCO3) that is rich in oxygen. When the organism dies its shell is deposited on the shallow sea floor and buried over time to create the limestone rock of the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.

Maintaining the Oxygen Cycle

With the increase in the level of atmospheric pollution and and large scale deforestation, experts worry about disturbing the balance in the atmosphere -- there might come a time when the amount of carbon di oxide is very large and there are very few plants and trees to covert it into Oxygen. This crisis will not know international boundaries. It is imperative that we plant more trees and reduce vehicular and industrial pollution.

Oxygen Cycle and environment

  • Oxygen cycle plays a vital role in decomposition of organic waste. When dead tissue (carbon compounds) decays by a combination of oxidation and microorganism decay, carbon dioxide is released back to the atmosphere.
  • A slower cycle occurs whenever mineral matter is oxidised, such as in the formation of rocks.
  • The content of biodegradable substances in waste waters is expressed by a special index called "biological oxygen demand" (BOD), representing the amount of oxygen needed by aerobic bacteria to decompose the waste. With not enough oxygen available for these bacteria, for example, to waste much in a body of water, they die and anaerobic bacteria that do not need oxygen take over.

These bacteria change waste material into H2S and other poisonous and foul-smelling substances.


  • Oxygen is the most common element by volume or mass (weight) on Earth.
  • In each breath taken by a human being, one-fifth of the molecules are oxygen
  • There are twelve trillion tonnes of oxygen in the air.
  • Oxygen is less common than hydrogen, helium, and neon in the Universe, but by far the most common element on the Earth.
  • Oxygen makes 90% of water (bymass), about 47% of the earth's crust, as well as 23% of dry air and 65% ofthe human body.
  • Plants produce approximately 10x as much oxygen during the day as they consume at night.
  • The night-time consumption of oxygen by plants can create low oxygen conditions in some water habitats


  • Oxygen Cycle
  • Climate
  • Environment Literacy
  • The Oxygen Cycle
  • Oxygen Cycling
  • Oxygen

See Also