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The Cycle of Oxygen Through an Ecosystem

Oxygen is a necessity for all living organisms. It supports aquatic life when dissolved in water. In the form of ozone in the stratosphere, it protects life on Earth by filtering out the sun's ultraviolet rays. Oxygen comprises 21 percent of atmospheric gases, and is the most common element in the Earth's crust and mantle. It is the third most common element in the universe after hydrogen and helium. It cycles through the Earth's ecosystems in a symbiotic relationship between bacteria, plants and animals.

Photodissociation
- Photodissociation is the breakdown of water vapor into hydrogen and oxygen using the sun's ultraviolet radiation. Cyanobacteria, a primitive organism on Earth, began this process 2 billion years ago. The hydrogen escaped into outer space leaving an increasing concentration of oxygen in the atmosphere. Oxygen concentrations in the atmosphere reached their current 21 percent level 545 million years ago and remain stable.
Weathering
- Oxygen is one of the most reactive elements in the atmosphere. It combines with other elements to produce the minerals of the lithosphere, as well as animal shells and bones. The weathering of soil and rocks that comprise the lithosphere by wind and water releases oxygen and carbon dioxide into the atmosphere.
Photosynthesis
- Photosynthesis is the main driver of the oxygen cycle. Plants and bacteria take up carbon dioxide and water vapor from the atmosphere and use the sun's radiation to convert these to energy in the form of sugars, and release oxygen back into the atmosphere. Chlorophyll in plant and bacteria cells is the catalyst for this process. This process reverses at night; the organisms take up oxygen from the atmosphere and release carbon dioxide.
Respiration
- Human and animal communities within the ecosystem inhale oxygen through their lungs and circulate it in their blood to burn sugars and produce energy. The organisms release carbon dioxide and water vapor as waste products. Fish absorb oxygen dissolved in water through their gills.
Decomposition
- Aerobic bacteria, organisms that live in oxygen, biodegrade the organic waste from plant and animals into stable inorganic waste. Carbon dioxide is a waste product of decomposition.

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