oxygen cycle

noun Ecology.
the process by which oxygen released into the atmosphere by photosynthetic organisms is taken up by aerobic organisms while the carbon dioxide released as a by-product of repiration is taken up for photosynthesis.

Origin:
1930–35

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Encyclopedia Britannica
Encyclopedia

oxygen cycle

circulation of oxygen in various forms through nature. Free in the air and dissolved in water, oxygen is second only to nitrogen in abundance among uncombined elements in the atmosphere. Plants and animals use oxygen to respire and return it to the air and water as carbon dioxide (CO2). CO2 is then taken up by algae and terrestrial green plants and converted into carbohydrates during the process of photosynthesis, oxygen being a by-product. The waters of the world are the main oxygen generators of the biosphere; their algae are estimated to replace about 90 percent of all oxygen used. Oxygen is involved to some degree in all the other biogeochemical cycles. For example, over time, detritus from living organisms transfers oxygen-containing compounds such as calcium carbonates into the lithosphere.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The use and reuse of oxygen is called the oxygen cycle.
The extreme depolarizing effect of the oxygen cycle must be taken into account.
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