When this word is used to describe an organism it means living or active only in the absence of air and/or oxygen. The oxygen is not present as a gas, dissolved in water or available in some other way. This means the organism is respiring anaerobically.

The opposite is Aerobic, which means taking place in the presence of air and/or oxygen as a gas, dissolved in water or available in some other way.

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Anaerobic organisms such as some bacteria get their oxygen from substances such as nitrates or sulphates that have nitrogen combined with oxygen or sulphur combined with oxygen. In the process they release nitrogen, nitrous oxide, ammonium or hydrogen sulphide provided there is oxidizable material such as organic matter, methanol or some other carbon compound available. Oxidizable means able to have oxygen added and to be combined with that oxygen

Anaerobic respiration is the process by which organisms release energy by breaking down food substances without consuming oxygen as a gas or in dissolved form. Anaerobic respiration is less efficient than aerobic and this is why aerobic organisms dominate on Earth.

Anaerobic conditions are where there is no dissolved or free oxygen. Anaerobic conditions often lead to what humans consider to be smells, such as from compost heaps that have become too wet and this has prevented oxygen getting in. Anaerobic bacteria and other organisms take over from the aerobic ones and the process goes in a different direction to the way the composter intended.

An anaerobic process takes place where there is no dissolved or free oxygen. Some processes are better as anaerobic than as aerobic, because the anaerobic digestion or processing organisms are more appropriate for the job you want done. Anaerobic processing is often used to reduce organic matter in wastes, such as in a septic tank to break down human sewage to the point where it is safe to put into the soil. It is also used to digest animal manure or human sewage to produce methane gas for heating or cooking.
Many normally aerobic organisms can respire anaerobically if there is insufficient oxygen for aerobic respiration while some organisms never use free oxygen.

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This page was updated on December 27, 2007