Anaerobic
This term refers to an environment lacking in molecular oxygen (O2), or to an organism, tissue, chemical reaction, or biological process that does not require oxygen. Anaerobic organisms can use a molecule other than O2 as the terminal electron acceptor in respiration.
These organisms can be either obligate, meaning that they cannot use O2, or facultative, meaning that they do not require oxygen but can use it if it is available.
Organic matter decomposition in poorly aerated environments, including water-logged soils, septic tanks, and anaerobically-operated waste treatment facilities, produces large amounts of methane gas. The methane can become an atmospheric pollutant, or it may be captured and used for fuel, as in "biogas"-powered electrical generators. Anaerobic decomposition produces the notorious "swamp gases" that have been reported as unidentified flying objects (UFOs).
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