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Methanogenesis is the formation of methane by microbes. This is an important and widespread form of microbial metabolism. In most
environments, it is the final step in the decomposition of organic matter.
Organisms capable of methogensis are called methanogens. These organisms have no nucleous or membrane-bound organelles (they are
procaryotes). Methanogens are considered to be a very old group of organisms, being members of the archaebacteria, also known as archaea
(depending on what taxonomic system is being used).
Methanogenesis is a form of anaerobic respiration. Methanogens do not use oxygen to breathe. In fact, oxygen is a deadly poison to methanogens,
and kills all methanogens in very tiny concentrations. The terminal electron acceptor in methanogenesis is not oxygen, but carbon. The carbon can
occur in a small number of organic compounds, all with low molecular weights. The two best described pathways involve the use of carbon dioxide
and acetic acid as terminal electron acceptors:
CO2 + 4 H2 => CH4 + 2H2O
CH3COOH => CH4 + CO2
However, methanogenesis has been established to use carbon from other small organic compounds, such as formic acid and methanol.
Methanogens cannot exist in the presence of oxygen, so they are only found in environments in which the oxygen has been depleted. Most
commonly these are environments experiencing the rapid decay of organic matter, such as wetland soils, the digestive tracts of animals, and
aquatic sediments. Methanogenesis occurs in other areas where oxygen is absent, although in the absence of decaying organic matter, such as the
terrestrial deep subsurface, deep-sea hydrothermal vents, and oil reservoirs.
Methanogenesis is the final step in the decay of organic matter. During the decay process, electron acceptors (such as oxygen, ferric iron, sulfate,
nitrate, and manganese) become depleted, while hydrogen (H2) and carbon dioxide accumulat