<<Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism. In most environments, it is the final step in the decomposition of biomass. Some humans produce flatus that contains methane. In one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane.
Methanogenesis in microbes is a form of anaerobic respiration. Methanogens do not use oxygen to respire; in fact, oxygen inhibits the growth of methanogens. 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 acetic acid and inorganic carbon dioxide as terminal electron acceptors:
- CO2 + 4 H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2
However, depending on pH and temperature, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid (formate), methanol, methylamines, tetramethylammonium, dimethyl sulfide, and methanethiol.
The presence of atmospheric methane has a role in the scientific search for extra-terrestrial life. The justification is that methane in the atmosphere will eventually dissipate, unless something is replenishing it. If methane is detected (by using a spectrometer for example) this may indicate that life is, or recently was, present. This was debated when methane was discovered in the Martian atmosphere by M.J. Mumma of NASA's Goddard Flight Center, and verified by the Mars Express Orbiter (2004) and in Titan's atmosphere by the Huygens probe (2005). This debate was furthered with the discovery of 'transient', 'spikes of methane' on Mars by the Curiosity Rover.
It is also argued that atmospheric methane can come from volcanoes or other fissures in the planet's crust and that without an isotopic signature, the origin or source may be difficult to identify.
On 13th April 2017, NASA confirmed that the dive of the Cassini orbiter spacecraft on 28 October 2005 discovered the Enceladus plume which has all the ingredients for methanogensis based life forms to feed from. Previous results, published in March 2015, suggested hot water is interacting with rock beneath the sea; the new findings support that conclusion and add that the rock appears to be reacting chemically. From these observations scientists have determined that nearly 98% of the gas in the plume is water, about 1% is hydrogen and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.