Methanogen
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Methanogens are archaea that produce methane as a metabolic byproduct. They are common in wetland, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for flatulence. They are also common in soils in which the oxygen has been depleted. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of the earth's crust, kilometers below the surface.
Methanogens are anaerobic. All methanogens are rapidly killed by the presence of oxygen. Some, called hydrotropic, use carbon dioxide as a source of carbon and, hydrogen as a source of energy (hydrogen functions as a reducing agent). Some of the carbon dioxide is reacted with the hydrogen to produce methane, which produces a electrochemical gradient across a membrane, which is used to generate ATP through chemiosmosis. In contrast, plants and algae use water as their reducing agent. Other methanogens use acetate (CH3COO-) as a source of carbon and a source of energy. This type of metabolism is referred to as "acetotrophic" or "aceticlastic," breaking down acetate to produce carbon dioxide and methane. Other methanogens are able to utilize methylated compounds such as methylamines, methanol, and methanethiol as well.
Ecologically, methanogens play the vital role in anaerobic environments of removing excess hydrogen and fermentation products that have been produced by other forms of anaerobic respiration. Methanogens typically thrive in environments in which all other electron acceptors (such as oxygen, nitrate, sulfate, and trivalent iron) have been depleted. In the deep rock they obtain their hydrogen from the thermal and radioactive breakdown of water.
There are over 50 species of methanogens, divided into three classes - the Methanobacteria, Methanococci, and Methanopyri. All are included among the Euryarchaeota. They are paraphyletic, and there is some reason to suppose the Methanococci are paraphyletic as well.
Methanogens have been discovered in several extreme environments on Earth - buried under kilometres of ice in Greenland and living in hot, dry desert soil. They have also been found to be the most common prokaryotes archaebacteria in deep subteranean habitats.
Live microbes making methane were found in a glacial ice core sample retrieved from three kilometres under Greenland by researchers from the University of California, Berkeley, US[1].
Another study[2] has also discovered methanogens in a harsh environment on Earth. Researchers studied dozens of soil and vapour samples from five different desert environments in Utah, Idaho and California in the US, and in Canada and Chile. Of these, five soil samples and three vapour samples from the vicinity of the Mars Desert Research Station in Utah were found to have signs of viable methanogens.[3]
Some scientists have proposed that the presence of methane in the Martian atmosphere may be indicative of native methanogens on that planet[4].
[edit] References
- ^ Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102).
- ^ Icarus (vol. 178, p. 277)cs:Methanogen
- ^ Extreme bugs back idea of life on Mars
- ^ Crater Critters: Where Mars Microbes Might Lurk
