Methanogens are members of Archaea, a relatively new domain in the classification of life. Related to both bacterial and eukaryotic cells, they are a unique group in all respects: metabolism, habitat, and physiology. Methanogens are united by methanogenesis, a catabolic specialization not found in any other archaeon.
History of Archaea and Methanogens
As the name implies, Archaeans, and therefore methanogens, are some of the oldest organisms on earth. There have been numerous molecular fossils found in the Messel Oil shale of Germany, where there are Miocene deposits. Molecular fossils are found by looking for the membranes formed from isoprene chains unique to Archaea. These do not decompose at high temperatures and make good markers for the presence of ancient Archaea.
These ancient life forms have also been found in the oldest known sediment on earth, the Isua district of Greenland, at 3.8 billion years old. This indicates that Archaea appeared within one billion years of the earth's formation, in an atmosphere rich in ammonia and methane.
They have also been found in Mesozoic, Paleozoic, and Precambrian sediments and it is thought that these initial habitants of earth were most likely methanogens.
Habitat of Methanogens
Methanogens inhabit some of the most extreme environments on earth, including:
- the rumen of ruminants living on hydrogen and carbon dioxide produced by other microbes, helping digest cellulose, as well as being necessary for protein synthesis.
- the muck of swamps and marshes, hydrothermal vents, porous rock, sewage sludge, the gut of termites.
- oil-contaminated groundwater at underground oil storage facilities.
They are further classified as:
- Thermophilic (heat loving) methanogens are found terrestrially in volcanic hot springs and sofataras, where temperatures span between 40 to 100 degrees Celsius, and in marine environments in undersea hydrothermal vents where, due to pressure, temperatures can reach 350 degrees Celsius.
- Psychrophily (cold loving) is rare among methanogens with only a few species being identified.
- Halophilic (salt loving) methanogens are found in salt marshes.
- Acidophilic (acid loving) methanogens, such as those that inhabit peat bogs where pH is less than four, are rare. Optimum pH for methanogenesis is six.
- There are some species of alkaliphilic (base loving) methanogens that exist at a pH exceeding eight.
Methanogens and Extraterrestrial Life
Because of their extraordinary ability to live in extreme oxygen deficient environments, methanogens make excellent contenders for inhabiting other planets.
Currently, the most likely candidates for this habitation are subsurface ecosystems on Mars and Jupiter's moon Europa.
Lidy Hot Springs in Idaho provides an analogous environment to the ones proposed on Mars and Europa. The Lidy Hot Springs' methanogen-dominated environment is unlike anything described on earth before, and is compatible with geochemical propositions for microbial populations that could reside in the subsurface of Mars or Europa.
Sources:
Chapelle, F.H. et al. A hydrogen-based subsurface microbial community dominated by methanogens. Nature. 415, 2002.
Galagan, J.E. et al. The genome of M. acetivorans reveals extensive metabolic and physiological diversity. Genome Research 12, 2002.
Watanabe, K., Kodoma, Y., Hamamura, N. Kaku, N. Diversity, abundance, and activity of archaeal populations in oil-contaminated groundwater accumulated at the bottom of an underground crude oil storage cavity. Applied and Environmental Microbiology. 68, 2002.
Woese, C.R., Kandler, O., and Wheelis, M.L. Toward a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences USA. 87, 1990.
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