methanogen

Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They were once classified as archaebacteria but archaebacteria have now been reclasified as archaea, a group quite distinct from bacteria. They are common in wetlands, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for the methane content of flatulence.^[1] In marine sediments biomethanation is generally confined to where sulfates are depleted, below the top layers.^[2] 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.

Physical description

Methanogens are usually coccoid or rod shaped. There are over 50 described species of methanogens, which do not form a monophyletic group, although all methanogens belong to Archaea. Methanogens are also anaerobic. Although methanogens cannot function under aerobic conditions they can sustain oxygen stresses for prolonged times^[3].

An exception is Methanosarcina barkeri, which contains a superoxide dismutase (SOD) enzyme and may survive longer.^[4] Some, called hydrogenotrophic, use carbon dioxide (CO₂) as a source of carbon, and hydrogen as a reducing agent. Some of the CO₂ is reacted with the hydrogen to produce methane, which produces an electrochemical gradient across a membrane, used to generate ATP through chemiosmosis. In contrast, plants and algae use water as their reducing agent.

Methanogens lack a polymer that is found in the cell walls of other prokaryotes. Some methanogens have a cell wall that is composed of pseudomurein. Other methanogens that don't have a pseudomurein have at least one paracrystalline array (S-layer) which is made up of proteins that fit together like a puzzle.

Methanogens and environments

Although most marine biogenic methane is the result of CO₂ reduction, a small amount is derived from acetate (CH₃COO^-).^[5] Archaea that catabolize this for energy are referred to as acetotrophic or aceticlastic. Methylotrophic archaea utilize methylated compounds such as methylamines, methanol, and methanethiol as well.

Methanogens play the vital ecological 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.

Methanogens have been found in several extreme environments on Earth - buried under kilometres of ice in Greenland and living in hot, dry desert soil. They can still reproduce from temperatures of 15 to 100 degrees Celsius. They are known 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.^[6]

Another study^[7] 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 United States, 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.^[8]

Some scientists have proposed that the presence of methane in the Martian atmosphere may be indicative of native methanogens on that planet.^[9]

Closely related to the methanogens are the anaerobic methane oxidizers, which utilize methane as a substrate in conjunction with the reduction of sulfate and nitrate.^[10] Most methanogens are autotrophic producers, but those which oxidize CH₃COO^- are classed as chemoheterotrophs instead.

Types of methanogens

• Methanobacterium bryantii
• Methanobacterium formicum
• Methanobrevibacter arboriphilicus
• Methanobrevibacter gottschalkii
• Methanobrevibacter ruminantium
• Methanobrevibacter smithii
• Methanocalculus chunghsingensis
• Methanococcoides burtonii
• Methanococcus aeolicus
• Methanococcus deltae
• Methanococcus jannaschii
• Methanococcus maripaludis
• Methanococcus vannielii
• Methanocorpusculum labreanum
• Methanoculleus bourgensis (Methanogenium olentangyi & Methanogenium bourgense)
• Methanoculleus marisnigri
• Methanofollis liminatans
• Methanogenium cariaci
• Methanogenium frigidum
• Methanogenium organophilum
• Methanogenium wolfei
• Methanomicrobium mobile
• Methanopyrus kandleri
• Methanoregula boonei
• Methanosaeta concilii
• Methanosaeta thermophila
• Methanosarcina acetivorans
• Methanosarcina barkeri
• Methanosarcina mazei
• Methanosphaera stadtmanae

• Methanospirillium hungatei
• Methanothermobacter defluvii (Methanobacterium defluvii)
• Methanothermobacter thermautotrophicus (Methanobacterium thermoautotrophicum)
• Methanothermobacter thermoflexus (Methanobacterium thermoflexum)
• Methanothermobacter wolfei (Methanobacterium wolfei)
• Methanothrix sochngenii

References

1. ^ Joseph W. Lengeler (1999). Biology of the Prokaryotes. Stuttgart: Thieme. p. 796. ISBN 0632053577. 
2. ^ J.K. Kristjansson, et al. (1982). "Different Ks values for hydrogen of methanogenic bacteria and sulfate-reducing bacteria: an explanation for the apparent inhibition of methanogenesis by sulfate". Arch. Microbiol. 131: 278–282. doi:10.1007/BF00405893. 
3. ^ V. Peters, R. Conrad, 1995, Methanogenic and other strictly anaerobic bacteria in desert soil and other oxic sois, Applied and Environmental Microbiology, 61: 1673-1676
4. ^ http://spacecenter.uark.edu/JillJabstract.doc
5. ^ M.J. Whiticar, et al. (1986). "Biogenic methane formation in marine and freshwater environments: CO₂ reduction vs. acetate fermentation — isotope evidence". Geochim. Cosmochim. Acta 50: 393–709. doi:10.1016/0016-7037(86)90346-7. 
6. ^ Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102)
7. ^ Icarus (vol. 178, p. 277)cs:Methanogen
8. ^ Extreme bugs back idea of life on Mars
9. ^ Crater Critters: Where Mars Microbes Might Lurk
10. ^ Thauer, R. K. and Shima, S., "Biogeochemistry: Methane and microbes", Nature, 2006, 440, 878-879