Anaerobic oxidation of methane
Encyclopedia
Anaerobic oxidation of methane (AOM) is a microbial process occurring mainly in anoxic marine sediments. During AOM methane
is oxidized with sulfate
as the terminal electron acceptor
: CH4 + SO42- → HCO3- + HS- + H2O
According to current knowledge, AOM is mediated by a syntrophic consortium http://www.thefreedictionary.com/syntrophy of methanotrophic archaea
and sulfate-reducing bacteria
. They often form small aggregates or sometimes voluminous mats. The archaeal partner is abbreviated ANME, which stands for "anaerobic methanotroph
". ANME's are very closely related to methanogenic archaea and recent investigations suggest that AOM is an enzymatic reversal of methanogenesis
. It is still poorly understood how the syntrophic partners interact and which intermediates are exchanged between the archaeal and bacterial cell. The research on AOM is hindered by the fact that the responsible organisms have not been isolated. This is because these organisms show very slow growth rates with a minimum doubling time of a few months.
Countless isolation efforts have not been able to isolate one of the anaerobic methanotrophs, a possible explanation can be that the ANME archaea and the SRB have an obligate syntrophic interaction and can therefore not be isolated individually.
In benthic marine areas with strong methane releases from fossil reservoirs (e.g. at cold seeps, mud volcano
es or gas hydrate deposits) AOM can be so high that chemosynthetic organisms like filamentous sulfur bacteria (see Beggiatoa
) or animals (clams, tube worms) with symbiont sulfide-oxidizing bacteria can thrive on the large amounts of hydrogen sulfide
that are produced during AOM. The production of bicarbonate
from AOM can result in the precipitation of calcium carbonate
or so-called authigenic
carbonates.
AOM is considered to be a very important process reducing the emission of the greenhouse gas
methane from the ocean into the atmosphere. It is estimated that almost 90% of all the methane that arises from marine sediments is oxidized anaerobically by this process.
Recent investigations have shown that some consortia of archaea and bacteria are also able to oxidize methane with nitrate
instead of sulfate.
Recent findings also suggest that nitrite reduction can be coupled to methane oxidation by a single bacterial species without the need for an archaeal partnerhttp://ucelinks.cdlib.org:8888/sfx_local?&url_ver=Z39.88-2004&url_ctx_fmt=info:ofi/fmt:kev:mtx:ctx&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.atitle=Nitrite-driven%20anaerobic%20methane%20oxidation%20by%20oxygenic%20bacteria&rft.auinit=K&rft.aulast=Ettwig&rft.date=2010&rft.epage=U94&rft.genre=article&rft.issn=0028-0836&rft.issue=7288&rft.spage=543&rft.stitle=NATURE&rft.title=NATURE&rft.volume=464&rfr_id=info:sid/www.isinet.com:WoK:WOS&rft.au=Butler%2C%20M&rft.au=Le%20Paslier%2C%20D&rft.au=Pelletier%2C%20E&rft.au=Mangenot%2C%20S&rft.au=Kuypers%2C%20M&rft_id=info:doi/10%2E1038%2Fnature08883].The organisms involved in the nitrate reducing process are not directly related to the ANME organisms found in marine sediments where methane oxidation is coupled to sulfate reduction.
In the literature two different abbreviations of anaerobic oxidation of methane can be found: AOM (anaerobic oxidation of methane) and AMO (anaerobic methane oxidation). AMO was considered to be grammatically incorrect, because it implies that anaerobic methane is oxidized.
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
is oxidized with sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
as the terminal electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
: CH4 + SO42- → HCO3- + HS- + H2O
According to current knowledge, AOM is mediated by a syntrophic consortium http://www.thefreedictionary.com/syntrophy of methanotrophic archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
and sulfate-reducing bacteria
Sulfate-reducing bacteria
Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide...
. They often form small aggregates or sometimes voluminous mats. The archaeal partner is abbreviated ANME, which stands for "anaerobic methanotroph
Methanotroph
Methanotrophs are bacteria that are able to metabolize methane as their only source of carbon and energy. They can grow aerobically or anaerobically and require single-carbon compounds to survive...
". ANME's are very closely related to methanogenic archaea and recent investigations suggest that AOM is an enzymatic reversal of methanogenesis
Methanogenesis
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...
. It is still poorly understood how the syntrophic partners interact and which intermediates are exchanged between the archaeal and bacterial cell. The research on AOM is hindered by the fact that the responsible organisms have not been isolated. This is because these organisms show very slow growth rates with a minimum doubling time of a few months.
Countless isolation efforts have not been able to isolate one of the anaerobic methanotrophs, a possible explanation can be that the ANME archaea and the SRB have an obligate syntrophic interaction and can therefore not be isolated individually.
In benthic marine areas with strong methane releases from fossil reservoirs (e.g. at cold seeps, mud volcano
Mud volcano
The term mud volcano or mud dome are used to refer to formations created by geo-excreted liquids and gases, although there are several different processes which may cause such activity. Hot water mixes with mud and surface deposits. Mud volcanoes are associated with subduction zones and about 700...
es or gas hydrate deposits) AOM can be so high that chemosynthetic organisms like filamentous sulfur bacteria (see Beggiatoa
Beggiatoa
Beggiatoa is a genus of bacteria in the order Thiotrichales. They are named after the Italian medic and botanist F.S. Beggiato. The organisms live in sulfur-rich environments...
) or animals (clams, tube worms) with symbiont sulfide-oxidizing bacteria can thrive on the large amounts of hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
that are produced during AOM. The production of bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
from AOM can result in the precipitation of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
or so-called authigenic
Authigenic
An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere by water or wind. Authigenic sediments are the main...
carbonates.
AOM is considered to be a very important process reducing the emission of the greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
methane from the ocean into the atmosphere. It is estimated that almost 90% of all the methane that arises from marine sediments is oxidized anaerobically by this process.
Recent investigations have shown that some consortia of archaea and bacteria are also able to oxidize methane with nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
instead of sulfate.
Recent findings also suggest that nitrite reduction can be coupled to methane oxidation by a single bacterial species without the need for an archaeal partnerhttp://ucelinks.cdlib.org:8888/sfx_local?&url_ver=Z39.88-2004&url_ctx_fmt=info:ofi/fmt:kev:mtx:ctx&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.atitle=Nitrite-driven%20anaerobic%20methane%20oxidation%20by%20oxygenic%20bacteria&rft.auinit=K&rft.aulast=Ettwig&rft.date=2010&rft.epage=U94&rft.genre=article&rft.issn=0028-0836&rft.issue=7288&rft.spage=543&rft.stitle=NATURE&rft.title=NATURE&rft.volume=464&rfr_id=info:sid/www.isinet.com:WoK:WOS&rft.au=Butler%2C%20M&rft.au=Le%20Paslier%2C%20D&rft.au=Pelletier%2C%20E&rft.au=Mangenot%2C%20S&rft.au=Kuypers%2C%20M&rft_id=info:doi/10%2E1038%2Fnature08883].The organisms involved in the nitrate reducing process are not directly related to the ANME organisms found in marine sediments where methane oxidation is coupled to sulfate reduction.
In the literature two different abbreviations of anaerobic oxidation of methane can be found: AOM (anaerobic oxidation of methane) and AMO (anaerobic methane oxidation). AMO was considered to be grammatically incorrect, because it implies that anaerobic methane is oxidized.