Methanogenesis

The methanogenesis (also: methane formation ) is the formation of methane by the metabolism of living organisms, which are known as methanogens or methane producers. She finds - with few exceptions - mostly in the last stage of the anaerobic microbial degradation of biomass instead. The most methanogens carbon dioxide and hydrogen set to methane. From simple organic C1 compounds such as formic acid, methanol and methylamines methane is formed. Acetic acid is converted by acetic acid -cleaving ( acetoclastic ) methanogens into methane and carbon dioxide. Other bacterial fermentation products such as lactic acid, propionic acid and butyric acid, however, can not be used as precursors for the methane formation.

In the literature, methanogenesis is mainly treated as a specific, anaerobic metabolic pathway of archaea, as a special form of anaerobic respiration. This use case the exergonic ( energy releasing ) methanogenesis as an energy source. Therefore, the article focuses on the anaerobic methane release in archaea.

• 3.1 methanogenesis from carbon dioxide and hydrogen 3.1.1 Reduction of carbon dioxide to methane
• 3.1.2 regeneration of coenzymes M and B

• 4.1 ATP synthesis
• 4.2 Energy yield

Importance

The methanogenesis is a central component of the carbon cycle of the earth, since the resulting degradation products, methane and carbon dioxide, if necessary, return to this cycle. Since these gases, especially methane, are powerful greenhouse gases, methanogenesis has also won in the prevention of global warming important. Probably the biotic methane production also plays a role in the formation of methane hydrate, whose economic use of interest.

Has an important meaning methanogenesis in the end and the end of the anaerobic food chain, as they at all possible the growth of many bacteria syntropher. These secondary leavener obtain their energy from the reaction of lactate, propionate, butyrate and simple organic compounds, in addition to carbon dioxide and acetate and hydrogen is produced. For thermodynamic reasons, however, these fermentation reactions are only possible if the resulting hydrogen is consumed rapidly again and the H2 partial pressure does not rise above 100 Pa. This is ensured by living in close proximity methanogens, which in turn require this hydrogen for methanogenesis. The transfer of hydrogen between syntrophic bacteria and archaea to, ie between different species, also called interspecies hydrogen transfer.

Since methanogens, associated with syntrophic bacteria in the human digestive tract occur, there methanogenesis has an influence on digestion. Approximately 10% of anaerobes in the human intestine are stadtmanae methanogens Methanobrevibacter smithii and the types Methanosphaera. These use the two products of bacterial fermentation hydrogen and formate for methanogenesis. A high concentration of hydrogen to inhibit the ATP - production of other bacteria. M. smithii builds under formation from methane, methanol, which is toxic to humans. Therefore, the methanogens have a positive impact on the human intestinal flora. Whether this also affect how much energy the person can take in from food, is still the subject of research.

Occurrence

The methane formation occurs in nature in exclusively anaerobic medium, in which there is a reduction of biomass. This can be, for example sediments of lakes and the sea, the rumen of cattle, the gut of termites and people, rice fields or swamps. Even wastewater treatment plant sludge basin as artificial plants for biological degradation are possible locations for methanogenesis. In these habitats prevail moderate ( mesophilic ) temperatures. The methanogenesis also occurs in environments with extremely high and low temperatures and at high salt or acid content, such as in geothermal systems. In all cases, the concentrations of sulfate, nitrate, manganese ( IV) must in these habitats - and iron ( III ) ions to be low, otherwise use bacteria such substances as external electron acceptors in anaerobic respiration and usable in this breathing for methanogens electron donors consume. The redox processes run with these electron acceptors namely preferably before the methanogenesis from and methanogens is thereby deprived of their energy source, and thus their livelihood. Under anaerobic conditions, carbon dioxide, the substrate of most methanogens, rarely limiting, since it is continuously released by fermentation reactions by socialized bacteria.

Classification

The methanogenic archaea is operated by, all belong to the department of the Euryarchaeota. Methanogenic archaea are found in the following five orders: Methanopyrales, Methanobacteriales, Methanococcales, Methanomicrobiales and Methanosarcinales. Here Methanopyrales the phylogenetically oldest, Methanosarcinales is against the phylogenetically youngest branch. 2008 a sixth order ( Methanocellales ) was discovered as occurring in the soil of rice fields archaea Methanocella paludicola and Methanocella arvoryzae operate a methanogenesis from carbon dioxide and hydrogen. Methanoplasmatales that are related to the thermal plasma valley, are proposed in the literature as a seventh order.

Methanopyrales, Methanobacteriales and Methanococcales to one of the class I, class II Methanomicrobiales to the methanogens. Methanosarcinales are class III methanogens.

Substrate diversity

In many habitats are methanogens end consumers in the so-called " anaerobic food chain ". In this chain, first, biopolymers such as proteins and polysaccharides such as cellulose in particular on oligomers into monomers (eg amino acids and carbohydrates) split. Lipids are terminated into their components (eg fatty acids). After bacteria ferment these cleavage products to simple carboxylic acids ( such as formate, acetate, propionate, lactate, and succinate ) to alcohols (such as ethanol, 2-propanol and butanol ) and other low molecular weight compounds (H2, CO2, and short-chain ketones). Syntrophic, acetogenic bacteria utilize a portion of these compounds and translate them to acetate and C1 compounds. In the last part of the anaerobic food chain, methanogenesis, these compounds are used as carbon, reductant and energy source, CH4 and CO2 are released often.

• Most methanogens operate methanogenesis with carbon dioxide (CO2) as a substrate, wherein the hydrogen (H2) is used as the primary reducing agent. One calls such as methanogens or wasserstoffoxidierend hydrogenotroph. To the obligatory ( exclusive ) hydrogenotrophic count the Methanopyrales, Methanobacteriales, Methanococcales and Methanomicrobiales that use only H2 and CO2 or formic acid ( HCOOH ) as substrates for methanogenesis. An exception among the Methanomicrobiales is Methanosphaera stadtmanae, which occurs in the human digestive tract. It is dependent on methanol, and hydrogen, as it can not utilize CO2. Model organisms under the hydrogenotrophic are Methanothermobacter thermoautotrophicus and Methanocaldococcus jannaschii (formerly Methanococcus jannaschii ).

• Carbon monoxide ( CO) may be used only by a few species for methanogenesis. M. thermoautotrophicus and Methanosarcina barkeri form of four molecules of CO, three molecules of CO2 and Methane. Methanosarcina acetivorans can also use CO as the substrate, parallel acetate and formate are formed. This type of acetogenesis in methanogens is called carboxidotrophe acetogenesis.

• The Methanosarcinales are the most versatile methanogens, they may use very different C1 compounds for methanogenesis. CO2 H2 next to use many kinds of C1 - compounds in which the carbon is contained as a methyl group, such as methanol, methyl amines ( mono-, di -, trimethylamine ) and Methylthiole ( dimethyl sulfide, methanethiol ). Methanosarcinales but can not implement formic acid.

• Acetate ( CH3COOH ), C2 is the only compound which can be used for methanogenesis. These are - as far as known so far - only the genera Methanosaeta and Methanosarcina ( Methanosarcinales ) capable. They are called acetoclastic methanogens or Acetoklaster. In this type of methanogenesis acetate is cleaved into CO2 and CH4. Although acetate is used by few archaea for methanogenesis, transmits the resulting methane with 66% of the annual methane production on Earth at. Thus, the methane formation of Acetoklaster is the largest biogenic source.

Distinction by cytochromes

Methanogens of the orders Methanosarcinales contain cytochromes, while this has not been found among the other orders. This, in addition to physiological and metabolic specific effects on methanogenic archaea as metabolize carbon dioxide and hydrogen to methane.

• Methanogens with cytochromes contain Methanophenazin. It is the universal electron carrier in the membrane, where it replaces the methanogens quinone, which occurs only at low levels and in other organisms is essential for the transport of electrons of the respiratory chain. Many Methanosarcinales grow on acetate and methylated compounds. If you use CO2 H2, the H2 partial pressure must be above 10 Pa. Methanogens with cytochromes grow slowly, the division rate is over 10 hours per division. So far, no representatives were discovered among methanogens with cytochromes, which grow under hyperthermophilic conditions.
• In methanogens without cytochromes, however Methanophenazin missing. In contrast to the Methanosarcinales those methanogens grow with CO2 H2 or formic acid and methylated compounds can not yet utilize acetate. An exception is the degenerate in humans M. stadtmanae, the methanol and hydrogen required for the growth. Cytochrome enough methanogenic without a H2 partial pressure below 10 Pa, to perform the methanation. Their doubling time is less than an hour per doubling. Among the methanogens without cytochromes there are many hyperthermophilic species.

Biochemical reactions

In the reduction of carboxyl groups (-COOH) and carbon dioxide to methane enzymes play an essential role with characteristic coenzymes. In particular, these are the coenzymes Tetrahydromethanopterin, coenzyme M, coenzyme F430 and F420, as well as specific electron or hydrogen carrier. These come partly seen only in methanogens.

Methanogenesis from carbon dioxide and hydrogen

Reduction of carbon dioxide to methane

Overview EC numbers

• Formylmethanofuran dehydrogenase EC 1.2.99.5
• Methenyl - H4MPT cyclohydrolase EC 3.5.4.27
• Methylene H4MPT dehydrogenase EC 1.5.99.9
• F420 -dependent methylene reductase EC 1.5.99.11
• F420 - independent methylene- reductase EC 1.12.98.2
• F420 -reducing hydrogenase EC 1.12.98.1
• Methyltransferase EC 2.1.1.86
• Cytosolic hydrogenase / reductase EC 1.8.98.1

Therefore carbon dioxide can be used as a substrate, a reactive amino group of the coenzyme Methanfuran (MFR), it is first subject to the. This produces N- Carboxymethanofuran, an unstable intermediate product, which is stable to the first intermediate, the N- Formylmethanofuran ( CHO MFR) is reduced. A Formylmethanofuran dehydrogenase ( MFR dehydrogenase ) catalyzes the two reactions and requires a reducing agent in the form of reduced ferredoxin. The data necessary for this reduction electrons originate either from hydrogen, which transmits a hydrogenase to oxidized ferredoxin. Alternatively they come from the oxidation of formate, the anion of formic acid, to carbon dioxide, which is a formate dehydrogenase catalyzed. Since the formation of CHO MFR endergonic, the necessary energy is tapped from the electrochemical ion gradients of the membrane.

The bound on MFR formyl group ( -CHO) is transmitted to Tetrahydromethanopterin ( H4MPT ) that structurally resembles the tetrahydrofolate (THF ) of other organisms. Then the bound H4MPT formyl gradually over N5, N10- methenyl - H4MPT and N5, N10 -methylene- H4MPT is reduced to methyl H4MPT ( - CH3). This process is fully reversible and can also run in the opposite direction. Reducing agent is F420H2 here. A cytosolic methenyl - H4MPT cyclohydrolase, methylene H4MPT dehydrogenase or a ( F420 -dependent ) methylene reductase catalyze these reactions. In Methanosarcinaarten is Tetrahydosarcinapterin before ( H4SPT ), which is very similar to the H4MPT.

In addition to the F420 - dependent methylene reductase some obligate hydrogenotrophic hydrogen use directly. These methylene reductase contains, in contrast to other hydrogenases neither iron-sulfur still nickel-iron cluster, it is "metal- free".

The universal F420H2 reducing agent is regenerated by oxidation of an iron-nickel containing F420 -reducing hydrogenase which needs hydrogen.

The methyl group of methyl - H4MPT is then transferred to the simplest coenzyme, coenzyme M ( CoM ). It arises methyl - CoM, in which the methyl group is attached to the sulfide rest of the coenzyme ( H3C -S- CoM ). The transfer takes place via a membrane-bound methyltransferase. This reaction is exergonic ( ΔG0 '= -29 kJ / mol. ) Methanogens utilize the energy released in this process to export about two sodium ions per conversion from the cell. Thus an electrochemically acting Natriumionkonzentrationsunterschied forms.

Methyl - CoM finally reacts with coenzyme B ( CoB ) to a mixed disulfide CoM -S -S- CoB and methane. This is the key reaction in methanogenesis. , The mixed disulfide is also called heterodisulfide. This reaction is catalyzed by a reductase - methyl -COM containing the cofactor F430.

In the balance sheet so that will be implemented in accordance with one molecule of carbon dioxide:

Regeneration of the coenzymes, and M B

The coenzymes M and B must be regenerated for a new passage. This is done by oxidation of CoM -S -S- CoB to CoM and CoB and is catalyzed by a Heterodisulfidreduktase. The electrons required for this reaction are taken either from hydrogen, reduced ferredoxin or F420H2. Wherein the reaction energy released ( ΔG0 '= -39 kJ mol -1).

In methanogens with cytochromes CoM -S -S- CoB is reduced to a membrane-bound Heterodisulfidreduktase. In obligate carbonaceous reducing methanogens this is a complex of three subunits ( HdrABC ) in Methanosarcina species it is composed of two subunits ( HdrDE ). For the reduction electrons are needed. Either hydrogen is oxidized to a membrane-bound hydrogenase, the prosthetic group contains ( Vho ), among other heme b. In parallel, protons are transported to the outside. The Hydrogenasekomplex example, was identified living in freshwater Ms. barkeri. Ms. acetivorans, a saltwater occurring archaeon, oxidized ferredoxin instead of hydrogen to a membrane-bound complex also (Ma- Rnf ) which has, inter alia, as a prosthetic group of cytochrome c. Here, sodium ions are transported to the outside. If Ms. acetivorans grows exclusively on carbon monoxide, a membrane-bound oxidized F420 - dehydrogenase complex ( Fpo ) reduced F420, in the process, protons are exported. The transfer of electrons from hydrogenase or to Heterodisulfidreduktase dehydrogenase complex is mediated by Methanophenazin. The reduction of CoM -S -S- CoB is exergonic, therefore also be simultaneously transported through this process protons outward, so that overall a proton motive force is built. These use the methanogens for the synthesis of ATP ( see section below).

In contrast, methanogens have no cytochromes neither Methanophenazin still a membrane-bound Heterodisulfidreduktase. For the oxidation of Heterodisulfides CoM -S -S- CoB they use a cytosolic hydrogenase / reductase, which requires hydrogen and the energy released coupled to the reduction of ferredoxin. However, no mechanism is based, in which the energy released could be coupled to build a proton -motive force - the Hetereosulfidreduktase is not membrane bound before. Therefore methanogens can only use the sodium ion concentration difference is established in the Methyltransferasereaktion without cytochromes.

Conversion of formate to methane

Formic acid or its anion, formate ( HCOO - ), can be used by about half of all methanogens as a substrate. In contrast to carbon dioxide, it is not directly transmitted to MFR, but first oxidized to carbon dioxide by formate dehydrogenase. The enzyme contains molybdenum and iron-sulfur cluster, it has been isolated from methanogenic archaea ( formicicium example, from Methanobacterium and Mc. Vannielii ). In the catalyzed reaction is simultaneously reduced F420 to F420H2. Carbon dioxide is then, as described above, reduced to methane.

Reducing agent as needed for the gradual implementation of carbon dioxide to methane for the conversion of formate to methane at four points. At two points, they are consumed directly in the form of F420H2 in the stepwise reduction of methenyl - H4MPT to methyl H4MPT. On the other two hydrogen is required for the cytosolic Heterodisulfidreduktase which couples the oxidation of CoM -S -S- CoB to CoM and CoB and the formation of reduced ferredoxin. Hydrogen can be generated either by the F420 -reducing hydrogenase, or alternatively by a nickel- free hydrogenase. The reduced ferredoxin formed during the reaction is required for Heterodisulfidreduktase MFR dehydrogenase in the input response.

Therefore, a total of four molecules F420H2 are required to utilize formate in methanogenesis. These are provided by four molecules of formic acid is oxidized to carbon dioxide. Three molecules of carbon dioxide are released and the fourth finally converted to methane. In the balance sheet becomes:

Methanogenesis with methylated C1 compounds

C1 compounds with a methyl group such as methyl amine ( CH3NH2 ) or methanol ( CH3OH ) are especially found in sea water or brackish water and are anaerobic degradation products of cellular constituents of certain plants and phytoplankton.

Since the carbon is already reduced more in the methyl group as in CO2, these compounds do not have to go through the entire path as that of the carbon dioxide. Therefore, they are fed in the lower third of the way into the methanogenesis in the form of CH3 - CoM. In addition to the direct route to methane methylated compounds are also oxidized to carbon dioxide. So there is an oxidative and reductive one branch. The reason is that the electrons for the reductive branch of the oxidation of methyl group must be taken into carbon dioxide, as the use of hydrogen from the environment ( as an electron source ) is often not possible.

A molecule of methanol is oxidized to carbon dioxide, for example, so that with the help of the released reducing equivalents three molecules are reduced to methane. This disproportionation occurs, for example in accordance with:

These two branches are also found in the reaction of methyl amines by Methanosarcina. Methyl amines are metabolized to methane, CO2, and ammonia (NH3), wherein three of the methyl groups to be reduced and methane is oxidized into carbon dioxide.

Here, the methyl group of the substrate is transferred to CoM and finally - as described above - reduced to methane. The transfer to CoM methyltransferases catalyze cytosolic, the need for the reaction Pyrrolysine than 22 amino acid and contain a corrinoid prosthetic group.

In the oxidative branch of the methyl group is transferred to H4MPT by a membrane-bound methyl - H4MPT - CoM methyltransferase. Since this reaction consumes energy ( endergonic is ), this electrochemical Natriumionengradient is tapped. Methyl- H4MPT then, in the reverse order as described above, is oxidized to formyl H4MPT, wherein at the same time reduces F420. The formyl group is linked to MFR and finally oxidized to carbon dioxide by the formyl dehydrogenase. Formally comply with the reactions of the oxidative branch of the reverse metabolism of carbon dioxide to CH3 - CoM.

For example, four molecules of methylamine are converted into:

General methylated C1 compounds are broken down according to:

( R = -SH,- OH,- NH2, - NHCH3 ,-N (CH3) 2 ,-N (CH3) 3 )

Cleavage of acetate to methane and carbon dioxide

Overview EC numbers

• Acetyl- CoA synthetase EC 6.2.1.1
• Acetate kinase EC 2.7.2.1
• Phosphotransacetylase EC 2.3.1.8

Acetate ( CH3COOH ) is the only C2 connection for Metanogenese who can implement only representatives of the genera Methanosaeta and Methanosarcina. Compared to all other methanogens, however, the greater part mainly of methane comes from the cleavage of acetate worldwide.

To be used as a substrate for methanogenesis, acetate is first "activated". This is done in that it is linked to coenzyme A, so that acetyl-CoA is formed. These two pathways have been identified:

• Done either directly by the activation of an acetyl -CoA synthetase, wherein the process is a molecule of ATP to AMP and pyrophosphate ( PPi) is cleaved. The acetyl- CoA synthetase are found in obligate acetotrophen methanogens of the genus Methanosaeta.
• Alternatively, the process is gradual: acetate is phosphorylated by an acetate kinase by ATP initially and causes acetyl phosphate. This reacts with coenzyme A to acetyl -CoA. A phosphotransacetylase catalyzed the second reaction.

Acetyl- CoA ( CH3 -CO- SCoA ) is for the rest split into three components: coenzyme A ( HS -CoA), the methyl group (- CH3) and the carboxyl group ( - CO). This reaction takes place (short CODH / ACS ) in CO-Dehydrogenase/Acetyl-CoA-Synthase-Komplex. Of the complex transferred to the methyl group H4MPT, which is converted into methane, as described above. CO is enzyme- bound oxidized to CO2, while the released electrons go to ferredoxin, which is required for the regeneration of coenzyme B and M. Cleavage of acetyl-CoA into three components corresponding to the formal inverse of reductive CoA path is formed in the acetyl -CoA. One molecule of carbon dioxide and one molecule of methane is formed from one molecule of acetate thus, according to:

Energy

ATP synthesis

During the methanogenesis both a proton, and a sodium ion concentration difference is generated, which at the same time leads to an energization of the cell membrane ( ΔμH ΔμNa ). Here, methanogens are the only organisms that build these two concentration differences parallel. Like anaerobic or aerobic respiration, the energy of both concentration differences for the synthesis of ATP by ATP synthase is utilized.

Archaea possess ATP synthases of type A1AO, bacteria, mitochondria and chloroplasts, the F1FO -ATP synthase and eukaryotes, the type V1VO. Here methanogens utilize a A1AO -ATP synthase. In the genome of Ms. barkeri and Ms. acetivorans even genes have been discovered for a bacterial F1FO -ATP synthase. However, it is not even sure if these are also read and are fully functional at all. Probably these genes have passed through horizontal gene transfer in the genome that archaea.

Whether the A1AO -ATP synthase in methanogenic archaea accepted both protons and sodium ions is not yet clear. By the presence of Na / H antiporter, the electrochemical sodium ion concentration difference can at any time be transformed into a proton motive force. So you have identified three of these transporters in the genome of Ms. mazei.

The precise structure of the ATP synthase is still the subject of research. Although A1AO -ATP synthases resemble eukaryotic type V1VO, but functionally different - they generate ATP, while the latter hydrolyze ATP to build a ion gradients and consume it. Most archaea have a rotor of 12 groups. The catalytic domain, is generated at the ATP has three binding sites. To meet four protons for the synthesis of a molecule ATP. An exception is the ATP synthase in Mc. janaschii and Mc. maripaludis, wherein said rotor element has only 8 groups. Thus satisfy an average of 2.6 protons for the synthesis of a molecule of ATP.

Energy yield

The reduction of carbon dioxide to methane by hydrogen is exergonic ( energy-free putting ). Under standard conditions at pH 7 is the change in Gibbs free energy ΔG0 ' depending on the citation -130, -131 or -135 kJ / mol CH4. Under such conditions, three molecules of ATP from ADP and Pi would in methanogenesis per molecule formed methane can be formed. The ΔG0' values ​​for the other methanogenic reactions are listed in the table above.

For the calculation of ΔG0 ' - provided concentrations of dissolved gases in equilibrium with the gas pressure of 105 Pa - in addition to a temperature of 25 ° C and a pH value of 7. This does not reflect the natural conditions, because such high gas concentrations occur in the habitats neither before, nor could they be maintained in the cell. Thus, the energy yield falls under natural conditions lower.

In most habitats prevails a H2 gas pressure of about 1-10 Pa. Under these conditions and pH = 7, the free energy change (? G ) is about -17 to -40 kJ / mol methane, which is less than an average of one molecule of ATP produced per molecule can be formed methane. In addition, the pH, the pressure and the temperature prevailing play a role in the calculation of? G. Thus, the change in free energy is obtained in the reduction of carbon dioxide to methane with hydrogen under standard conditions ( 25 ° C) of -131 kJ / mole to -100 kJ / mol, and when a temperature of 100 ° C is present.

Even with the use of other C1 compounds is? G ' low, so many methanogens just outside the " thermodynamic limit " grow.

Evolution

Genomic analyzes show that methanogenesis was established early in Euryarchaeota and only after elimination of Thermococcales. This is supported by the fact that all methanogens share the same homologous enzymes and cofactors for the central methanogenic pathway. In addition, the methanogenesis in evolution is likely to have occurred only once, since horizontal gene transfer between methanogens can not be established. So lie between the orders Methanopyrales, Methanobacteriales, Methanococcales ( Class I methanogens ) and Methanomicrobiales (Class II methanogens ) and Methanosarcinales ( class III methanogens ) orders that can not perform methanogenesis, such as the thermal plasma Tales, Archaeoglobales and Halobacteriales. While still enzymes for the first steps of methanogenesis example, can be detected in A. fulgidus. The archaeon missing enzymes for the last two steps, as well as for the coenzyme M reductase. Probably the archaea have lost independently in these three systems, the ability to methanogenesis in the course of evolution.

Why "suddenly" methanogenesis occurred in Euryarchaeota quite early and is still subject of research. On the origin of methanogenesis, there are various theories. One of them says that the last common ancestor of all archaea itself was a methanogenic organism. Some archaea operate methanogenesis in environments extreme salt and acidity as well as high temperatures. Since it is precisely these environmental conditions have probably prevailed even after the formation of the Earth, methanogenic archaea could have counted among the first life forms. Accordingly, but would have the ability to both methanogenesis in all Crenarchaeota as well as in all other non- methanogenic lines independently have been lost, which is considered unlikely.

Another theory suggests that the origin of methanogenesis possibly lies in the oxidation of methane, that is in the opposite pathway. This also methanotroph above organisms oxidize methane to carbon dioxide, and this aerobic and anaerobic archaea occurs in bacteria. However, contradicted by a contrary assumption: This states that such methanotrophic archaea have emerged rather from methanogens. Because it has been postulated that the methanogenesis, anaerobic methanotrophy the archaea and aerobic methanotrophy the bacteria have emerged from a common pathway that was originally used in the last common ancestor ( MCRA, engl. For most recent common ancestor ) for the detoxification of formaldehyde.

A new theory considers the role of pyrrolysine ( Pyl ) in the methyl - corrinoid pathway of Methanosarcinales, can enter into the methanogenesis by methyl amines. The methyl group of the methyl amines is transmitted by a specific methyltransferase to a corrinoid protein contained (see section above). Methyltransferases contain the 22 amino acid pyrrolysine in the catalytically active site. Pyrrolysine was so well established as in no other enzyme. Since the entire Pyl machinery phylogenetically considered to be very old, it is believed that this comes through horizontal gene transfer from probably several Donorlinien, which now all are either extinct or have not yet been discovered. However, this also implies that the donor line, from which comes the Pyl machinery, had already reached a certain degree of diversity at the time as yet exist a common ancestor of the three domains.

Only in Methanosarcinales cytochromes were found. They have a broader substrate spectrum than methanogens without cytochromes, use them as acetate. It is assumed that the methanogenesis is evolves late on acetate. For growth on carbon dioxide / hydrogen need Methanosarcinales high H2 concentrations, so that they are always cheated at lower gas pressures of methanogens without cytochromes. This led, in the course of evolution means that some Methanosarcinales such as Ms. acetivorans, Methanolobus tindarius or Methanothrix soehngenii, have completely lost the ability to use carbon dioxide as a substrate with the concomitant use of hydrogen.

Aerobic methane release

Even under aerobic conditions, a biogenic methane release was observed. 2006 has been postulated that living plants and dead plant material beitragten up to 40 % to the global biologically produced methane amount. However, this was revised by subsequent measurements, in the emerged that plants produce only a relatively small fraction of methane. Moreover, it appears this is not to be a specific pathway. Instead, for example, results in high UV stress to spontaneous destruction of biomass, which methane is formed. Also dissolved methane may be released to the plant and discharged to the atmosphere in water.

Also for marine microorganisms such as bacteria, aerobic methanogenesis was postulated. This can methylphosphonic acid (MPS ) split by a specific lyase phosphonate and methane. However, MPS was not demonstrated either free in marine ecosystems, yet it is a naturally occurring compound. A possible source of methylphosphonic could archaeon Nitrosopumilus maritimus be generated the polysaccharides which are associated with the MPS and have a metabolic pathway which can convert phosphoenolpyruvate to MPS.

A new in vitro enzymatic mechanism of bacterial SAM-dependent lyase has been shown of the ribose -1 -phosphonate -5-phosphate and ribose methane -1 ,2- cyclic phosphate -5-phosphate cleaved. When the concentration of phosphonates is low, can be cleaved under aerobic conditions with the lyase unreactive carbon-phosphorus compounds; this methane is released.

You may also saprotroph mushrooms put metabolically specific free methane from methionine.

Application

Methane still contains a large part of the energy that was stored in the starting product biomass. This is done in various technical applications too. Thus, the formation of methane for the production of fermentation gases (biogas, sewage gas, landfill gas) is in fermenters of biogas plants, anaerobic digesters of sewage treatment plants and landfill bodies used. The biomass that is used would be otherwise difficult or impossible with other methods used for energy.

The use of methane in technical applications, such as a device connected to a biogas plant cogeneration (CHP) is effected by oxidation with oxygen: