Methanogenesis

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Parent
Anaerobic respiration.
Biosynthesis of alkanes.
Metabolism of methane
Subordinate
from acetate
from CO 2
from methanol
from methylamines
Gene Ontology
QuickGO

The methanogenesis (also methane formation ) is the formation of methane by the metabolism of living organisms , as methanogens or methane producers are designated. With a few exceptions, it largely takes place in the last stage of the anaerobic , microbial degradation of biomass . Most methane generators convert carbon dioxide and hydrogen into methane. Methane is also formed from simple organic C 1 compounds such as formic acid , methanol and methylamines . Acetic acid is converted into methane and carbon dioxide by acetic acid-splitting (acetoclastic) methane-forming agents. Other bacterial fermentation products such as lactic acid , propionic acid and butyric acid , on the other hand, cannot be used as starting materials for methane formation.

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

meaning

Methanogenesis is a central component of the earth's carbon cycle , as the breakdown products that arise, methane and possibly carbon dioxide, are returned to this cycle. Since these gases, especially methane, are effective greenhouse gases, methanogenesis has also gained in importance in preventing global warming . The biotic methane production presumably also plays a role in the formation of methane hydrate , the economic use of which is of interest.

Methanogenesis plays an important role in the process and at the end of the anaerobic food chain, as it is what enables many syntrophic bacteria to grow in the first place. These secondary fermenters gain their energy from the conversion of lactate, propionate, butyrate and simple organic compounds, whereby hydrogen is produced in addition to carbon dioxide and acetate. For thermodynamic reasons, however, these fermentation reactions are possible only if the resulting hydrogen is consumed rapidly again and the H 2 - partial pressure of not more than 100 Pa increases. This is ensured by methanogens living in close proximity, which in turn need this hydrogen for methanogenesis. The transfer of hydrogen between syntrophic bacteria and the archaea, i.e. between different species , is also referred to as interspecies hydrogen transfer.

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

Occurrence

Methanogens occur in the rumen of cattle.

Methane formation occurs naturally in predominantly anaerobic environments in which biomass is broken down. These can be, for example, sediments from lakes and the sea, the rumen of cattle , the intestines of termites and humans, rice fields or swamps . They are also associated with bacteria in order to use their metabolic products, as in the breakdown of wet wood in Clostridium butyricum . Sewage treatment sludge basins as artificial systems for biological degradation are also possible places for methanogenesis. In these habitats there are moderate temperatures suitable for mesophilic organisms. Methanogens have also been discovered in soils that contain oxygen; they can deal with the associated oxidative stress. Methanogenesis also occurs in environments with extremely high and low temperatures (e.g. Methanogenium frigidum ) as well as with high salt or acid contents, e.g. in geothermal systems. In all cases, the concentrations of sulfate , nitrate , manganese (IV) and iron (III) ions must be low in these habitats , as otherwise bacteria use these substances as external electron acceptors in anaerobic respiration and in this respiration those that can be used for methanogens Consume electron donors . The redox processes with these electron acceptors take place preferentially before methanogenesis and the methanogens are thereby deprived of their energy source and thus their livelihood. Methane production in the oceans is therefore comparatively low, since the sulfates dissolved in the oceans are converted into hydrogen sulfide in the sediments by sulfate-reducing bacteria with the consumption of hydrogen ( desulfurication ).

Under anaerobic conditions, carbon dioxide, the substrate of most methanogens, is rarely a limiting factor, as it is continuously released through fermentation reactions by associated bacteria. Most methanogens prefer a neutral pH value, exceptions are, for example, Methanocalculus alkaliphilus and Methanosalsum natronophilum , which grow optimally in the alkaline environment at a pH value of 9.5, and Methanoregula booneii , whose pH optimum in acidic conditions is 5.1. Methanosalsum natronophilum tolerates a higher salt content than Methanocalculus alkaliphilus at the same pH value .

Classification

Methanogenesis is carried out by archaea, all of which belong to the department of the Euryarchaeota . There they are listed in the classes Methanobacteria , Methanococci , “ Methanomicrobia ” and Methanopyri . Methanogenic archaea can be found in the following six orders : Methanopyrales , Methanobacteriales , Methanococcales , Methanomicrobiales , Methanosarcinales and Methanocellales . Here Methanopyrales the phylogenetically oldest, Methanosarcinales however, the phylogenetically youngest branch. The sixth order ( Methanocellales ) discovered in 2008 can be traced back to the archaea Methanocella paludicola and Methanocella arvoryzae that occur in the soil of rice fields . These operate methanogenesis from carbon dioxide and hydrogen. Methanoplasmatales , which are related to the Thermoplasmatales , were proposed in the literature as the seventh order, but then renamed Methanomassiliicoccales .

Methanopyrales , Methanobacteriales and Methanococcales are class I methanogens , Methanomicrobiales class II methanogens. Methanosarcinales are class III methanogens.

Substrate diversity

Overview diagram of the anaerobic food chain of microorganisms, in which methane and carbon dioxide are produced in the last step by archeal methanogens.

In many habitats, methanogens are end users in the so-called “anaerobic food chain”. In this chain, biopolymers such as proteins and in particular polysaccharides such as cellulose are split into monomers (for example amino acids and carbohydrates ) via oligomers . Lipids are broken down into their components (e.g. fatty acids ). Bacteria then ferment these cleavage products to form simple carboxylic acids (such as formate , acetate , propionate , lactate and succinate ), alcohols (such as ethanol , 2-propanol and butanol ) and other low-molecular compounds (H 2 , CO 2 and short-chain ketones ). Syntrophic, acetogenic bacteria use some of these compounds and convert them to acetate and C 1 compounds. In the last part of the anaerobic food chain, methanogenesis, these compounds are used as a carbon, reductant and energy source, with CH 4 and mostly CO 2 being released.

  • Most methanogens carry out methanogenesis with carbon dioxide (CO 2 ) as a substrate, in which hydrogen (H 2 ) is used as the primary reducing agent. Such methanogens are called hydrogen-oxidizing or hydrogenotropic . The obligatory (exclusive) hydrogenotrophs include the Methanopyrales , Methanobacteriales , Methanococcales and Methanomicrobiales , which only use H 2 and CO 2 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 because it cannot use CO 2 . Model organisms among the hydrogenotrophs are Methanothermobacter thermautotrophicus and Methanocaldococcus jannaschii (formerly Methanococcus jannaschii ). In methanomassiliicoccales , no methanogenesis in this regard has been detected so far. Hydrogenotrophic methanogenesis occurs particularly in sediments of the deep sea and in the intestines of termites, humans and animals. The resulting methane contributes around 33% to the annual methane production on earth.
  • Only a few species can use carbon monoxide (CO) for methanogenesis. M. thermoautotrophicus and Methanosarcina barkeri form three molecules of CO 2 and one molecule of methane from four molecules of CO . Also Methanosarcina acetivorans may use CO as a substrate, wherein parallel acetate and formate are formed. This type of acetogenesis in methanogens is called carboxidotrophic acetogenesis .
  • The Methanosarcinales are the most versatile methanogens, they can use very different C 1 compounds for methanogenesis. In addition to CO 2 + H 2 , many types of C 1 compounds use carbon as a methyl group, such as methanol, methylamines ( mono- , di- , trimethylamine ) and methylthiols ( dimethyl sulfide , methanethiol ). Methanosarcinales cannot convert formic acid.
  • Acetate (CH 3 COOH) is the only C 2 compound that can be used for methanogenesis. As far as known so far, only the genera Methanosaeta and Methanosarcina ( Methanosarcinales ) are capable of doing this. They are called acetoclastic methanogens or acetoclasters . In this type of methanogenesis, acetate is split into CO 2 and CH 4 . Although acetate is only used by a few archaea for methanogenesis, the resulting methane contributes 66% to the annual methane production on earth. The methane formation of the acetoklasters is therefore the largest biogenic source. Acetoclasts occur mainly in digestion towers / biogas plants, rice fields or swamps.
  • N- methylated amines with a C 2 carbon chain are also used for methanogenesis by some methanogens of the genus Methanococcoides (belongs to the Methanosarcinales ). However, only the methyl groups are used in these compounds. For example, choline or dimethylaminoethanol (DMAE) is broken down into ethanolamine and the released methyl group is used in methanogenesis. DMAE is broken down by Methanococcoides methylutens and Methanococcoides burtonii , among others . Also, betaine serves some Methanococcoides species as the substrate analogous to the choline a methyl group is liberated and dimethylglycine degraded. Whether methanogens can also use methylated amines with longer side chains is still being investigated. Methylotrophic methanogenesis occurs particularly in the sea or in hypersaline, sulphate-rich sediments.
Reaction in methanogenesis ΔG 0 ' [kJ / mol CH 4 ] organism
Carbon dioxide type
CO 2 + 4 H 2 → CH 4 + 2 H 2 O −135 most of the methanogens
4 HCOOH → CH 4 + 3 CO 2 + 2 H 2 O −130 many hydrogenotropic methanogens
CO 2 + 4 C 3 H 8 O → CH 4 + 4 C 3 H 6 O + 2 H 2 O −37 some hydrogenotropic methanogens
4 CO + 2 H 2 O → CH 4 + 3 CO 2 −196 Methanothermobacter and Methanosarcina
with methyl compounds
4 CH 3 OH → 3 CH 4 + CO 2 + 2 H 2 O −105 Methanosarcina and other methylothrophic methanogens
CH 3 OH + H 2 → CH 4 + H 2 O −113 Methanomicrococcus Blatticola and Methanosphaera
2 (CH 3 ) 2 S + 2 H 2 O → 3 CH 4 + CO 2 + 2 H 2 S −49 some methylothrophic methanogens
4 CH 3 NH 2 + 2 H 2 O → 3 CH 4 + CO 2 + 4 NH 3 −75 some methylothrophic methanogens
2 (CH 3 ) 2 NH + 2 H 2 O → 3 CH 4 + CO 2 + 2 NH 3 −73 some methylothrophic methanogens
4 (CH 3 ) 3 N + 6 H 2 O → 9 CH 4 + 3 CO 2 + 4 NH 3 −74 some methylothrophic methanogens
4 CH 3 NH 3 Cl + 2 H 2 O → 3 CH 4 + CO 2 + 4 NH 4 Cl −74 some methylothrophic methanogens
with acetate (acetic acid)
CH 3 COOH → CH 4 + CO 2 −33 Methanosarcina and Methanosaeta
with N-methylated amines with a C 2 side chain
4 (CH 3 ) 3 N + CH 2 CH 2 OH + 6 H 2 O → 4 H 2 NCH 2 CH 2 OH + 9 CH 4 + 3 CO 2 + 4 H + −63 some methanosarcina
2 (CH 3 ) 2 NCH 2 CH 2 OH + 2 H 2 O → 2 H 2 NCH 2 CH 2 OH + 3 CH 4 + 3 CO 2 −47 some methanosarcina
4 (CH 3 ) 3 N + CH 2 COO - + 2 H 2 O → 4 (CH 3 ) 2 NH + CH 2 COO - + 3 CH 4 + CO 2 −240 some methanosarcina

Differentiation by means of cytochromes

Methanogens of the order Methanosarcinales contain cytochromes , while these were not found in the other orders. In addition to physiological effects, this also has metabolic-specific effects on how methanogenic archaea metabolize carbon dioxide and hydrogen to methane.

Structural formula of methanophenazine; the long side chain serves as a membrane anchor.
  • Methanogens with cytochromes contain methanophenazine . It is the universal electron carrier in the membrane of these methanogens and replaces quinone there , which only occurs in low concentrations and is essential in other organisms for the transport of electrons in the respiratory chain . Many Methanosarcinales grow on acetate and methylated compounds. If you use CO 2 + H 2 , the H 2 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 have been discovered among methanogens with cytochromes that grow under hyperthermophilic conditions.
  • Methanophenazine is missing in methanogens without cytochromes. In contrast to the Methanosarcinales , those methanogens grow with CO 2 + H 2 or formic acid and cannot utilize methylated compounds or acetate. An exception is M. stadtmanae , which decays in humans and requires methanol and hydrogen for growth. For methanogens without cytochromes, an H 2 partial pressure of less than 10 Pa is sufficient to carry out methanogenesis. Your doubling time is less than an hour per doubling. Many hyperthermophilic species are found among methanogens without cytochromes.

Biochemical reactions

In the reduction of carboxy groups (–COOH) and of carbon dioxide to methane, enzymes with characteristic coenzymes play an essential role. In particular, these are the coenzymes tetrahydromethanopterin , coenzyme M , coenzyme F 430 and F 420 , as well as special electron or hydrogen carriers. Some of these only occur with methane generators. The complex biochemical process requires over 200 genes that code for the corresponding enzymes and coenzymes.

Methanogenesis from carbon dioxide and hydrogen

Reduction of carbon dioxide to methane

Biochemical pathway of archaeal methanogenesis from carbon dioxide and hydrogen H 2 , coupled with ADP phosphorylation. Abbreviations: MF: Methanofuran . THM: tetrahydromethanopterin. CoM: Coenzyme M. CoB: Coenzyme B. B12: B12 cofactor. Ni: hydrogenase with nickel cofactor. Mo: dehydrogenase with molybdenum cofactor. F 420 and F 430 : electron carrier factor 420 and 430, respectively.

Overview of EC numbers

Coenzyme M, the simplest coenzyme in methanogenesis.
Coenzyme B forms the mixed disulfide with coenzyme M, thereby releasing methane.

So that carbon dioxide can be used as a substrate, it is first linked to one reactive amino group of the coenzyme methanfuran (MFR). This creates N -carboxymethanofuran , an unstable intermediate product, which is reduced to the first stable intermediate, N -formylmethanofuran (CHO-MFR). A formylmethanofuran dehydrogenase (MFR dehydrogenase) catalyzes these two reactions and requires a reducing agent in the form of reduced ferredoxin . The electrons required for this reduction come either from hydrogen, which a hydrogenase transfers to oxidized ferredoxin. Alternatively, they get from the oxidation of formate, the anion of formic acid, to carbon dioxide, which catalyzes a formate dehydrogenase. Since the formation of CHO-MFR is endergonic , the necessary energy is tapped from the membrane's electrochemical ion gradient.

The formyl group (-CHO) bound to MFR is transferred to tetrahydromethanopterin (H 4 MPT), which is structurally similar to tetrahydrofolate (THF) of other organisms. The formyl group bound to H 4 MPT is then gradually reduced via N 5 , N 10 -methenyl-H 4 MPT and N 5 , N 10 -methylene-H 4 MPT to methyl-H 4 MPT (-CH 3 ). This process is completely reversible and can also take place in the opposite direction. The reducing agent here is F 420 H 2 . A cytosolic methenyl-H 4 MPT cyclohydrolase, methylene H 4 MPT dehydrogenase or an (F 420 -dependent) methylene reductase catalyze these reactions. In Methanosarcinaarten there is tetrahydosarcinapterin (H 4 SPT), which is very similar to H 4 MPT.

In addition to the F 420 -dependent methylene reductase, some obligatory hydrogenotrophs use hydrogen directly. In contrast to other hydrogenases, this methylene reductase contains neither iron-sulfur nor nickel- iron clusters, it is "metal-free".

The universal reducing agent F 420 H 2 is regenerated after oxidation by an iron-nickel containing F 420 -reducing hydrogenase, which requires hydrogen.

The methyl group from methyl-H 4 MPT is then transferred to the simplest coenzyme, coenzyme M (CoM). The result is methyl-CoM, in which the methyl group is linked to the sulfide residue of the coenzyme (H 3 C – S-CoM). The transfer takes place via a membrane-bound methyl transferase. This reaction is exergonic (ΔG 0 ' = −29 kJ / mol.) Methanogens use the energy released in this process to export around two sodium ions per conversion from the cell. This creates an electrochemically active sodium ion concentration difference.

Methyl-CoM eventually reacts with Coenzyme B (CoB) to form a mixed disulfide , CoM-S – S-CoB, and methane. This is the key reaction in methanogenesis. The mixed disulfide is also known as heterodisulfide. This reaction is catalyzed by a methyl coenzyme M reductase , which contains the cofactor F 430 .

In the balance, a molecule of carbon dioxide is converted according to:

Regeneration of coenzymes M and B

The coenzymes M and B have to be regenerated for a new run. This takes place through a reduction of CoM-S-S-CoB to CoM and CoB and is catalyzed by a heterodisulfide reductase. The electrons required for this reaction come from either hydrogen, reduced ferredoxin or F 420 H 2 . Energy is released during the reaction (ΔG 0 ' = −39 kJ · mol −1 ).

In methanogens with cytochromes, CoM-S – S-CoB is reduced on a membrane-bound heterodisulfide reductase. In obligatory carbon-reducing methanogens this is a complex with three subunits (HdrABC), in Methanosarcina species it is made up of two subunits (HdrDE). Electrons are required for the reduction. Either hydrogen is oxidized on a membrane-bound hydrogenase that contains, among other things, heme b as a prosthetic group (Vho). At the same time, protons are transported outwards. For example, the hydrogenase complex has been identified in freshwater Ms. barkeri . Ms. acetivorans , an archaeon found in salt water, oxidizes ferredoxin instead of hydrogen to a membrane-based complex (Ma-Rnf), which u. a. Has cytochrome c as a prosthetic group. Sodium ions are transported to the outside. If Ms. acetivorans grows exclusively on carbon monoxide, a membrane-bound F 420 dehydrogenase complex (Fpo) oxidizes reduced F 420 , during which protons are exported. The transfer of electrons from the hydrogenase or dehydrogenase complex to the heterodisulfide reductase is mediated by methanophenazine. The reduction of CoM-S – S-CoB is exergonic, so this process also simultaneously transports protons to the outside, so that overall a proton motor force is built up. These use the methanogens to build up ATP (see section below ).

Cytosolic hydrogenase / reductase model. Here, hydrogen is oxidized on an iron-nickel-containing subunit (MvhA), the released electrons reach ferredoxin and coenzyme B and M.

In contrast, methanogens without cytochromes have neither methanophenazine nor a membrane-bound heterodisulfide reductase. For the oxidation of the heterodisulfide CoM-S – S-CoB, they use a cytosolic hydrogenase / reductase, which requires hydrogen and couples the energy released to reduce ferredoxin. However, there is no underlying mechanism in which the released energy could be coupled to build up a proton motor force - the heterosulfide reductase is not bound to the membrane. Therefore, methanogens without cytochromes can only use the sodium ion concentration difference that is built up in the methyltransferase reaction.

Conversion of formate to methane

Structural formula of formate

Formic acid or its anion, formate (HCOO - ), can be used as a substrate by around half of all methanogens. In contrast to carbon dioxide, it is not transferred directly to the MFR, but is first oxidized to carbon dioxide by a formate dehydrogenase. The enzyme contains molybdenum and iron-sulfur clusters; it has already been isolated from methanogenic archaea (for example from Methanobacterium formicicium and Mc. Vannielii ). In the catalyzed reaction, F 420 is reduced to F 420 H 2 at the same time . Carbon dioxide is then reduced to methane, as described above.

As for the step-by-step conversion of carbon dioxide to methane, reducing agents are also required in four places for the conversion of formate to methane. In two places they are consumed directly in the form of F 420 H 2 in the gradual reduction of methenyl-H 4 MPT to methyl-H 4 MPT. The other two require hydrogen for the cytosolic heterodisulfide reductase, which couples the oxidation of CoM-S – S-CoB to CoM and CoB and the formation of reduced ferredoxin. Hydrogen can either be generated by the F 420 -reducing hydrogenase or alternatively by a nickel-free hydrogenase. The reduced ferredoxin formed in the heterodisulfide reductase reaction is required for the MFR dehydrogenase in the initial reaction.

Therefore a total of four molecules of F 420 H 2 are required to utilize formate in methanogenesis. These are provided by oxidizing four molecules of formic acid to carbon dioxide. Three molecules of carbon dioxide are released, and the fourth is finally converted into methane. The balance thus results:

Methanogenesis with methylated C 1 compounds

C 1 compounds with a methyl group such as methylamine (CH 3 NH 2 ) or methanol (CH 3 OH) occur in particular in seawater or brackish water and are anaerobic breakdown products of cellular components of certain plants and phytoplankton .

Since the carbon in the methyl group is already more reduced than in CO 2 , these compounds do not have to go all the way through as with carbon dioxide. They are therefore fed into the methanogenesis in the lower third of the way in the form of CH 3 -CoM. In addition to the direct route to methane, methylated compounds are also oxidized to carbon dioxide. So there is an oxidative and a reductive branch. This is because the electrons for the reductive branch have to be taken from the oxidation of the methyl group to carbon dioxide, since the use of hydrogen from the environment (as an electron source) is often not possible.

For example, one molecule of methanol is oxidized to carbon dioxide, so that three molecules are reduced to methane with the help of the released reduction equivalents. This disproportionation takes place z. B. according to:

These two branches also occur in the conversion of methylamines by Methanosarcina . Methylamines are metabolized to methane, CO 2 and ammonia (NH 3 ), with three of the methyl groups being reduced to methane and one being oxidized to 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 is catalyzed by cytosolic methyltransferases, which require pyrrolysine as the 22nd amino acid for the reaction and contain a corrinoid as a prosthetic group.

In the oxidative branch, the methyl group is transferred to H 4 MPT by a membrane-bound methyl-H 4 MPT-CoM methyl transferase. Since this reaction consumes energy (it is endergonic), the electrochemical sodium ion gradient is tapped for this. Methyl-H 4 MPT is then, in reverse order to that described above, oxidized to formyl-H 4 MPT, with F 420 being reduced at the same time . The formyl group is coupled to MFR and finally oxidized to carbon dioxide by formyl dehydrogenase. Formally, the reactions of the oxidative branch correspond to the reverse metabolism of carbon dioxide to CH 3 -CoM.

For example, four molecules of methylamine are converted into:

In general, methylated C 1 compounds are broken down according to:

(with R = –SH, –OH, –NH 2 , –NHCH 3 , –N (CH 3 ) 2 , –N (CH 3 ) 3 + )

Cleavage of acetate to methane and carbon dioxide

Overview of EC numbers

Overview of acetoclastic methanogenesis using the example of Methanosarcina. During the process, a difference in the concentration of protons and sodium ions is built up, which is tapped for the synthesis of ATP. CODH / ACS = carbon monoxide dehydrogenase / acetyl-CoA synthase complex; Hdr = membrane-bound heterodisulfide reductase complex.

Acetate (CH 3 COOH) is the only C 2 compound for methanogenesis that only representatives of the genera Methanosaeta and Methanosarcina can implement. Compared to all other methane generators, however, the overwhelming majority of methane worldwide comes from the breakdown of acetate.

In order to be used as a substrate for methanogenesis, acetate is first “activated”. This is done by attaching it to coenzyme A so that acetyl-CoA is formed. Two metabolic pathways were identified:

  • Either the activation takes place directly through an acetyl-CoA synthetase , during which a molecule of ATP is split into AMP and pyrophosphate (PP i ). Acetyl-CoA synthetase is found in obligate acetotrophic methanogens of the genus Methanosaeta .
  • Alternatively, the process takes place step by step: Acetate is first phosphorylated by an acetate kinase using ATP , which creates acetyl phosphate. This reacts with coenzyme A to form acetyl-CoA. A phosphotransacetylase catalyzes the second reaction.

Acetyl-CoA (CH 3 -CO-SCoA) is split into three components for the further course: Coenzyme A (HS-CoA), the methyl group (-CH 3 ) and the carboxy group (-CO). This reaction takes place in the CO dehydrogenase / acetyl CoA synthase complex (CODH / ACS for short). The complex transfers the methyl group to H 4 MPT, which is converted to methane as described above. CO is oxidized to CO 2 bound by enzymes , the electrons released in the process reach ferredoxin, which is required for the regeneration of coenzyme B and M. The splitting of acetyl-CoA into three components corresponds formally to the reversal of the reductive CoA path , in which acetyl-CoA is formed. One molecule of acetate is thus formed into one molecule of carbon dioxide and one molecule of methane, according to:

Energy generation

ATP synthesis

In the course of methanogenesis, both a proton and a sodium ion concentration difference are generated, which at the same time leads to an energization of the cell membrane (Δµ H + , Δµ Na + ). Methanogens are the only organisms that build up these two concentration differences in parallel. As with anaerobic or aerobic respiration, the energy of both concentration differences is used to build up ATP by an ATP synthase .

Archaea have ATP synthases of the A 1 A O type , bacteria, mitochondria and chloroplasts the F 1 F O -ATP synthase and eukaryotes the V 1 V O type . Methanogens use an A 1 A O -ATP synthase. In the genome of Ms. barkeri and Ms. acetivorans , genes for a bacterial F 1 F O -ATP synthase were discovered. However, it is not even certain whether these can also be read and are functionally complete at all. These genes probably got into the genome of those archaea through horizontal gene transfer .

It is not yet clear whether the A 1 A O -ATP synthase in methanogenic archaea accepts both protons and sodium ions. Due to the presence of an Na + / H + antiporter, the electrochemical sodium ion concentration difference can be converted into a proton motor force at any time. Three of these transporters have been identified in the genome of Ms. mazei .

The exact structure of the ATP synthase is still the subject of research. A 1 A O -ATP synthases are similar to eukaryotic type V 1 V O , but are functionally different - they generate ATP, while the latter hydrolyze ATP to build up an ion gradient and thus use it. Most archaea have a rotor of 12 groups . The catalytic domain at which ATP is produced has three binding sites. Four protons are thus sufficient to synthesize one molecule of ATP. The exception is the ATP synthase in Mc. janaschii and Mc. maripaludis , where the rotor element only has 8 groups. This means that an average of 2.6 protons are sufficient for the synthesis of one molecule of ATP.

Energy yield

The reduction of carbon dioxide to methane by hydrogen is exergonic (energy releasing). Under standard conditions at pH 7, the change in Gibbs energy ΔG 0 'is −130, −131 or −135 kJ / mol CH 4, depending on the literature . Under such conditions, three molecules of ATP could be formed from ADP and P i for each molecule of methane formed in methanogenesis . The ΔG 0 ' values ​​for the other methanogenic reactions are listed in the table above.

For the calculation of ΔG 0 ' , in addition to a temperature of 25 ° C and a pH value of 7, concentrations of the dissolved gases in equilibrium with gas pressures of 10 5 Pa are assumed. However, this does not correspond to the natural conditions, because such high gas concentrations do not occur in the habitats, nor can they be maintained in the cell. This means that the energy yield is lower under natural conditions.

In most habitats there is an H 2 gas pressure of around 1–10 Pa. Under these conditions and pH = 7, the change in free energy (ΔG) is around −17 to −40 kJ / mol methane, which means that less than an average of one molecule of ATP can be formed per molecule of methane generated. In addition, the pH value , the prevailing pressure and also the temperature play a role in calculating ΔG . For example, the change in free energy in the reduction of carbon dioxide to methane by hydrogen under standard conditions (25 ° C) falls from −131 kJ / mol to −100 kJ / mol when the temperature is 100 ° C.

Even when other C 1 compounds are used, ΔG 'is low, so that many methanogens grow close to the “thermodynamic limit”.

evolution

Genomic analyzes show that methanogenesis was established early in Euryarchaeota and only after the Thermococcales had split off . This is substantiated by the fact that all methanogens share the same homologous enzymes and cofactors for the central methanogenic metabolic pathway. In addition, methanogenesis probably only occurred once in evolution , since horizontal gene transfer between the methanogens cannot be demonstrated. Thus, between the orders Methanopyrales , Methanobacteriales , Methanococcales (class I methanogens) as well as Methanomicrobiales (class II methanogens) and Methanosarcinales (class III methanogens) are orders that cannot perform methanogenesis, e.g. B. the Thermoplasmatales , Archaeoglobales and Halobacteriales . It is true that enzymes for the first steps of methanogenesis can still be detected in A. fulgidus , for example . The archaeon lacks enzymes for the last two steps, including coenzyme M reductase. Probably the archaea in these three orders lost the ability to methanogenesis independently of one another in the course of evolution.

Why methanogenesis occurred quite early and “suddenly” in Euryarchaeota remains the subject of research. There are various theories about the origin of methanogenesis. One of them says that the last common ancestor of all archaea was itself a methanogenic organism. Some archaea carry out methanogenesis in environments with extreme salt and acid content and high temperatures. Since it is precisely these environmental conditions that presumably prevailed even after the earth was formed, methanogenic archaea could have been among the first forms of life. As a result, however, the ability to methanogenesis must have been lost in all Crenarchaeota as well as in all other non-methanogenic lines independently of each other, which is considered to be quite improbable.

According to another theory, methanogenesis may originate in the oxidation of methane, i.e. in the reverse metabolic pathway. These organisms, also known as methanotrophs , oxidize methane to carbon dioxide and water, which happens aerobically in bacteria and anaerobically in archaea. However, an opposite assumption speaks against this: This says that such methanotrophic archaea have more likely emerged from methanogenic archaea. It was postulated that the methanogenesis, the anaerobic methanotrophy of the archaea and the aerobic methanotrophy of the bacteria arose from a common metabolic pathway, which in the last common ancestor (MCRA, English for most recent common ancestor ) originally served to detoxify formaldehyde .

A new theory considers the role of pyrrolysine (Pyl) in the methyl corrinoid pathway of the Methanosarcinales , through which methylamines can enter into methanogenesis. The methyl group of these methylamines is transferred to a corrinoid-containing protein by a specific methyltransferase (see section above). Methyltransferases contain the 22nd amino acid pyrrolysine in the catalytically active center. Pyrrolysine was detected as well as in no other enzyme. Since the entire Pyl machinery is considered to be very old in terms of phylogenetic history, it is assumed that it comes from probably several donor lines through horizontal gene transfer, all of which have either become extinct or have not yet been discovered. However, this also assumes that the donor line from which the Pyl machinery originates had already achieved a certain degree of diversity at the time when a common ancestor of our three domains still existed.

Cytochromes were only found in Methanosarcinales . They have a broader substrate spectrum than methanogens without cytochromes; they also use acetate, for example. It is believed that methanogenesis from acetate developed late. Presumably, the acetate kinase genes required for the use of acetate only entered the methanogenic archaea through horizontal gene transfer from a cellulose- degrading acetogenic bacterium belonging to the clostridia .

When growing on carbon dioxide + hydrogen, methanosarcinales require high H 2 concentrations so that methanogens without cytochromes always take advantage of them at lower gas pressures. In the course of evolution, this led to the fact that some Methanosarcinales , such as Ms. acetivorans , Methanolobus tindarius and Methanothrix soehngenii , have completely lost the ability to use carbon dioxide as a substrate while using hydrogen. Since methanogenesis with carbon dioxide and hydrogen is very widespread, it is assumed that this form is the most original.

Other types of biological methane release

A biogenic methane release was also observed under aerobic conditions. In 2006 it was postulated that living plants and dead plant material contribute up to 40% to the global amount of biologically produced methane. This was however revised by later measurements, from which it emerged that plants only produce a comparatively very small proportion of methane. In addition, it does not appear to be a specific metabolic pathway. Instead, for example, high UV stress leads to the spontaneous destruction of biomass, as a result of which methane is formed. In addition, methane dissolved in water could be released in the plant and released into the atmosphere.

Structural formula of methylphosphonic acid

Aerobic methanogenesis has also been postulated for marine microorganisms such as bacteria. These can split methylphosphonic acid (MPS) into phosphonate and methane using a special lyase . However, MPS has not been found free in marine ecosystems, nor is it a naturally occurring compound. A possible source of methylphosphonic acid could be the archaeon Nitrosopumilus maritimus , which produces polysaccharides linked to MPS and has a metabolic pathway that can convert phosphoenolpyruvate to MPS.

In-vitro a new enzymatic mechanism for a bacterial SAM -dependent lyase was shown, which cleaves ribose-1-phosphonate-5-phosphate to methane and ribose-1,2-cyclic phosphate-5-phosphate. If the concentration of phosphonates is low, the lyase can cleave unreactive carbon-phosphorus compounds under aerobic conditions; this releases methane.

It is possible that saprotrophic fungi also release methane from methionine in a metabolically specific manner .

The bacterium Rhodopseudomonas palustris can convert N 2 together with CO 2 and protons to methane, ammonia and H 2 in a single enzymatic reaction using a pure iron-containing nitrogenase .

application

Digestion tower of a sewage treatment plant for methane formation

The methane formed microbially from biomass contains a large part of the energy that was stored in the starting product. This is used in various technical applications. In fermenters in biogas plants, digestion towers in sewage treatment plants and in landfill bodies, methane formation is used to generate fermentation gases ( biogas , sewage gas , landfill gas ). The biomass used would be difficult or impossible to use for energy with other processes.

The use of methane in technical applications such as B. a block-type thermal power station (CHP) connected to a biogas plant , takes place through oxidation with oxygen:

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This version was added to the list of articles worth reading on July 26, 2013 .