Anaerobic methane oxidation

Three different mechanisms of anaerobic methane oxidation are known. In the first variant (above) the methane oxidation of anaerobic methanotrophic (ANME) archaea of ​​strains 1 and 2a, b, c is carried out, in symbiosis with sulfate-reducing bacteria (SRB). In the second variant (middle) methane is oxidized by means of nitrate, in symbiosis with anammox bacteria, which convert the nitrite formed. In the third mechanism (below) methane is also oxidized by means of nitrate, but the nitrite formed is then used by NC10 bacteria to oxidize further methane. In contrast to the first two mechanisms, methane oxidation is carried out by both partners.

The anaerobic methane oxidation is a metabolic process which various microorganisms in symbiosis is performed. This process takes place in oxygen-free sea water - and fresh water - sediments instead. Methane is not using oxygen , but by means of sulphate , metal oxides , nitrate or nitrite oxidized .

Methane oxidation with sulfate

This process represents a symbiosis of methanotrophic archaea with sulfate-reducing bacteria . The two organisms usually combine to form aggregates or they appear as voluminous mats . Co-cultures can be maintained in the laboratory, but the doubling time is several months. The archaea are called ANME (for anaerobic methanotrophs) and are closely related to methanogenic archaea. The metabolism of anaerobic methane oxidation is the opposite of methane formation . Based on genetic and enzymatic studies, it is assumed that very similar enzymes catalyze the respective reverse reaction . The link between methane oxidation and sulphate reduction is made possible , according to the latest hypothesis, through electrical conductivity . Electrons flow via multi-heme c -type cytochromes from the methanotrophic archaea to the sulfate-reducing bacteria.

The reaction equation is:

${\displaystyle \mathrm {CH_{4}+SO_{4}^{2-}\longrightarrow HCO_{3}^{-}+HS^{-}+H_{2}O} }$

Methane oxidation with metal oxides

Anaerobic methane oxidation coupled with the reduction of iron and manganese oxides has been reported, but this process could not be assigned to a specific organism. It could be shown that ANME, which normally coexist with sulfate-reducing bacteria, are able to transfer electrons from methane to various artificial oxidizing agents . Based on this discovery, it can be speculated that methane oxidation with metal oxides must also work and thus represents an alternative metabolism for ANMEs which normally coexist with sulfate-reducing bacteria.

Methane oxidation with nitrate

Methane oxidation with nitrate is carried out by Methanoperedens nitroreducens (ANME-2d). The genome of this organism has been deciphered and shows that all genes for the metabolism of reverse methanogenesis are present. Cultivation in the laboratory has so far only been successful in co-culture, e.g. B. with anammox bacteria, which efficiently remove the metabolite nitrite.

The reaction equation is:

${\displaystyle \mathrm {CH_{4}+4\ NO_{3}^{-}\longrightarrow CO_{2}+4\ NO_{2}^{-}+2\ H_{2}O} }$

Methane oxidation with nitrite

Methane oxidation with nitrite is carried out by the bacterium Candidatus Methylomirabilis oxyfera . It is believed that this bacterium makes oxygen internally. The actual methane activation is therefore not anaerobic. The metabolism takes place in the same way as aerobic methane oxidation.

The reaction equation is:

${\displaystyle \mathrm {3\ CH_{4}+8\ NO_{2}^{-}+8\ H^{+}\longrightarrow 3\ CO_{2}+4\ N_{2}+10\ H_{2}O} }$

Environmental relevance

Anaerobic methane oxidation converts the strong greenhouse gas methane into the less strong greenhouse gas CO ₂ . It is estimated that up to 300 million tons of methane are oxidized with sulfate each year.

See also

• Anammox

Individual evidence

1. ^ ^{A b} Shawn E. McGlynn, Grayson L. Chadwick, Christopher P. Kempes, Victoria J. Orphan: Single cell activity reveals direct electron transfer in methanotrophic consortia. In: Nature . 526, 2015, pp. 531-535, doi: 10.1038 / nature15512 .
2. ↑ ^{a b} Gunter Wegener, Viola Krukenberg, Dietmar Riedel, Halina E. Tegetmeyer, Antje Boetius .: Intercellular wiring enables electron transfer between methanotrophic archaea and bacteria. In: Nature . 526, 2015, pp. 587-590, doi: 10.1038 / nature15733 .
3. ↑ ^{a b c} Mohamed F. Haroon, Shihu Hu, Ying Shi, Michael Imelfort, Jurg Keller, Philip Hugenholtz, Zhiguo Yuan, Gene W. Tyson: Anaerobic oxidation of methane coupled to nitrate reduction in a novel archaeal lineage. In: Nature. 500, No. 7464, 2013, pp. 567-570, doi: 10.1038 / nature12375 .
4. ↑ Ashna A. Raghoebarsing et al: A microbial consortium couples anaerobic methane oxidation to denitrification. In: Nature. 440, No. 7086, 2006, pp. 918-921, doi: 10.1038 / nature04617 .
5. ↑ Katrin Knittel, Antje Boetius: Anaerobic Oxidation of Methane: Progress with an Unknown Process. In: Annual Review of Microbiology . 63, No. 1, 2009, pp. 311-334, doi: 10.1146 / annurev.micro.61.080706.093130 , PMID 19575572 .
6. ↑ Katja Nauhaus, Melanie Albrecht, Marcus Elvert, Antje Boetius, Friedrich Widdel: In vitro cell growth of marine archaeal-bacterial consortia during anaerobic oxidation of methane with sulfate. In: Environmental Microbiology. 9, No. 1, 2007, pp. 187-196, doi: 10.1111 / j.1462-2920.2006.01127.x .
7. ↑ Steven J. Hallam, Nik Putnam, Christina M. Preston, John C. Detter, Daniel Rokhsar, Paul M. Richardson, Edward F. DeLong: Reverse Methanogenesis: Testing the Hypothesis with Environmental Genomics. In: Science . 305, No. 5689, 2004, pp. 1457-1462, doi: 10.1126 / science.1100025 , PMID 15353801 .
8. ↑ ^{a b} Silvan Scheller, Meike Goenrich, Reinhard Boecher, Rudolf K. Thauer, Bernhard Jaun: The key nickel enzyme of methanogenesis catalyses the anaerobic oxidation of methane. In: Nature. 465, No. 7298, 2010, pp. 606-608, doi: 10.1038 / nature09015 .
9. ↑ Sahand Pirbadian, Mohamed Y. El-Naggar: Multistep hopping and extracellular charge transfer in microbial redox chains. In: Physical Chemistry Chemical Physics. 14, 2012, pp. 13802-13808, doi: 10.1039 / c2cp41185g
10. ↑ Emily J. Beal, Christopher H. House, Victoria J. Orphan: Manganese- and Iron-Dependent Marine Methane Oxidation. In: Science . 325, 2009, pp. 184-187, doi: 10.1126 / science.1169984 .
11. ↑ Silvan Scheller, Hang Yu, Grayson L. Chadwick, Shawn E. McGlynn, Victoria J. Orphan .: Artificial electron acceptors decouple archaeal methane oxidation from sulfate reduction. In: Science . 351, 2016, pp. 703–707, doi: 10.1126 / science.aad7154 .
12. ↑ Amelia-Elena Rotaru, Bo Thamdrup .: A new diet for methane oxidizers. In: Science . 351, 2016, pp. 658–659, doi: 10.1126 / science.aaf0741 .
13. ↑ Katharina F. Ettwig et al: Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. In: Nature. 464, No. 7288, 2010, pp. 543-548, doi: 10.1038 / nature08883 .