5.1.6: Methanotrophy

Methanotrophy occurs when microorganisms use methane as their electron donor and sole source of carbon. Methane oxidation can be coupled to oxygen \(\left(\text{O}_{2}\right)\) reduction, as shown in the following example reaction: \[\text{CH}_{4} + 2 \ \text{O}_{2} \longleftrightarrow 2 \ \text{H}_{2} \text{O} + \text{CO}_{2}\]Aerobic methanotrophs belong to the bacterial phyla Proteobacteria and Verrucomicrobia (Guerrero-Cruz et al., 2021).

In the absence of oxygen, anaerobic oxidation of methane (AOM) can be coupled to reduction of nitrite, nitrate, manganese, iron, and sulfate (Guerrero-Cruz et al., 2021) as illustrated in the following example reactions: \[\begin{align} & \text{CH}_{4} + 1.6 \ \text{H}^{+} + 1.6 \ \text{NO}_{3}^{-} \longleftrightarrow 2.8 \ \text{H}_{2} \text{O} + \text{CO}_{2} + 0.8 \ \text{N}_{2} \\ & 3 \ \text{CH}_{4} + 8 \ \text{H}^{+} + 8 \ \text{NO}_{2}^{-} \longleftrightarrow 10 \ \text{H}_{2} \text{O} + 3 \ \text{CO}_{2} + 4 \ \text{N}_{2} \\ & \text{CH}_{4} + 8 \ \text{Fe}^{3+} + 2 \ \text{H}_{2} \text{O} \longleftrightarrow \text{CO}_{2} + 8 \ \text{Fe}^{2+} + 8 \ \text{H}^{+} \\ & \text{CH}_{4} + \text{H}^{+} + \text{SO}_{4}^{2-} \longleftrightarrow 2 \ \text{H}_{2} \text{O} + \text{CO}_{2} + \text{HS}^{-} \end{align}\]

Anaerobic oxidation of methane coupled with sulfate reduction is catalyzed through the combined effort of anaerobic methanotrophic (ANME) archaea and sulfate-reducing bacteria. The anaerobic methanotrophic archaea oxidize the methane and transfer the electrons to the sulfate reducers, who put them on sulfate (Boetius et al., 2000; Martens and Berner, 1974; Timmers et al., 2016). Anaerobic methanotrophic archaea are related to methanogens of orders Methanosarcinales and Methanomicrobiales. The reaction catalyzed by anaerobic methanotrophic archaea uses the same biochemical pathway as methanogenesis, but in reverse (Timmers et al., 2017).

Similarly, bacteria and archaea can also both be involved in AOM coupled to nitrate reduction. Members of the archaeal family Methanoperedenaceae (formerly ANME group 2d) are capable of coupling anaerobic oxidation of methane to the reduction of nitrate to ammonia (Haroon et al., 2013), and bacteria in the candidate genus Methylomirabilis can couple anaerobic oxidation of methane to nitrite reduction (Versantvoort et al., 2018). The nitrite used by Methylomirabilis can be produced as an intermediate in nitrate reduction by Methanoperedenaceae, although the nitrate could come from other sources as well. Thus, the two groups do not necessarily need to work together, as is the case for anaerobic oxidation of methane coupled with sulfate reduction.

Where nitrate is not available, members of archaeal family Methanoperedenaceae are also capable of coupling anaerobic oxidation of methane to reduction of iron and manganese in ferrihydrite and birnessite, respectively (Ettwig et al., 2016). As with anaerobic oxidation of methane coupled to nitrate reduction, their activity is not dependent on a bacterial partner.

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