5.1.4: Methanogenesis

Microbial methanogenesis refers to methane production by microbial reactions. For the purposes of this book, we specifically refer to methanogens as microorganisms that form methane for energy production. In addition to these groups, there are also microorganisms that form methane as byproduct of their activities, which are discussed in Section 13.4.

Methanogenesis is traditionally assumed to occur where electron acceptors other than carbon dioxide are not available, although that appears to be an oversimplification. For example, several studies have found methanogenesis active alongside microbial iron reduction (Marquart et al., 2019). As a second example, methanogenesis and sulfate reduction have also been observed to coexist when substrates are available that one group can consume but the other cannot (Oremland and Polcin, 1982). Either way, use of methanogenesis as an energy source is thought to be limited to anoxic environments, including anoxic microenvironments within systems that are otherwise oxic (Angle et al., 2017).

All known microorganisms that use methanogenesis as an energy source are classified in Domain Archaea, Kingdom Euryarchaeota. Six orders within Euryarchaeota are long recognized to contain methanogens (Whitman et al., 2006): Methanococcales, Methanopyrales, Methanobacteriales, Methanosarcinales, Methanomicrobiales, Methanocellales. More recently, Thermoplasmatales has also been shown to contain methanogens (Borrel et al., 2013). Methanogens are thought to use a relatively small number of electron donors (Costa and Leigh, 2014). They can make methane by oxidizing dihydrogen \(\left(\text{H}_{2}\right)\) or formate \(\left(\text{HCOO}^{-}\right)\) and reducing carbon dioxide to methane \(\left(\text{CH}_{4}\right)\):

\[\begin{align} & \text{HCOO}^{-} + \text{H}^{+} \longleftrightarrow 0.25 \ \text{CH}_{4} + 0.75 \ \text{CO}_{2} + 0.5 \ \text{H}_{2} \text{O} \\ & \text{H}_{2} + 0.25 \ \text{CO}_{2} \longleftrightarrow 0.25 \ \text{CH}_{4} + 0.5 \ \text{H}_{2} \text{O} \end{align}\]This pathway is known as carbon dioxide reduction or hydrogenotrophic methanogenesis and most methanogens are thought to be capable of it.

Secondly, methanogens can make methane from acetate: \[\text{CH}_{3} \text{COO}^{-} + \text{H}^{+} \longleftrightarrow \text{CH}_{4} + \text{CO}_{2}\]

The reaction, known as acetoclastic methanogenesis, splits acetate into a carboxyl group, which is oxidized to carbon dioxide, and a methyl group, which is reduced to methane. Only members of the order Methanosarcinales can catalyze the reaction. About 1 gigaton of methane is estimated to be generated each year and roughly two-thirds of that sum is thought to be produced by acetoclastic methanogenesis (Thauer, 1998). Thus, Methanosarcinales species appear to be the biggest producers of microbial methane on Earth.

Lastly, methanogens can also make methane from methyl compounds such as methanol, methylamines, and dimethylsulfide. An example reaction with methanol \(\left(\text{CH}_{3} \text{OH}\right)\) is as follows: \[\text{CH}_{3} \text{OH} \longleftrightarrow 0.75 \ \text{CH}_{4} + 0.25 \ \text{CO}_{2} + 0.5 \ \text{H}_{2} \text{O}\]Use of methyl compounds to make methane is known as methylotrophic methanogenesis.

These three pathways are thought to be the main pathways of methanogenesis as a form of catabolism. However, it can be hard to generalize about the capabilities of microorganisms. They can do many things, and much remains to be learned. Kurth et al. (2021) recently demonstrated that a strain of Methermicoccus shengliensis was able to make methane by consuming methoxylated aromatic compounds, which are widespread in the subsurface. In addition, methanogens can also receive electrons from other microorganisms through direct interspecies electron transfer (DIET). In the latter case, electrons are transferred from one group to the methanogen via conductive pili and direct contact between cells (Rotaru et al., 2014b, 2014a). Both of these pathways have the potential to have broad environmental significance, but whether that is the case has not yet been demonstrated. Either way, these examples are good reminders that the three main pathways are not the only options available for methanogenic catabolism.