10.2.5: Interactions among iron reducers, methanogens, and methanotrophs

In addition to sulfate reducers, microorganisms capable of iron reduction can also interact with methanogenic microorganisms. They can compete with one another for electron donors (Fig. \(10.4\)), with their reactions segregated in to distinct zones (Achtnich et al., 1995; Lovley and Goodwin, 1988) or occurring simultaneously within the same zone (Flynn et al., 2013; Herndon et al., 2015; Jakobsen and Postma, 1999; Küsel et al., 2008; Marquart et al., 2019; Metje and Frenzel, 2007; Paul et al., 2006). They can also interact syntrophically through interspecies electron transfer (IET). During this interaction, ironreducing microorganisms can consume electron donors and then transfer electrons to methanogens who use them to reduce carbon dioxide to methane (Rotaru et al., 2014b, 2014a), as described in Chapter 5.

Where iron reduction and methanogenesis are both occurring, methane and ferric iron must be available, which has the potential to support anaerobic oxidation of methane (AOM) by anaerobic methanotrophic (ANME) archaea (Fig. \(10.4\)). If the methanogen benefits from consumption of methane by the ANME archaea, then their interaction would be syntrophic. However, if there is no gain for the methanogen, then the interaction is a form of commensalism.

Similarly to the work of Paper et al. (2021), Marquart et al. (2019) hypothesized that pH may also influence interactions of iron reducers with methanogens based on data from laboratory experiments. At low pH, where energy available for iron reduction is often high, they interpreted that the interactions between each group were primarily defined by competition for electron donors. However, at basic pH, where energy available for iron reduction can be low, Marquart et al. hypothesized that iron reducers increasingly transfer electrons to methanogens rather than ferric minerals and thus interact syntrophically. As with the hypotheses defined by Paper et al. (2021), additional research is needed to fully test this hypothesis.

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