Evidence for Methanogenesis in ANME Archaea

Abstract:

Methanogenesis, unlike many higher energy-yielding metabolisms, can be exergonic in the reverse direction at reasonable concentrations of products and reactants. This is because the ΔG_f for hydrogenotrophic methanogenesis, 4H₂ + CO₂ → CH₄ + H₂O, is quite low, and also because hydrogen exerts a fourth-power control on the reaction quotient. This means that common sedimentary metabolisms such as sulfate reduction and metal reduction can draw hydrogen concentrations low enough as a result of their metabolism to make reverse methanogenesis, or anaerobic methane oxidation, exergonic and potentially a useful energy source for life. Previous work has provided evidence for reverse methanogenesis in marine sediments, but an organism capable of both forward and reverse methanogenesis has not been identified. We present evidence in support of the theory that anaerobic methanotrophs (ANME), which are commonly found in methane producing environments, are capable of growth under methanogenic conditions in anoxic incubations of marine sediments from Cape Lookout Bight, NC. Coupled to a large body of literature studying the participation of ANME archaea in methanotrophy (as well as having molecular machinery for methanogenesis), we propose that these difficult-to-culture organisms can also perform methanogenesis when conditions dictate that methanogenesis is exergonic.