PH DEPENDENT INTERACTIONS BETWEEN IRON REDUCTION AND METHANOGENESIS SUGGEST A NEW MODEL FOR METHANOGENESIS

Traditional models assume that methanogenesis and iron reduction are segregated into distinct zones defined by competition. Several studies show, however, that the reactions can occur simultaneously and that the microbes driving the reactions can even interact syntrophically through interspecies electron transfer. Using results from bioreactor experiments and natural systems, we propose a new model for methanogenesis that links competition and syntrophy. In bioreactors with goethite as a source of ferric iron, we found that the balance between iron reduction and methanogenesis varied significantly with pH. Compared to acidic bioreactors, electron donor oxidation in alkaline bioreactors was 85% lower for iron reduction and 61% higher for methanogenesis. Thus, methanogenesis displaced iron reduction considerably at alkaline pH. Geochemistry data collected from U.S. aquifers demonstrate that a similar pattern also exists on a broad spatial scale in natural settings. Bioenergetics calculations show that variation in the thermodynamic drive of iron reduction parallels changes in the balance between each reaction. At acidic pH, iron reduction has an energy advantage over methanogenesis but at alkaline pH, the opposite is true. Despite much less iron reduction in alkaline reactors than acidic reactors, the relative abundance of iron reducers differed little between each set of reactors. Sequences that classified in Geobacteraceae, a family of bacteria associated with dissimilatory metal reduction, were the largest group overall and accounted for 19 and 15% of the sequences from acidic and alkaline reactors, respectively. We account for these observations by hypothesizing that, as pH increases, interactions between iron reducers and methanogens transition from competition to syntrophy in response to variation in the thermodynamic drive of iron reduction.