Thermodynamic Control on Terminal Electron Transfer and Methanogenesis

Terminal electron accepting processes (TEAPs) control the fate of elements in anoxic environments. This study focuses on thermodynamic regulation of H2-dependent TEAPs. H2-dependent methanogenesis and sulfate reduction operate near free energy thresholds (ΔGc) and can be inhibited by changes in thermodynamic conditions, whereas more 'potent' TEAPs occur far from their energy thresholds and lower H2 concentrations to levels that exclude other TEAPs. Metabolic free energy thresholds depend on microbial physiology and occur when the energy conserved by ATP generation approaches the thermodynamic driving force. A model analysis for peat-sand mixtures suggests that acetoclastic methanogenesis can be inhibited by CH4 and dissolved inorganic carbon (DIC) accumulation, lowering the free energy (ΔGr) toward an energy threshold (ΔGc), which was identified by inverse modeling near - 25 kJ mol-1. Inhibition was sensitive to ΔGc and acetate concentrations, so that ΔGc ± 5 kJ mol-1 and a range of 1 to 100 µmol L-1 acetate lead to strongly differing steady state CH4 concentrations in the model results.