Reactive Transport Modeling to Reveal Syntrophic Electron Transport Mechanism and Size Constraints in Anaerobic Methanotrophic Consortia

Anaerobic oxidation of methane (AOM) acts as a significant sink for methane that plays an important role in global carbon cycle and impacts global warming. A three-dimension model is developed to investigate the underlying mechanisms of sulfate reduction coupled AOM in deep-sea sediments. Our reactive transport models simulated activities across a range of aggregate sizes and archaeal and bacterial cell arrangements. Model results were compared to measured AOM rates and intra-aggregate activity patterns determined by FISH-nanoSIMS.

For small aggregates (1.5 - 12.5 μm radius) that exchange of H₂ or small organic molecules between archaea and bacteria, activity was limited by the build-up of metabolites. In contrast, electron transfer through the delivery of disulfide from methane oxidizing archaea to bacteria for disproportionation and direct interspecies electron transfer (DIET) yielded cell specific rates and archaeal activity distributions that were consistent with observations from single cell resolved FISH-nanoSIMS analyses. Comparing model simulations to the heterogeneous intra-aggregate activity patterns observed in a large aggregate (25 μm radius) suggests that the aggregate either operates via a mixed diffusive / direct electron transfer with little voltage loss, or via DIET but limited by voltage loss from ohmic resistance.

Our novel integration of both intra-aggregate and environmental data provides powerful constraints on the model results. These modeling efforts can be used to guide further empirical and theoretical explorations into the identity and kinetics of extracellular redox-active components within AOM consortia.