IRON REDUCTION: A SLIPPERY RUNG ON THE THERMODYNAMIC LADDER

Iron reduction, sulfate reduction, and methanogenesis are three of the most common microbial reactions in anoxic environments. The microorganisms that survive by deriving energy from these processes can be active simultaneously or, through competition, as spatially-segregated zones dominated by a single process. The extent to which environmental factors influence their interactions, however, are poorly known. Field characterizations and laboratory experiments show that pH is a primary driver influencing iron-reducing microbes' interactions with sulfate reducers and methanogens in aquifers. Under acidic conditions, iron-reducing microbes out-compete and exclude sulfate reducers and methanogens. As pH increases, microbes gain less energy from iron reduction relative to sulfate reduction and/or methanogenesis. Furthermore, increased sorption of ferrous ions to ferric minerals slows the kinetics of iron reduction, making it a less viable source of energy for microbial growth. By coupling with sulfate reducers, however, iron-reducing microbes can persist in alkaline systems leading to the precipitation of iron sulfides and potentially cryptic sulfur cycling. In the absence of sulfur, while the overall rate and extent of iron reduction itself decreases we hypothesize that iron-reducing microbes can survive by transferring electrons directly to methanogens rather than ferric minerals. Taken together, these findings show that pH can influence the relationship of iron reducers with sulfate reducers and methanogens by impacting both the thermodynamics and kinetics of this method of microbial metabolism. Moreover, these results suggest that iron-reducing microbes depend strongly on both sulfate reducers and methanogens to survive under alkaline conditions.