Simultaneous Iron Oxide stimulation of Sulfate Reduction and Methanogenesis

Sulfate reducing bacteria (SRB) and methanogenic archaea are generally thought to exist mutually exclusively unless in syntrophic association. When in competition for the same substrate (e.g., acetate, hydrogen gas), methanogenesis is outcompeted by the more thermodynamically favorable sulfate reduction reactions due to the higher uptake affinity by SRB enzymes. However, mounting evidence points to the co-occurrence and activity of both mechanisms in many anaerobic environments warranting further investigations into the biogeochemical factors that could be in action. Previous observations indicated SRB and methanogenic archaea co-occurrence in our subsurface ground water field site at the Oak Ridge Reservation in TN, USA. To examine this further, we developed syntrophic butyrate degrading cultures under sulfate reducing and/or methanogenesis conditions with or without different forms of iron oxides (conductive hematite and insulative ferrihydrite). We hypothesize that highly prevalent and conductive Fe oxides at this location act as conduits of electron transfer, facilitating electron flow via extracellular electron transfer under anaerobic environments. Interestingly, while both hematite and ferrihydrite stimulated sulfate reduction by reducing soluble sulfide levels; methanogenesis was stimulated only by hematite. To examine how hematite supports both sulfate reduction and methanogenesis, we will use synchrotron-based Fe K-edge EXAFS and S K-edge XANES to shed light on the molecular scale identity of the secondary iron and sulfur minerals formed while 16S analysis will be carried out to understand the microbial communities involved. To our knowledge, this is the first study to report how crystalline hematite can promote two anaerobic metabolisms simultaneously, thereby mediating the co-existence of SRB and methanogens in subsurface terrestrial environments.