CARBON DIOXIDE AS A CONTROL ON COMPETITION BETWEEN IRON- AND SULFATE-REDUCING MICROORGANISMS

Understanding controls on competition between iron and sulfate-reducing microorganisms is important because they are widespread and strongly influence the biogeochemistry of aqueous systems. Here we examine CO₂ as a control on competition between each group using semi-continuous bioreactors inoculated with a mixed-microbial community from a freshwater aquifer. We performed two sets of experiments: one with low CO₂ partial pressure (~0.02 atm) in the headspace of the reactors and one with high CO₂ partial pressure (~1 atm). The aqueous medium consisted of groundwater amended with small amounts of acetate (250 µM), phosphate (1 µM), and ammonium (50 µM) to stimulate microbial activity. Each reactor was also supplied with synthetic goethite (1 mmol) and sulfate (500 µM influent concentration), electron acceptors for iron and sulfate reducers, respectively. Our results demonstrate that iron reducers were better able to compete with sulfate reducers in reactors with high CO₂ content. Mass-balance calculations and pyrosequencing results show that sulfate reducers were dominant in reactors with low CO₂ content. They consumed 85% of the acetate after acetate consumption stabilized while iron reducers consumed only 15% on average. In contrast, iron reducers were dominant during that same interval in reactors with high CO₂ content, consuming at least 90% of the acetate while sulfate reducers consumed a negligible amount (<1%). Thermodynamic calculations suggest that this shift in microbial activity occurred in response to differences in the free energy yield of iron reduction between reactors. Additional research is currently being conducted to examine this possible mechanism in more detail.

This material is based upon work supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.