CO₂-induced shift in microbial activity affects carbon trapping and water quality in anoxic bioreactors

Microbial activity is a potentially important yet poorly understood control on the fate and environmental impact of CO₂ that leaks into aquifers from deep storage reservoirs. In this study we examine how variation in CO₂ abundance affected competition between Fe(III) and SO42--reducers in anoxic bioreactors inoculated with a mixed-microbial community from a freshwater aquifer. We performed two sets of experiments: one with low CO₂ partial pressure (~0.02atm) in the headspace of the reactors and one with high CO₂ partial pressure (~1atm). A fluid residence time of 35days was maintained in the reactors by replacing one-fifth of the aqueous volume with fresh medium every seven days. The aqueous medium was composed of groundwater amended with small amounts of acetate (250μM), phosphate (1μM), and ammonium (50μM) to stimulate microbial activity. Synthetic goethite (1mmol) and SO42- (500μM influent concentration) were also available in each reactor to serve as electron acceptors. Results of this study show that higher CO₂ abundance increased the ability of Fe(III) reducers to compete with SO42- reducers, leading to significant shifts in CO₂ trapping and water quality. Mass-balance calculations and pyrosequencing results demonstrate that SO42- reducers were dominant in reactors with low CO₂ content. They consumed 85% of the acetate after acetate consumption reached steady state while Fe(III) reducers consumed only 15% on average. In contrast, Fe(III) reducers were dominant during that same interval in reactors with high CO₂ content, consuming at least 90% of the acetate while SO42- reducers consumed a negligible amount (<1%). The higher rate of Fe(III) reduction in the high-CO₂ bioreactors enhanced CO₂ solubility trapping relative to the low-CO₂ bioreactors by increasing alkalinity generation (6X). Hence, the shift in microbial activity we observed was a positive feedback on CO₂ trapping. More rapid Fe(III) reduction degraded water quality, however, by leading to high Fe(II) concentration.