Changes in microbial community function during a period of reoxidation in a groundwater recirculation system
A test groundwater recirculation system was constructed at the U.S. DOE's Field Research Center (FRC) in Oak Ridge, TN to examine the applicability of in situ U(VI) bioremediation of a U-contaminated site. Ethanol was injected into the system to biostimulate the microbial community and a reduction in U concentration has been achieved. To examine the effect of dissolved oxygen (DO) on U(VI) reduction and the stability and re-oxidation of U in this system, air-saturated tap water (9-12 mg L-I DO) was introduced for a period of 77 d. Geochip, a comprehensive SOmer microarray containing probes for genes involved in the geochemical cycling of N, S, and C as well as genes related to metal reduction and resistance and to organic contaminant degradation, was used to examine changes in the microbial functional community in two monitoring wells before, during, and after the reoxidation period. The well located closest to the injection and extraction wells (101-2), showed a greater increase in DO (2 mg L-1) than the well located further away (102-2; 0.4-0.5 mg L-1). An increase in U(VI) occurred during the reoxidation period and was associated with a decrease in the relative abundance of cycthrome c genes. At 40 d post reoxidation, 101-2 showed an increase in the relative abundance of genes involved in denitrification, nitrate reduction, methane and ammonium oxidation and a decrease for those associated with cytochromes, N fixation and sulfate reduction. By 77 d post reoxidation, the relative abundance of denitrification and ammonia oxidation genes had returned to pre-oxidation levels. At 40 d post reoxidation, 102-2 showed an increase in the relative abundance of genes involved in methane oxidation and nitrate reduction. A decrease was observed with genes associated with ammonification, cytochromes, denitrification, N fixation, and sulfate reduction. The relative abundance of these genes returned to pre-oxidation levels by 77 d post reoxidation. Changes in the functional community were similar in the two wells; however, the community in 101-2 appeared to be affected for a longer period of time, most likely due to the higher levels of DO as compared to 102-2.