Microbial incorporation of 13C-labeled acetate at the field scale:: Detection of microbes responsible for reduction of U(VI)

A field-scale acetate amendment experiment was performed in a contaminated aquifer at Old Rifle, CO to stimulate in situ microbial reduction of U(VI) in groundwater. To evaluate the microorganisms responsible for microbial uranium reduction during the experiment, C-13-labeled acetate was introduced into well bores via bio-traps containing porous activated carbon beads (Bio-Sep). Incorporation of the C-13 from labeled acetate into cellular DNA and phospholipid fatty acid (PLFA) biomarkers was analyzed in parallel with geochemical parameters. An enrichment of active delta-proteobacteria was demonstrated in downgradient monitoring wells: Geobacter dominated in wells closer to the acetate injection gallery, while various sulfate reducers were prominent in different downgradient wells. These results were consistent with the geochemical evidence of Fe(III), U(VI), and SO42- reduction. PLFA profiling of bio-traps suspended in the monitoring wells also showed the incorporation of 13C into bacteria] cellular lipids. Community composition of downgradient monitoring wells based on quinone and PLFA profiling was in general agreement with the C-13-DNA result. The direct application of 13C label to biosystems, coupled with DNA and PLFA analysis, which combined detailed taxonomic description with a quantitative measure of metabolic diversity, allowed identification of the metabolically active portion of the microbial community during reduction of U(VI).