THE USE OF QUANTITATIVE PCR TO EXAMINE CONTROLS ON BIOGEOCHEMICAL PROCESSES AT AN AQUIFER CONTAMINATED WITH LANDFILL LEACHATE (NORMAN,OK)

Geochemical research at an aquifer contaminated with landfill leachate suggests that the dominant biogeochemical processes are iron reduction, sulfate reduction, and methanogenesis. However, distributions of chemical constituents are not always explained by the observed activity, abundance, and composition of the microbial community. Our current goal was to evaluate quantitative PCR (qPCR) as a potential technique to better understand the controls on the biogeochemistry at this site. We measured geochemical concentrations, biogeochemical rates, and the abundances of genes indicative of iron reduction (16s rRNA gene for Geobacter spp.), sulfate reduction (dissimilatory sulfite reductase, dsr), and methanogenesis (methyl coenzyme M-reductase, mcrA). In order to compare qPCR to more traditional enumeration techniques, we also used most probable number (MPN) analyses to enumerate iron-reducers (FeRB), sulfate-reducers (SRB), and methanogens. Iron reduction was detectable through much of the plume (rates = 0.05 mg Fe(III) g sed^-1 day^-1) and was significantly correlated with reducible iron as determined by oxalate extraction (r²=0.64, p<0.01). FeRB were enumerated with the MPN technique (10¹-10² FeRB ml^-1 but qPCR abundances of Geobacter spp. were at least two orders of magnitude higher, suggesting that the MPN technique significantly underestimated this population. SRB and dsr genes were present in the contaminant plume and showed no correlation with in situ sulfate concentrations. Sulfate reduction rates were negligible, suggesting that the activity of the SRB community may be limited by factors other than sulfate. In contrast, methane concentrations were positively correlated with the methane production rate (r²=0.79, p<0.01) and the mcrA abundances (r²=0.74, p<0.01). However, the MPN analyses failed to detect methanogens at any of the sites except the landfill wells, including those with high concentrations of methane. These results suggest that qPCR, a rapid relatively inexpensive technique, is more sensitive than MPN analyses for enumerating potentially important microorganisms. The addition of qPCR to our integrated methodological approach also provided significant insight into the complex relationship between the composition of the microbial community and the geochemistry of this aquifer.