16S RRNA GENE LIBRARIES TO CHARACTERIZE THE MICROBIAL COMMUNITY STRUCTURE AT PETROLEUM-CONTAMINATED AQUIFERS EXHIBITING ANOMALOUS HIGH ELECTRICAL CONDUCTIVITIES

To understand the complex biogeophysical interactions at subsurface petroleum spills, field studies and laboratory experiments using combined geophysical and microbiological methods were performed. Measurements with vertical resistivity probes have shown anomalous high conductivities at a hydrocarbon contaminated site undergoing biodegradation. Microbiological culture-based methods showed the highest activity of hydrocarbon-degrading microorganisms in the zone of highest electrical conductivities. To evaluate whether the vertical changes in conductivity parallel changes in the microbial community structure, 16S rRNA gene libraries were constructed along a vertical gradient from a well in the free-product phase and from a control well. Typically for an aged oil spill, the signature of a methanogenic community was identified with aliphatic and aromatic hydrocarbon degraders, methylotrophic, and iron and sulfur reducing bacteria being detected at characteristic depths. At the control site, a diverse community with various common soil bacteria was found to be present. To simulate a subsurface oil spill in the laboratory, 36 inches long and 12 inches wide PVC columns were constructed that were amenable to vertical resistivity measurements and microbiological sampling. Increased electrical conductivities were measured in the column that was subjected to petroleum contamination and had been inoculated with a hydrocarbon-degrading microbial community. In contrast, the non-contaminated control column showed decreased bulk conductivities. 16S rRNA gene libraries were constructed from the experimental and control columns at the beginning (2 months) and at the end (20 months) of the experiment. It was shown that the microbiological changes preceded the geoelectrical changes and after 20 months, an anaerobic, hydrocarbon degrading community was established in the contaminated column. Since changes in the geophysical properties of contaminated sediments parallel changes in the microbial community composition, we suggest evaluating the use of geoelectrical methods as a tool to guide microbiological sampling for microbial ecological studies and for the monitoring of natural or engineered bioremediation processes.