Bacterial community evidence for anaerobic degradation of petroleum hydrocarbons in cold climate groundwater

There is currently limited scientific data to assess whether groundwater bacterial communities in fractured rock environments can degrade petroleum hydrocarbon plumes in cold regions. The former Colomac Mine is located in the Canadian Shield in the Northwest Territories (mean annual air temperature of − 5 °C) and is currently listed on the Federal Contaminated Sites inventory. Groundwater in fractured rock beneath the former fuel tank-farm at the mine site is contaminated by petroleum hydrocarbons. The objectives of this study were to investigate the bacterial community structure in the groundwater associated with hydrocarbon contamination, and to probe for potential anaerobic microbial processes involved in intrinsic bioremediation. Groundwater monitoring wells previously installed at the site were used to collect samples for chemical, isotopic and microbial analyses. Denaturing gradient gel electrophoresis (DGGE) analysis identified a relatively high bacterial diversity in the least contaminated locations, but as the hydrocarbon contamination increased, bacterial diversity decreased. Sequence analysis of the 16S rRNA gene demonstrated that the bacteria belonged to a wide range of genera, such as Pseudomonas, Thiobacillus, and Geobacter, all of which have been associated with the anaerobic degradation of hydrocarbons. Two putative nitrate-reducing genes were detected in samples with high nitrate reducing activity. Both chemical and microbiological results indicated the presence of microbial anaerobic processes by using nitrate, manganese(IV), ferric iron and sulfate as electron acceptors and suggest that these anaerobic processes play an important role in the biodegradation of dissolved petroleum hydrocarbons in the groundwater at the Colomac site. Our results also revealed that more than one biogeochemical process linked to hydrocarbon degradation could be present in a single borehole and these processes could vary spatially at this site.