Genetic analyses of microbial polychlorinated biphenyl degradation in natural and engineered systems

Polychlorinated biphenyls (PCBs) are carcinogenic, persistent, and bioaccumulative contaminants that pose risks to human and environmental health. PCB biodegradation by indigenous microbial communities could be a cost-effective and an environmental-friendly bioremediation strategy for in situ PCB removal. A comprehensive understanding of the microbial PCB degradation at the contaminated site is required for the acceptance and optimization of using microbial PCB degradation as the site clean-up strategy. This thesis describes investigations of the aerobic and anaerobic microbial degradation of PCBs under both field and laboratory conditions.

The microbial PCB degradation potential in sediments from Indiana Harbor and Ship Canal (IHSC), a site that was historically contaminated by PCBs, was explored by analyzing the PCB congener distributions and microbial communities in two core sediment samples. PCB congener analysis suggested the possibility of in situ dechlorination in deep sediments. Molecular analysis of biomarker genes revealed the potential of both aerobic and anaerobic PCB degradation in sediments. Microbial communities were characterized by the combination use of terminal restriction fragment length polymorphism (T-RFLP), clone library, and pyrosequencing. These methods elucidated the dominant role of Proteobacteria, especially Acidovorax and Acinetobacter in sediments.

To improve the microbial PCB degradation, phytoremediation with switchgrass (Panicum vigratum) was employed under laboratory conditions. Congener analysis showed that both phytoextraction and microbial PCB degradation contributed to the enhanced PCB removal in the presence of switchgrass. Bioaugmentation with Burkholderia xenovorans LB400 was performed to further promote aerobic PCB degradation. The presence of LB400 was associated with improved degradation of PCB 52, but not PCB77 or PCB 153. Increased abundance of the biphenyl dioxygenase gene, which is indicative of aerobic PCB degradation, and its transcript were observed after bioaugmentation, suggesting active aerobic PCB degradation.

To promote the anaerobic PCB degradation, redox cycling (alternating flooding and non-flooding) was performed. Redox cycling was found to improve the removal of PCB 153 in unplanted soils and to increase the dechlorinating Chloroflexi population. Characterization of the microbial community by T-RFLP and clone library revealed that Proteobacteria and Acidobacteria were dominant. Species that contain dechlorination potential were identified, including Geobacter and Clostridium, suggesting that their possible role in PCB dechlorination.

The research described in this thesis provides scientific knowledge and evidence for the feasibility of employing bioremediation including natural attenuation, phytoremediation, and bioaugmentation to clean up PCB contamination. Such information will be critical in selecting and optimizing remediation strategies for PCB contaminated sites.