Enrichment of microbial reductive dechlorination of polychlorinated biphenyls in sediments

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) is a promising mechanism for in situ bioremediation of widespread PCB contamination. This dissertation presents research on the enhancement of dechlorination, the observation of complete dechlornation, the identification of PCB-dechlorinating microorganisms, and the role of sediment in PCB dechlorination. In Baltimore Harbor (estuarine) sediment cultures, the addition of low levels of electron donors (Fe0 or H2) decreased the lag prior to PCB dechlorination, while the addition of high levels of Fe0 or H2 increased the lag period. The inhibition of PCB dechlorination by an alkaline pH, high [Fe2+], or high [HS-] indicated that PCB dechlorinators were sensitive to environmental conditions. The sensitivity of PCB dechlorinators was further evidenced by the different dechlorination patterns exhibited by replicate cultures derived from the same Palos Verdes (marine) sediment. Dechlorination at all positions occurred, but doubly flanked meta and para chlorines were preferentially removed. In one replicate, dichlorobiphenyl was the end product whereas in another further dechlorination to biphenyl was observed. Bacterial community analyses found slightly different community structures between the two replicate PCB-dechlorinating cultures. The effect of added inorganic carbon on the dechlorination pattern and extent in cultures enriched from Hudson River (freshwater) sediment was also studied. With H2 as an electron donor, lower levels of added bicarbonate resulted in extensive dechlorination, whereas the addition of high concentrations of bicarbonate limited the extent of dechlonnation. Analysis of the bacterial community structure identified the involvement of three populations phylogenetically related to known dehalorespirers, including one Dehalococcoides-like species and two Dehalobacter restrictus-like species. Dehalococcoides-like species were also observed in Palos Verdes and Baltimore Harbor sediment and were correlated to the removal of doubly flanked chlorines. The effect of sediment origin on PCB dechlorination was also investigated. Although it was observed that the PCB-dechlorinating activity of mixed cultures could be augmented by mixing enrichment cultures, the sediment properties had a significant impact on PCB dechlorination. This indicates that bioaugmentation might be a possibility for PCB-contaminated sediment, but more work is required to fully understand the role of sediment and trace nutrients on dechlorination.