Microbial reductive dechlorination of pre-existing PCBs and spiked 2,3,4,5,6-pentachlorobiphenyl in anaerobic slurries of a contaminated sediment of Venice Lagoon (Italy)

Reductive dechlorination of polychlorinated biphenyls (PCBs) pre-existing (at ∼1 mg kg−1) in a marine sediment of Porto Marghera (Venice Lagoon, Italy) was investigated in anaerobic slurries developed in water of the same contaminated site. Some microcosms were pasteurized whereas others were amended with 2-bromoethanesulfonic acid, molybdate or eubacteria-inhibiting antibiotics (without and in the presence of exogenous carbon sources) to preliminarily characterize the microbial populations involved in the process. Bioconversion of highly chlorinated PCBs into tri- and di-chlorinated, ortho-substituted biphenyls was detected from the 11th week of incubation both in the non-amended and in the pasteurized microcosms, where a significant consumption of sulfate and no methane production were observed. Conversely, no significant PCB transformation was detected in the microcosms with molybdate, where no sulfate consumption and a significant methane evolution occurred. Neither was PCB transformation observed in the microcosms supplemented with antibiotics and exogenous carbon sources, where a strong methane evolution and no sulfate consumption were recorded until the 11th week. The addition of exogenous 2,3,4,5,6-pentachlorobiphenyl showed preferential dechlorination at the meta and para positions, and did not significantly influence the onset of pre-existing PCB dechlorination. These results indicate that endogenous PCBs pre-existing in the marine sediment underwent reductive dechlorination. They also suggest that the process was not ‘primed’ upon 2,3,4,5,6-pentachlorobiphenyl addition, and was likely to be mediated by sulfate-reducing, spore-forming bacteria.