PCB reductive dehalogenation in sediments of venice lagoon: preliminary characterization of the dechlorinating microflora

It has been documented that PCBs might undergo in situ microbial dechlorination in a wide variety of anaerobic freshwater sediments mostly under methanogenic conditions, although recent studies have shown a possible role of Dehalococcoides-like microrganisms in the process. Recently, we demonstrated the occurrence of microbial-mediated reductive dechlorination processes towards weathered PCBs in marine sediments of the Porto Marghera area of Venice lagoon. The process is a) directed towards meta- and para-chlorines of PCB molecule, b) not primed by the addition of exogenous PCBs, and c) apparently mediated by indigenous sulfate-reducing spore-forming bacteria. To gain deeper insights on the latter finding, native microflora was sub-cultured 5 times in the presence of exogenous PCBs (5 spiked coplanar PCBs and then Aroclor 1254) and decreasing amounts of sterile sediment. Increasing PCB-dechlorination rate along with remarkable increases in sulfate-reduction and a progressive decline in methanogenic activity were observed throughout subculturing. The culture resulting from the last transfer displayed a marked activity towards Aroclor 1254. The addition of H2 and short chain organic acids did not affect the process. However, when the same agents were applied together with vancomycin or ampicillin, higher dechlorination rates and extents were observed. This finding seems to suggest that Dehalococcoides-like bacteria are also involved in PCB dechlorination, even though sulfate-reducing bacteria cannot be excluded. Molecular analysis of the cultures, currently in progress, will allow to have a deeper insight on PCB dechlorinating populations occurring in Venice lagoon sediments.