A Chloroflexi bacterium dechlorinates polychlorinated biphenyls in marine sediments under in situ-like biogeochemical conditions

We investigated the reductive dechlorination of Aroclor 1254 PCBs by a coplanar PCB-dechlorinating microbial community enriched from an actual site contaminated marine sediment of the Venice lagoon in sterile slurry microcosms of the same sediment suspended in its site water, i.e., under biogeochemical conditions that closely mime those occurring in situ. The culture dechlorinated more than 75% of the penta- through hepta-chlorinated biphenyls to tri- and tetra-chlorinated congeners in 30 weeks. The dechlorination rate was reduced by the addition of H2 and short chain fatty acids, which stimulated sulfate-reduction and methane production, and markedly increased by the presence of vancomycin or ampicillin. DGGE analysis of 16S rRNA genes on PCB-spiked and PCB-free cultures ruled out sulfate-reducing and methanogenic bacteria and revealed the presence of a single Chloroflexi phylotype closely related to the uncultured bacteria m-1 and SF1 associated to PCB dechlorination. These findings suggest that a single dechlorinator is responsible for the observed extensive dechlorination of Aroclor 1254 and that a Chloroflexi species similar to those already detected in freshwater and estuarine contaminated sediments mediates PCB dechlorination in the marine sediment adopted in this study under biogeochemical conditions resembling those occurring in situ in the Brentella Canal of Venice Lagoon.