BACKGROUND: Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) in contaminated sediments is characterized by long lag periods and low rates. Zerovalent iron is a source of cathodic hydrogen that has been proposed as an easily amendable agent to stimulate PCB microbial dechlorination. The objective of this study was to evaluate the effect of nanoscale zerovalent iron (NZVI) particles on the reductive dechlorination of Aroclor 1254 PCBs and on the indigenous microbial community in a marine sediment under in situ-like biogeochemical conditions. RESULTS: A 30-weeks lag phase followed by a modest dechlorination (5.0 +/- 0.4 mol%) of hepta- through penta-chlorinated PCBs occurred in NZVI-free cultures during 36 weeks of incubation. NZVI (6.7 g kg-1) reduced the lag phase of PCB dechlorination by 10 weeks, leading to a four-fold increase of the dechlorination extent at the end of the incubation. NVZI exerted some toxic effect on sulphate reducing bacteria, which were transiently inhibited until its complete oxidation occurred, and favoured the enrichment of a phylotype closely related to the PCB dehalorespiring bacterium Dehalobium chlorocoercia DF-1, probably via the simultaneous partial inhibition of sulphate reducing bacteria and the release of molecular hydrogen. Finally, DGGE analysis showed that NZVI did not affect markedly the biodiversity of the indigenous microbial community. CONCLUSION: NZVI displayed a very high biostimulation effect on PCB microbial dechlorination and a very low impact on the sediment indigenous microbial community. Supplementation with NZVI particles might thus be a sustainable effective strategy to intensify PCB reductive dechlorination processes in marine sediments. Copyright (c) 2012 Society of Chemical Industry