Water content as a primary parameter determines microbial reductive dechlorination activities in soil.

Organohalide-respiring bacteria (OHRB) remove halogens from a variety of organohalides, which have been utilized for in situ remediation of different contaminated sites, e.g., groundwater, sediment and soil. Nonetheless, dehalogenation activities of OHRB and consequent remediation efficiencies can be synergistically affected by water content, soil type and inoculated/indigenous OHRB, which need to be disentangled to identify the key driving parameter and to elucidate the underlying mechanism. In this study, we investigated the impacts of water content (0-100%), soil type (laterite, brown soil and black soil) and inoculated OHRB (Dehalococcoides mccartyi CG1 and a river sediment culture) on reductive dechlorination of perchloroethene (PCE) and polychlorinated biphenyls (PCBs), as well as on associated microbial communities. Results suggested that the water content as a primary rate-limiting parameter governed dechlorination activities in environmental matrices, particularly in the soil, possibly through mediation of cell-to-organohalide mobility of OHRB. By contrast, interestingly, organohalide-dechlorinating microbial communities were predominantly clustered based on soil types, rather than water contents or inoculated OHRB. This study provided knowledge on the impacts of major parameters on OHRB-mediated reductive dechlorination in groundwater, sediment and soil for future optimization of in situ bioremediation of organohalides.