Organohalide-Respiring Bacteria in Polluted Urban Rivers Employ Novel Bifunctional Reductive Dehalogenases to Dechlorinate Polychlorinated Biphenyls and Tetrachloroethene

Polluted
urban river sediments could be a sink of persistent and
toxic polychlorinated biphenyls (PCBs) in urban areas and provide
desired growth niches for organohalide-respiring bacteria (OHRB).
In this study, microcosms were set up with surface sediments of nationwide
polluted urban rivers in China, of which 164 cultures could dechlorinate
tetrachloroethene (PCE) to dichloroethenes (DCEs) and to vinyl chloride
and/or ethene. Further in vivo tests showed extensive
PCB dechlorination with different pathways in 135 PCE pregrown cultures.
Taking reductive dechlorination of PCB180 (2345-245-CB) as an example,
121 and 14 cultures preferentially removed flanked para- and meta-chlorines, respectively. Strikingly,
all in vitro assays with the 135 PCE pregrown cultures
showed identical PCB dechlorination pathways with their living cultures,
implying the involvement of bifunctional reductive dehalogenases (RDases)
to dechlorinate both PCBs and PCE. Further 16S rRNA and RDase gene-based
analyses, together with enantioselective dechlorination of chiral
PCBs, suggested that Dehalococcoides and Dehalogenimonas in the 135 cultures largely employed distinctively
different novel bifunctional RDases to catalyze PCB/PCE dechlorination.
Quantitative assessment of the community assembly process with the
modified stochasticity ratio (MST) indicated three different stages
in enrichment of OHRB. The second stage, as the only one controlled
by stochastic processes (MST > 0.5), required extra attention in
monitoring
community successional patterns to minimize stochastic variance for
enriching the PCB/PCE-dechlorinating OHRB.