Microbially mediated dechlorination of the PCB Aroclor 1254 in Taihu Lake sediment microcosms: Deciphering...

Microbially mediated dechlorination has been proposed as a promising strategy for the remediation of polychlorinated biphenyls (PCBs) contaminated sediments. Here, the microbial dechlorination and detoxification of Aroclor 1254, the second most widely produced PCB mixture with the highest toxicity among all commercial PCB mixtures, were comprehensively investigated in Taihu Lake sediment microcosms through congener-specific analysis and multiple molecular biological techniques, as well as the biostimulation via lactate (LA) and polylactic acid (PLA). The total mass and the toxic equivalents (TEQs) of Aroclor 1254 significantly decreased by 31.7 % (from 39.80 ± 0.24 to 27.17 ± 0.38 mg/kg) and 99.1 % (from 129.46 ± 1.28 to 1.14 ± 0.05 pg/g) within the 66 weeks of incubation, respectively. The dechlorination pathways of para-flanked meta and single-flanked para were identified as the predominant pathways. Additionally, ortho dechlorination pathways targeting unflanked ortho chlorines were observed, including PCB25 (24-3-CB)→PCB13 (3-4-CB), PCB31 (25-4-CB)→PCB13, and PCB28 (24-4-CB)→PCB15 (4-4-CB). Dehalococcoides and harbored RDase genes pcbA1, pcbA4, and pcbA5 increased upon the initiation of PCB dechlorination and subsequently maintained at high levels, ranging from 10⁶ to 10⁸ copies/mL. Moreover, by the end of incubation, the dechlorination extent stimulated via LA and PLA increased 9.0 % and 13.8 %, respectively, compared to that in microcosms without carbon sources. PLA for biostimulation exhibited a more sustained and long-term enhancement of PCB dechlorination than LA did. These findings elucidate the critical role of the intrinsic microbial detoxification capacity and advance the understanding of optimal biostimulation strategies for remediating PCB-contaminated sites.