Assessment of natural attenuation of polychlorinated biphenyls in harbor sediments based on analysis of congeners and environmental DNA: Anaerobic PCB-dechlorinating bacteria as key contributors

Polychlorinated biphenyls (PCBs) are toxic pollutants that are distributed worldwide. Indigenous microorganisms are believed to contribute significantly to their degradation. To better understand how environmental microorganisms degrade toxic pollutants, such as PCBs, we identified the indigenous bacteria involved in the natural attenuation of PCBs and PCB congener profiles in sediments from harbors on Awaji Island, Hyogo, Japan. High-resolution gas chromatography and high-resolution mass spectrometry (HRGC/HRMS) were used to analyze the PCB congener profiles. The sediment samples contained 2,200-3,800 PCBs/g dry, while the sediment sample with 100 pg PCBs/g dry served as a control harbor. The profiles of PCB congener composition in each homolog group using cosine theta (cos θ) analysis suggested that the polluted harbor with 2,200–3,800 PCBs/g dry was primarily affected by multiple commercial PCB products. Furthermore, the main congeners in the polluted harbor exhibited a positional preference that was not observed in the control harbor or in commercial PCB products; this suggests the possibility of microbial degradation rather than merely the presence of multiple commercial PCB products. Additionally, 16S ribosomal RNA (rRNA) next-generation sequencing (NGS) analysis revealed similar alpha-diversity but different bacterial community compositions between the two harbors, particularly in the sediments, with the presence of PCB-dechlorinating Dehalococcoidaceae species in the polluted harbor sediments. Polymerase chain reaction (PCR) amplification revealed the presence of PCB-dechlorinating bacteria, namely Dehalogenimonas sp., only in the polluted harbor. These findings suggest that anaerobic PCB-dechlorinating bacteria such as Dehalogenimonas sp. play a key role in the natural attenuation of PCBs by dechlorination in polluted harbors. Overall, we not only provided insight into the biodegradation process by indigenous microorganisms and the potential for the natural attenuation of PCBs in polluted environments but also demonstrated that the combination of PCB congener analysis using HRGC/HRMS and microbial analysis using environmental DNA is a powerful tool for assessing PCB pollution and indicating natural attenuation in the environment.