Aquatic sediments constitute a large sink for legacy PCB mixtures (e.g., Aroclors) released into the environment. Anaerobic reductive dechlorination of PCBs in sediments is a naturally occurring pathway for transformation of PCBs to lesser chlorinated and possibly less toxic products. Microorganisms that participate in natural attenuation of PCBs are poorly understood. The genera Dehalococcoides, Dehalogenimonas, and Dehalobacter contain known PCB dechlorinators. These bacteria use reductive dehalogenases (RDases) to catalyze PCB biodegradation, but only a few “PCB dehalogenase” genes are known. The environmental relevance of these genera and the RDase genes they employ is also unclear. In an initial study of 27 PCB-contaminated sediment samples from a wastewater lagoon, five samples displayed evidence of naturally occurring PCB dechlorination processes (i.e., their congener profiles deviated significantly from the dominant Aroclor 1248 profile). A quantitative PCR (qPCR) survey of reductive dehalogenase genes in the samples yielded 11 RDase genes, eight of which were most closely related to RD14, an RDase gene found in PCB-dechlorinating Dehalococcoides mccartyi strain CG5. To better understand anaerobic PCB natural attenuation processes, we constructed anaerobic microcosms with the five hotspot sediment samples and incubated them without exogenous electron donor for 417 days. SPME PDMS-fiber passive samplers were incorporated periodically into the microcosms to track pore water PCB congener profile dynamics. Pore water PCB congener profiles generally showed increases in PCB 4 and PCB 19, both of which are common reductive dechlorination end-products. Using qPCR, we observed a 10-1000 fold increase in Dehalococcoides 16S rRNA gene abundance (up to 1.5x107 genes/g sediment), while Dehalogenimonas and Dehalobacter 16S rRNA gene abundances decreased or did not change. Previously identified PCB RDase gene (i.e., PCBA1, PCBA4, and PCBA5) abundances were below 4x104 genes/g sediment. However, RD14 abundances increased in accordance with Dehalococcoides 16S rRNA genes. This suggests that a Dehalococcoides mccartyii strain harboring putative PCB RDase gene RD14 is growing on endogenous electron donor in microcosms while respiring on PCB congeners. Overall, this study has yielded insights into genera and RDase genes participating in PCB natural attenuation processes. This information will be useful in developing improved PCB sediment bioremediation strategies.

Growth of Dehalococcoides mccartyii and increased abundance of reductive dehalogenase genes in anaerobic PCB-contaminated sediment microcosms

2019 Spring National Meeting

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