Dechlorination of commercial PCBs and other multiple halogenated compounds by a sediment-free culture containing Dehalococcoides and Dehalobacter

Abstract

At the contaminated sites, polychlorinated biphenyls (PCBs) frequently coexist with other halogenated compounds, such as polybrominated diphenyl ethers (PBDEs), chloroethanes, and chloroethenes. The presence of multiple halogenated compounds usually poses toxicity to dehalogenating microbes, because few cultures are capable of detoxifying a broad spectrum of halogenated compounds. In this study, a sediment-free culture, designed as AD14, is able to sequentially remove halogens from PCBs and other cocontaminants. Culture AD14 dechlorinated the commercial PCB mixture-Aroclor 1260-mainly by removing flanked para- and doubly flanked meta-chlorines. It also dehalogenated octa-brominated diphenyl ether mixture predominantly to tetra-BDEs, 2,4,6-trichlorophenol (2,4,6-TCP) to 4-CP, and tetrachloroethene (PCE)/1,2-dichloroethane (1,2-DCA) completely to ethene. When applied to a mixture of the above-mentioned compounds, culture AD14 stepwise removed halogens from 2,4,6-TCP, 1,2-DCA, PCE, PBDEs, and PCBs. Illumina sequencing analysis of 16S rRNA genes showed that only two known dechlorinating genera, Dehalococcoides and Dehalobacter, were present in culture AD14. Quantitative real-time PCR analysis showed that the 16S rRNA gene copies of Dehalococcoides and Dehalobacter increased from 1.14 × 10(5) to 7.04 × 10(6) copies mL(-1) and from 1.15 × 10(5) to 8.20 × 10(6) copies mL(-1) after removing 41.13 μM of total chlorine from PCBs. The above results suggest that both Dehalobacter and Dehalococcoides could be responsible for PCB dechlorination. Although two Dehalococoides mccartyi strains with identical 16S rRNA genes were isolated from the PCBs-dechlorinating mixed culture using trichloroethene (TCE) and vinyl chloride (VC) as alternatives to PCBs, the two isolates are incapable of dechlorinating PCBs. In all, culture AD14 is promising for bioremediation applications at sites cocontaminated with PCBs and other halogenated compounds.