Dechlorination of Commercial PCBs and Other Multiple Halogenated Compounds by a Sediment-Free Culture ContainingDehalococcoidesandDehalobacter

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.

Genomic characterization of three unique Dehalococcoides that respire on persistent polychlorinated biphenyls

“…Chloroethenes are known to support the growth of Dehalococcoides (13) and so can potentially serve as an alternative substrate for their enrichment. However, this process also has drawbacks, with the possibility of the obtained strain losing PCB dechlorination activity during the enrichment process (13). Therefore, monitoring the transfer process through genomic approaches can help to guide the enrichment process and increase the odds.…”

“…The dechlorinating bacteria failed in growth competition with other bacteria in PCB-fed cultures due to their low cell densities resulting from the extremely low bioavailability of PCBs (10,14). To circumvent this problem, we previously used TCE and vinyl chloride instead of PCBs to enrich and isolate PCB-dechlorinating bacteria from culture AD14 (13). Even though we obtained two Dehalococcoides isolates from the culture, the two strains did not show PCB dechlorination activity, possibly as a result of loss of PCB-dechlorinators or PCB RDase genes during the enrichment process.…”

“…Completely defined, anaerobic mineral salts medium for bacterial enrichment and isolation was prepared as described (12,13,20). Enrichment of PCB-dechlorinating Dehalococcoides was achieved by sequential transfers in 100 mL medium amended with sodium lactate (10 mM) and the chlorinated substrate, 30 ppm (80.65 μM) Aroclor 1260 or 0.7 mM PCE.…”

“…PCBs were extracted with isooctane and quantified by gas chromatography (GC) equipped with an electron capture detector and a DB-5 capillary column as described (12,13). Chlorinated ethenes were analyzed by GC using flame ionization detector and a GS-GasPro column (13).…”

“…Correspondingly, to date, only three bacterial strains showed PCB dechlorination activity, Dehalobium chlorocoercia DF-1 (7), Dehalococcoides mccartyi 195 (8), and D. mccartyi CBDB1 (9), none of which have been shown to be capable of respiring on the commercial PCBs as is needed for in situ PCB bioremediation and for identification of key functional genes (10). On the other hand, several bacterial genera have been implicated in PCB dechlorination, including Dehalococcoides, Dehalogenimonas, and Dehalobacter (11)(12)(13)(14), and these serve as a rich environmental resource for isolation and comparative study of the genes and processes underlying organohalide respiration.…”