Microbial Reductive Dechlorination of Aroclor 1260 in Baltimore Harbor Sediment Microcosms Is Catalyzed by Three Phylotypes within the Phylum Chloroflexi

Abstract

The specific dechlorination pathways for Aroclor 1260 were determined in Baltimore Harbor sediment microcosms developed with the 11 most predominant congeners from this commercial mixture and their resulting dechlorination intermediates. Most of the polychlorinated biphenyl (PCB) congeners were dechlorinated in the meta position, and the major products were tetrachlorobiphenyls with unflanked chlorines. Using PCR primers specific for the 16S rRNA genes of known PCB-dehalogenating bacteria, we detected three phylotypes within the microbial community that had the capability to dechlorinate PCB congeners present in Aroclor 1260 and identified their selective activities. Phylotype DEH10, which has a high level of sequence identity to Dehalococcoides spp., removed the double-flanked chlorine in 234-substituted congeners and exhibited a preference for para-flanked meta-chlorines when no double-flanked chlorines were available. Phylotype SF1 had similarity to the o-17/DF-1 group of PCB-dechlorinating bacteria. Phylotype SF1 dechlorinated all of the 2345-substituted congeners, mostly in the double-flanked meta position and 2356-, 236-, and 235-substituted congeners in the ortho-flanked meta position, with a few exceptions. A phylotype with 100% sequence identity to PCB-dechlorinating bacterium o-17 was responsible for an ortho and a double-flanked meta dechlorination reaction. Most of the dechlorination pathways supported the growth of all three phylotypes based on competitive PCR enumeration assays, which indicates that PCB-impacted environments have the potential to sustain populations of these PCB-dechlorinating microorganisms. The results demonstrate that the variation in dechlorination patterns of congener mixtures typically observed at different PCB impacted sites can potentially be mediated by the synergistic activities of relatively few dechlorinating species.

Genome sequencing of a single cell of the widely distributed marine subsurface Dehalococcoidia, phylum Chloroflexi

“…These isolates are unified by their ability to grow via organohalide respiration, that is, they use halogenated organic compounds as terminal electron acceptors while using hydrogen as an electron donor in an anaerobic respiration (Tas et al, 2010). In addition, other relatively closely related DEH, including some marine phylotypes, have been implicated in organohalide respiration by enrichment or stable-isotope probing experiments Watts et al, 2005;Bedard et al, 2007;Fagervold et al, 2007;Kittelmann and Friedrich 2008a, b). Nevertheless, there are numerous 16S rRNA gene sequences within the whole DEH clade that are considerably divergent from these organohalide-respiring organisms.…”

Integrated Forensics Approach to Fingerprint PCB Sources in Sediments using Rapid Sediment Characterization (RSC) and Advanced Chemical Fingerprinting (ACF)

“…Retaining a fourth end-member improved the fit of these two congeners and resulted in an end-member that suggested evidence of a small amount of dechlorination in a few HPS samples. Other studies have reported preferential enrichment of PCB 49 and PCB99 in studies of dechlorination of Aroclor 1260 in estuarine environments (Fagervold et al, 2007). A hint of this can be seen in Figure 15 where PCB49 and PCB99 are elevated in the modeled EndMember 3 bar chart fingerprint when compared to the homogenized mixture of Aroclors 1260/1254/1248 shown directly below.…”

Quantifying Enhanced Microbial Dehalogenation Impacting the Fate and Transport of Organohalide Mixtures in Contaminated Sediments

“…Fagervold (2007) reported that three different phylotypes within the Chloroflexi increased in response to Aroclor 1260 in Baltimore Harbor sediment, suggesting that a consortium of dechlorinating bacteria with various specificities may result in more extensive dechlorination of mixed PCBs. In addition to the above studies, the tetrachloroethene dehalorespirer, Dehalococcoides ethenogenes strain 195, dechlorinated 2,3,4,5,6-PCB (Fennell et al, 2004), although growth was not tested.…”

“…Effective, economical methods for remediation of PCB contaminated sediments are lacking, however, progress has been made in understanding potential for biotransformation of PCBs in recent years. Discovery that anaerobic bacteria of the phylum Chloroflexi, including the genus Dehalococcoides and more distantly-related taxons, can dechlorinate PCBs may offer a promising avenue for bioremediation (Wu et al, 2000;Cutter et al, 2001;Fennell et al, 2004;Watts et al, 2005;Fagervold et al, 2005;Fagervold et al, 2007;Bedard et al, 2007;May et al, 2008;; and see review by Bedard, 2008). In this study we evaluated whether stimulating native Dehalococcoides populations or bioaugmentation could increase dechlorination of low concentrations of weathered PCBs, and whether the treatment effects persist over a relatively long time frame (> 1 year).…”