Identification of a Bacterium That Specifically Catalyzes the Reductive Dechlorination of Polychlorinated Biphenyls with Doubly Flanked Chlorines

Abstract

A microorganism whose growth is linked to the dechlorination of polychlorinated biphenyls (PCBs) with doubly flanked chlorines was identified. Identification was made by reductive analysis of community 16S ribosomal DNA (rDNA) sequences from a culture enriched in the presence of 2,3,4,5-tetrachlorobiphenyl (2,3,4,5-CB), which was dechlorinated at the para position. Denaturing gradient gel electrophoresis (DGGE) analysis of total 16S rDNA extracted from the culture led to identification of three operational taxonomic units (OTUs 1, 2, and 3). OTU 1 was always detected when 2,3,4,5-CB or other congeners with doubly flanked chlorines were present and dechlorinated. Only OTUs 2 and 3 were detected in the absence of PCBs and when other PCBs (i.e., PCBs lacking doubly flanked chlorines) were not dechlorinated. Partial sequences of OTUs 2 and 3 exhibited 98.2% similarity to the sequence of "Desulfovibrio caledoniensis" (accession no. DCU53465). A sulfate-reducing vibrio isolated from the culture generated OTUs 2 and 3. This organism could not dechlorinate 2,3,4,5-CB. From these results we concluded that OTU 1 represents the dechlorinating bacterium growing in a coculture with a Desulfovibrio sp. The 16S rDNA sequence of OTU 1 is most similar to the 16S rDNA sequence of bacterium o-17 (89% similarity), an ortho-PCB-dechlorinating bacterium. The PCB dechlorinator, designated bacterium DF-1, reductively dechlorinates congeners with doubly flanked chlorines when it is supplied with formate or H 2 -CO 2 (80:20).Polychlorinated biphenyls (PCBs) remain an environmental concern because of their chemical stability and potential toxicity to humans and wildlife. Although production of PCBs has been banned in much of the industrial world, these compounds are still used to some extent in electrical equipment and can be found in the environment many years after they are released. For these reasons microbial degradation and dechlorination of PCBs have been investigated in great detail (reviewed in references 1, 5, 24, and 25). Aerobic PCB-degrading microorganisms degrade the biphenyl ring, but usually they do not transform the more extensively chlorinated PCB congeners that dominate many commercial PCB mixtures. In contrast, microbial dechlorination of PCBs under anaerobic conditions results in the dechlorination of PCB congeners that are more extensively chlorinated and more resistant to aerobic attack. Although there have been several reports concerning the microbes that catalyze the aerobic biodegradation of PCBs, dechlorinating anaerobes have eluded isolation and identification. This has impeded further research into the mechanisms of anaerobic PCB dechlorination.Recently, two PCB-dechlorinating enrichment cultures derived from estuarine sediment were established in a defined and sediment-free medium. One of these cultures is capable of ortho PCB dechlorination (8), and the other (the DF culture) is restricted to dechlorination of PCB congeners with doubly flanked chlorines (27). The available evidence suggests that different mic...

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