ABSTRACTVinyl chloride (VC) is a human carcinogen that accumulates in soil and groundwater due to incomplete dechlorination of chlorinated ethenes. Some strains of obligate organohalide respiring Dehalococcoides mccartyi can synthesize the VC reductase that catalyzes the dechlorination of VC to ethene. The gene encoding the VC reductase, vcrA, is found on a mobile genetic element called the vcrA-Genomic Island (GI) that may participate in horizontal gene transfer. We designed an experiment to try to induce horizontal gene transfer of the vcrA-GI by mixing two enrichment cultures: one containing the donor D. mccartyi strain with the vcrA-GI that could not fix nitrogen and the second containing the recipient strain devoid of the vcrA-GI that could fix nitrogen. Therefore, mixing the two cultures in medium without ammonium while providing VC as the sole electron acceptor was hypothesized to select for a mutant strain of D. mccartyi that could both fix nitrogen and respire VC. However, after over 4 years of incubation, no evidence for horizontal gene transfer of the vcrA-GI was found. Rather, we observed VC-dechlorinating activity attributed to the TCE reductase, TceA, in the recipient strain. We also observed that D. mccartyi can grow by scavenging low concentrations of fixed nitrogen sources. During this experiment we identified two additional D. mccartyi strains in the KB-1 TCE-enriched culture that could fix nitrogen. The presence of multiple strains of D. mccartyi with distinct phenotypes may enhance bioaugmentation success, but here it may have undermined attempts to force horizontal gene transfer of the vcrA-GI.IMPORTANCEDehalococcoides mccartyi are a powerful bioremediation tool for the degradation of chlorinated solvent contamination in soil and groundwater. Only a few D. mccartyi strains have the ability to dechlorinate toxic chlorinated compounds like vinyl chloride. Interestingly, the genetic ability to dechlorinate vinyl chloride is theorized to be shared among D. mccartyi strains. In this study we attempted to promote the genetic transfer of vinyl chloride degrading ability from one D. mccartyi strain to another. Although we did not observe this exchange, our findings suggest there may be restrictions of genetic transfer between specific clades or sub-groups of D. mccartyi strains. Developing our understanding of genetic transfer among D. mccartyi strains could allow for enhanced degradation of chlorinated solvent contamination in situ.
Application of an iterative source localization strategy at a chlorinated solvent site
