Hongyan Wang,Huijuan Jin,Jingjing Wang,Xin Wang,Xiuying Li,Jun Yan,Yi Yang
Organohalide-respiring bacteria (OHRB) have been found in various environments and play an indispensable role in the biogeochemical cycling and detoxification of halogenated organic compounds (HOCs). Currently, few ORHB have been reported to perform reductive dechlorination under high salinity conditions, indicating a knowledge gap on the diversity of OHRB and the survival strategy of OHRB in saline environments (e.g., estuarine, marine). This study reports the characterization of an enrichment culture dominated by a new Dehalogenimonas population strain W derived from estuarine sediments, which demonstrates the capability to dechlorinate 1,2-dichloroethane (1,2-DCA) to ethene under elevated salinity (≥5.1% NaCl, w/v). Metagenomic and proteomic analyses revealed that the distinctive high-salinity dechlorination of strain W is primarily attributed to a putative reductive dehalogenase (RDase) DdeA, which shares >91.4% amino acid identity with the dihaloeliminating RDase DcpA from other Dehalogenimonas strains. Additionally, ectoine biosynthesis enzymes (EctABC) contribute to the strain’s salt tolerance. These findings underscore the potential of OHRB, particularly Dehalogenimonas, to detoxify HOCs in high-salinity environments, such as estuarine and marine ecosystems, by employing compatible solutes as an adaptive mechanism.
Microbial reductive dechlorination has been considered an effective process for the clean-up of organohalide-contaminated sites. Heavy metal ions are commonly present as co-contaminants in various organohalide-contaminated sites. To understand the impacts of heavy metal ions on the environmental fate of organohalides, we investigated the effects of Zn2+, Cu2+ and Cd2+ on reductive dechlorination of tetrachloroethene (PCE) and 1,2-dichloroethane (1,2-DCA) in sediment microcosms and transferred enrichment cultures. PCE and 1,2-DCA-dechlorinating enrichment cultures could be consecutively transferred in the presence of up to 10 mg/L Cu2+ or 10 mg/L Zn2+; by comparison, up to 50 mg/L Cd2+ had minor impacts on the microbial reductive dechlorination of PCE and 1,2-DCA. The inhibitory effects of tested heavy metal ions on microbial reductive dechlorination ranked in descending order are Zn2+, Cu2+, and Cd2+. Community profiling and principal component analysis indicate that the concentration and type of contaminants (e.g., heavy metal ions, organohalides) shaped the microbial community structure, an observation similar to a prior report. The enrichment of certain organohalide-respring bacteria (e.g., Dehalococcoides, Dehalogenimonas) during continuous transfers exposed to heavy metal ions suggests that they are capable of tolerating high concentrations of heavy metal ions. Our findings provide insights into the impacts of heavy metal ions on microbial reductive dechlorination and may be helpful for in situ bioremediation at sites contaminated with organohalides and heavy metals.
4-Hydroxy-chlorothalonil (4-OH-TPN), the main metabolite of the widely used fungicide chlorothalonil, has recently been reported to be reductively dechlorinated by an anaerobic consortium containing Dehalogenimonas. While the pure culture of the Dehalogenimonas has not been isolated and the reductive dehalogenase genes (rdhAs) responsible for the 4-OH-TPN dechlorination remain unclear. The assembled genome of Dehalogenimonas contained a 1,2-dichloroethane (1,2-DCA) reductive dehalogenase gene (dcpA) ortholog. In this study, by alternating the substrate from 4-OH-TPN to 1,2-DCA, a pure culture of Dehalogenimonas, designated as 4OHTPN1, was finally isolated. Strain 4OHTPN1 potentially represents a novel species in the genus Dehalogenimonas and contains 33 rdhA genes and 4 rdhB genes. The novel rdhA26 gene (designated htpnA) was identified as the key gene involved in 4-OH-TPN dechlorination. Its transcriptional and protein expression levels were 25.0-fold and 10.2-fold higher than the average of all rdhA genes when the culture was solely amended with 4-OH-TPN. In contrast, the dcpA ortholog, rdhA25, exhibited 10.1-fold and 9.8-fold higher transcriptional and protein expression levels when only amended with 1,2-DCA, highlighting a substrate-specific regulatory response to a distinct electron acceptor. This study expands our knowledge of the rdhA gene for 4-OH-TPN dechlorination in the pure culture of Dehalogenimonas.
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