Abstract
1,2-Dichloroethane (1,2-DCA) is one of the most abundant manmade chlorinated organic contaminants in the world. Reductive dechlorination of 1,2-DCA by organohalide-respiring bacteria (OHRB) can be impacted by other chlorinated contaminants such as chloroethenes and chloropropanes that can co-exist with 1,2-DCA at contaminated sites. The aim of this study was to evaluate the effect of chloroethenes and 1,2-dichloropropane (1,2-DCP) on 1,2-DCA dechlorination using sediment cultures enriched with 1,2-DCA as the sole chlorinated compound (EA culture) or with 1,2-DCA and tetrachloroethene (PCE) (EB culture), and to model dechlorination kinetics. Both cultures contained Dehalococcoides as most predominated OHRB, and Dehalogenimonas and Geobacter as other known OHRB. In sediment-free enrichments obtained from the EA and EB cultures, dechlorination of 1,2-DCA was inhibited in the presence of the same concentrations of either PCE, vinyl chloride (VC), or 1,2-DCP; however, concurrent dechlorination of dual chlorinated compounds was achieved. In contrast, 1,2-DCA dechlorination completely ceased in the presence of cis-dichloroethene (cDCE) and only occurred after cDCE was fully dechlorinated. In turn, 1,2-DCA did not affect dechlorination of PCE, cDCE, VC, and 1,2-DCP. In sediment-free enrichments obtained from the EA culture, Dehalogenimonas 16S rRNA gene copy numbers decreased 1–3 orders of magnitude likely due to an inhibitory effect of chloroethenes. Dechlorination with and without competitive inhibition fit Michaelis-Menten kinetics and confirmed the inhibitory effect of chloroethenes and 1,2-DCP on 1,2-DCA dechlorination. This study reinforces that the type of chlorinated substrate drives the selection of specific OHRB, and indicates that removal of chloroethenes and in particular cDCE might be necessary before effective removal of 1,2-DCA at sites contaminated with mixed chlorinated solvents.
Highlights
Understanding biodegradation bottlenecks has been a major objective in efforts to harness the metabolic potential of microorganisms for bioremediation of sites contaminated with organic pollutants (Atashgahi et al 2018; Meckenstock et al 2015; Vandermaesen et al 2016)
Kinetic modeling using the same culture revealed that 1,2-DCA dechlorination was strongly inhibited by cisdichloroethene, and efficient 1,2-DCA dechlorination occurred only when cDCE was completely dechlorinated to vinyl chloride (VC) (Mayer-Blackwell et al 2016)
The 16S ribosomal RNA (rRNA) gene numbers of Dehalobacter, Desulfitobacterium, and Sulfurospirillum in both EA and EB sediment cultures were below 106 copies/mL, representing less than 0.1% of the total bacterial 16S rRNA gene number (Fig. 2b and d)
Summary
Understanding biodegradation bottlenecks has been a major objective in efforts to harness the metabolic potential of microorganisms for bioremediation of sites contaminated with organic pollutants (Atashgahi et al 2018; Meckenstock et al 2015; Vandermaesen et al 2016). Organohalide respiration (OHR) is an example of a microbial metabolism that has been successfully harnessed for engineered remediation of sites contaminated with chlorinated solvents (Atashgahi et al 2017; Edwards 2014; Ellis et al 2000) This process is mediated by organohalide-respiring bacteria (OHRB) belonging to distinct genera within the phyla Chloroflexi (e.g., Appl Microbiol Biotechnol (2019) 103:6837–6849. During dechlorination of co-mingled organohalogens, bioattenuation of specific chlorinated solvents has been shown to be prone to inhibition due to the inhibitory effect of dechlorination intermediates on OHRB, their reductive dehalogenase enzymes, and their syntrophic partners (Chan et al 2011; Dillehay et al 2014; Grostern et al 2009; Mayer-Blackwell et al 2016). An improved understanding of such inhibitory effects can aid in designing bioremediation approaches for sites contaminated with a mixture of chloroethenes, chloroethanes, and/or chloropropanes (Dillehay et al 2014; Field and Sierra-Alvarez 2004; MayerBlackwell et al 2016)
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