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The organohalide respiring bacteria, Dehalococcoides mccartyi, are pertinent to bioremediation of perchloroethene (PCE) and trichloroethene (TCE), due to their unique metabolic capability to transform chlorinated ethenes to the non-toxic, ethene. Despite their widespread environmental distribution, biostimulation of Dehalococcoides in soil/sediment microcosm studies and during in situ bioremediation has failed to consistently promote reductive dechlorination of PCE/TCE beyond cis-dichloroethene (cis-DCE), commonly attributing this observation to the lack of strains with cis-DCE and vinyl chloride-reducing abilities. Consistent with this observation, in our study, microcosms established with garden soil and mangrove sediment stalled at cis-DCE, even after an extended incubation (200 days) and multiple biostimulation events. Only microcosms containing PCE-contaminated groundwater sediment produced ethene as the end-reduced product. Transfers from cis-DCE stalled microcosms to fresh medium in the absence of soil or sediment, however, yielded complete dechlorination of TCE to ethene, thus, unveiling the “true” biological potential of the endogenous Dehalococcoides. We hypothesized that biostimulation of Dehalococcoides in our cis-DCE-stalled microcosms was impeded by soil or sediment components serving as terminal electron acceptors for growth of microbes competing with Dehalococcoides for electron donor (H2). Therefore, dilution of the soil/sediment and competing microorganisms fostered the growth of Dehalococcoides and the production of ethene in the microcosm transfers. Our findings support this hypothesis through several lines of evidence. We found that i) Proteobacteria classes which dominated soil/sediment microbial communities became undetectable in the enrichment cultures, ii) the enrichment cultures developed contained up to 109 Dehalococcoides mccartyi cells mL−1 and achieved close to dechlorination of 0.5 mmol L−1 TCE to >80% ethene in 1.7 days, iii) methanogenesis drastically decreased in the enrichment cultures, and iv) garden soil and mangrove sediment microcosms bioaugmented with their respective enrichment cultures produced ethene. Our results provide an alternate explanation to “unsuccessful” microcosm experiments, providing a new perspective from which to better understand contaminated site assessments and to improve the bioremediation process. |