Microbial Chain Elongation And Subsequent Fermentation Of Elongated Carboxylates As H-2-Producing Processes For Sustained Reductive Dechlorination Of Chlorinated Ethenes

In situ anaerobic groundwater bioremediation of trichloroethene (TCE) to nontoxic ethene is contingent on organohalide-respiring Dehalococcoidia, the most common strictly hydrogenotrophic Dehalococcoides mccartyi (D. mccartyi). The H-2 requirement for D. mccartyi is fulfilled by adding various organic substrates (e.g., lactate, emulsified vegetable oil, and glucose/molasses), which require fermenting microorganisms to convert them to H-2. The net flux of H-2 is a crucial controlling parameter in the efficacy of bioremediation. H-2 consumption by competing microorganisms (e.g., methanogens and homoacetogens) can diminish the rates of reductive dechlorination or stall the process altogether. Furthermore, some fermentation pathways do not produce H-2 or having H-2 as a product is not always thermodynamically favorable under environmental conditions. Here, we report on a novel application of microbial chain elongation as a H-2-producing process for reductive dechlorination. In soil microcosms bioaugmented with dechlorinating and chain-elongating enrichment cultures, near stoichiometric conversion of TCE (0.07 +/- 0.01, 0.60 +/- 0.03, and 1.50 +/- 0.20 mmol L-1 added sequentially) to ethene was achieved when initially stimulated by chain elongation of acetate and ethanol. Chain elongation initiated reductive dechlorination by liberating H-2 in the conversion of acetate and ethanol to butyrate and caproate. Syntrophic fermentation of butyrate, a chain-elongation product, to H-2 and acetate further sustained the reductive dechlorination activity. Methanogenesis was limited during TCE dechlorination in soil microcosms and absent in transfer cultures fed with chain-elongation substrates. This study provides critical fundamental knowledge toward the feasibility of chlorinated solvent bioremediation based on microbial chain elongation.