Acetylene fuels reductive dechlorination of TCE by Dehalococcoides/Pelobacter-containing microbial consortia

Abstract:

Groundwater contamination by trichloroethene (TCE) poses a threat to health and leads to the generation of vinyl chloride (VC), a carcinogen. Dehalococcoides mccartyi is the only bacterium that can completely dechlorinate TCE to ethene (C₂H₄). Acetylene (C₂H₂) occurs in TCE-contaminated sites as a consequence of chemical degradation of TCE. Yet acetylene inhibits a variety of microbial processes including methanogesis and reductive dechlorination. Pelobacter acetylenicus and related species can metabolize acetylene via acetylene hydratase and acetaldehyde dismutatse thereby generating acetate and H₂ as endproducts, which could serve as electron donor and carbon source for growth of D. mccartyi.

We found that 1mM acetylene (aqueous) inhibits growth of D. mccartyi strain 195 on 0.3 mM TCE, but that the inhibition was removed after 12 days with the addition of an acetylene-utilizing isolate from San Francisco Bay, Pelobacter strain SFB93. TCE did not inhibit the growth of this Pelobacter at the concentrations tested (0.1-0.5 mM) and TCE was not consumed by strain SFB93. Co-cultures of strain 195 with strain SFB93 at 5% inoculation were established in 120 mL serum bottles containing 40 mL defined medium. TCE was supplied at a liquid concentration of 0.1 mM, with 0.1 mM acetylene and N₂/CO₂ (90:10 v/v) headspace at 34 °C. Co-cultures were subsequently transferred (5% vol/vol inoculation) to generate subcultures after 20 µmol TCE was reduced to VC and 36 µmol acetylene was depleted. Aqueous H₂ ranged from 114 to 217 nM during TCE-dechlorination, and the cell yield of strain 195 was 3.7 ±0.3 × 10⁷ cells µmol^-1 Cl^- released. In a D. mccartyi-containing enrichment culture (ANAS) under the same conditions as above, it was found that inhibition of dechlorination by acetylene was reversed after 19 days by adding SFB93.

Thus we showed that a co-culture of Pelobacter SFB93 and D. mccartyi 195 could be maintained with C₂H₂ as the electron donor and carbon source while TCE served as the electron acceptor. Inhibition by C₂H₂ of reductive dechlorination in both the D. mccartyi isolate and the enrichment culture ANAS were observed, but the inhibition was eliminated by adding Pelobacter SFB93 to the cultures. These results will help facilitate the optimization of TCE-bioremediation at contaminated sites containing both TCE and C₂H₂.