Effects of Sulfate and Sulfide on Dehalococcoides Strain 195 and Desulfovibrio vulgaris Hildenborough Ability to Biodegrade Trichloroethene

Abstract:

Trichloroethene (TCE) is a prevalent groundwater contaminant across the USA. Dehalococcoides is the only known bacterium that can reductively dechlorinate TCE to the benign end product ethene. Sulfate is also a common ubiquitous compound in groundwater. Little research has been conducted on the toxicity of sulfate or sulfide on Dehalococcoides and its effect on dechlorination. This study evaluated the inhibitory effects of high sulfate/sulfide concentrations on pure cultures Dehalococcoides mccartyi 195 (strain 195), sulfate reducer Desulfovibrio vulgaris Hildenborough (DVH) and the syntrophic co-culture of strain 195 with DVH.

Strain 195 was maintained in defined medium with H₂ as electron donor, acetate as carbon source, and 78 μmol TCE as the electron acceptor. 2.5 mM sulfate showed no inhibitory effect on strain 195 dechlorination rates (5.3 μmol d^-1); whereas, 2mM sulfide completely inhibited ethene production and only 14 μmol of cis-DCE was produced. DVH was maintained using lactate as the energy and carbon source and sulfate as the electron acceptor. High sulfate concentrations (30 mM) did not show an inhibitory effect on DVH growth and 90% of the amended sulfate was reduced within 24 hours. The cell yield of DVH was 1.3±0.1x10⁸ mL^-1, which was comparable to the control (1.5±0.2x10⁸ mL^-1). Similarly, 5mM sulfide did not show an inhibitory effect on the cell growth of DVH (1.8±0.3x10⁸ mL^-1).

In the syntrophic co-culture containing strain 195 and DVH, DVH fermented lactate to H₂ and acetate, and strain 195 used H₂ as the electron donor for dechlorination. When 1 mM sulfate was amended to the consortia, the TCE dechlorination rate (13 μmol d^-1) was similar to the control (no sulfate addition); whereas, the cell number of DVH increased from 8.7±0.4x10⁷ (control) to 2.6±0.7x10⁸ (1 mM sulfate). TCE dechlorination was completely inhibited when 5mM sulfate was amended, and only 10% of the total TCE added was dechlorinated to cis-DCE (8μmol). Accordingly, little growth of strain 195 was observed and DVH cell number in the 5mM bottles (7.7±0.6x10⁸ mL^-1) was 9 times higher than those maintained in control bottles (8.7±0.4x10⁷). The results showed sulfide had a significant inhibitory effect on Dehalococcoides’ dechlorination and cell growth rates in this study.