Hydrogen thresholds and steady-state concentrations associated with microbial arsenate respiration.

H2 thresholds for microbial respiration of arsenate (As(V)) were investigated in a pure culture of Sulfurospirillum arsenophilum. H2 was consumed to threshold concentrations of 0.03-0.09 nmol/L with As(V) as terminal electron acceptor, allowing for a Gibbs free-energy yield of 36-41 kJ per mol of reaction. These thresholds are among the lowest measured for anaerobic respirers and fall into the range of denitrifiers or Fe(III)-reducers. In sediments from an arsenic-contaminated aquifer in the Red River flood plain, Vietnam, H2 levels decreased to 0.4-2 nmol/L when As(V) was added under anoxic conditions. When As-(V) was depleted, H2 concentrations rebounded by a factor of 10, a level similar to that observed in arsenic-free controls. The sediment-associated microbial population completely reduced millimolar levels of As(V) to arsenite (As-(III)) within a few days. The rate of As(V)-reduction was essentially the same in sediments amended with a pure culture of S. arsenophilum. These findings together with a review of observed H2 threshold and steady-state values suggest that microbial As(V)-respirers have a competitive advantage over several other anaerobic respirers through their ability to thrive at low H2 levels.

Temperature dependence of anaerobic TCE-dechlorination in a highly enriched Dehalococcoides-containing culture.

Temperature dependence of reductive trichloroethene (TCE) dechlorination was investigated in an enrichment culture (KB-1), using lactate or propionate as electron donors at a temperature interval from 4 to 60 degrees C. Dechlorination was complete to ethene at temperatures between 10 and 30 degrees C (lactate-amended) and between 15 and 30 degrees C (propionate-amended). Dechlorination stalled at cis-1,2-dichloroethene (cDCE) at 4 degrees C (lactate-amended), at and below 10 degrees C (propionate-amended), and at 40 degrees C with both electron donors. No dechlorination of TCE was observed at 50 and 60 degrees C. Concentrations of Dehalococcoides had increased or remained constant after 15 days of incubation at temperatures involving complete dechlorination to ethene. Temperature dependence of dechlorination rates was compared using zero order degradation kinetics and a Monod growth-rate model for multiple electron acceptor usage with competition. Maximum growth rates (mu) and zero order degradation rates were highest for TCE dechlorination at 30 degrees C with lactate as substrate (mu(TCE) of 7.00+/-0.14 days(-1)). In general, maximum growth rates and dechlorination rates of TCE were up to an order of magnitude higher than rates for utilization of cis-dichloroethene (cDCE, mu(c)(DCE) up to 0.17+/-0.02 days(-1)) and vinyl chloride (VC, mu(VC) up to 0.52+/-0.09 days(-1)). Temperature dependence of maximum growth rates and degradation rates of cDCE and VC were similar and highest at 15-30 degrees C, with growth rates on cDCE being lower than on VC. This study demonstrates that bioaugmentation of chlorinated ethene sites may be more efficient at elevated temperatures.

Experimental evidence for a lack of thermodynamic control on hydrogen concentrations during anaerobic degradation of chlorinated ethenes.

Hydrogen (H2) concentrations during reductive dechlorination of cis-dichloroethene (cDCE) and vinyl chloride (VC) were investigated with respectto the influence of parameters entering the Gibbs free energy expression of the reactions. A series of laboratory experiments was conducted employing a mixed, Dehalococcoides-containing enrichment culture capable of complete dechlorination of chlorinated ethenes. The objective was to investigate whether a constant energy gain controls H2 levels in dechlorinating systems, thereby evaluating the applicability of the partial equilibrium approach to microbial dechlorination at contaminated sites. Variations in the temperature between 10 and 30 degrees C did not affect the H2 concentration in a fashion that suggested thermodynamic control through a constant energy gain. In another set of experiments, H2 levels at constant ionic strength were independent of the chloride concentration between 10 and 110 mmol chloride per liter. These findings demonstrate that the partial equilibrium approach is not directly applicable to the interpretation of reductive degradation of chlorinated ethenes. We also present recalculated thermodynamic properties of aqueous chlorinated ethene species that allow for calculation of in-situ Gibbs free energy of dechlorination reactions at different temperatures.

Methanosarcina spp. drive vinyl chloride dechlorination via interspecies hydrogen transfer.

Two highly enriched cultures containing Dehalococcoides spp. were used to study the effect of aceticlastic methanogens on reductive vinyl chloride (VC) dechlorination. In terms of aceticlastic methanogens, one culture was dominated by Methanosaeta, while the other culture was dominated by Methanosarcina, as determined by fluorescence in situ hybridization. Cultures amended with 2-bromoethanesulfonate (BES), an efficient inhibitor of methanogens, exhibited slow VC dechlorination when grown on acetate and VC. Methanogenic cultures dominated by Methanosaeta had no impact on dechlorination rates, compared to BES-amended controls. In contrast, methanogenic cultures dominated by Methanosarcina displayed up to sevenfold-higher rates of VC dechlorination than their BES-amended counterparts. Methanosarcina-dominated cultures converted a higher percentage of [2-(14)C]acetate to (14)CO(2) when concomitant VC dechlorination took place, compared to nondechlorinating controls. Respiratory indices increased from 0.12 in nondechlorinating cultures to 0.51 in actively dechlorinating cultures. During VC dechlorination, aqueous hydrogen (H(2)) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H(2) levels increased by a factor of 10 to 100, indicating active hydrogen production from acetate oxidation. This process was thermodynamically favorable by means of the extremely low H(2) levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H(2) as electron donor, in cultures both with and without BES. These findings all indicate that Methanosarcina (but not Methanosaeta), while cleaving acetate to methane, simultaneously oxidizes acetate to CO(2) plus H(2), driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.

Effects of sulfate on anaerobic chloroethene degradation by an enriched culture under transient and steady-state hydrogen supply.

Complete anaerobic dechlorination of chlorinated solvents such as trichloroethene (TCE) is essential for bioremediation of chloroethene-contaminated sites. We studied the influence of sulfate on microbial dechlorination of TCE to ethene both under transient and steady-state conditions, encompassing the range of hydrogen (H2) levels commonly found at contaminated sites. The results show that sulfate at a concentration of 2.5 mM limits microbial dechlorination by a mixed anaerobic culture by reducing the rate under steady-state hydrogen supply (a few nM H2), implying a H2 limited dechlorination. Conversely, sulfate did not affect dechlorination when rapid fermentation of lactate resulted in transient buildup of H2 to levels around two orders of magnitude higher compared to steady-state conditions. This has important implications both for optimizing culture conditions for dehalogenating microorganisms and for the efficiency of cleanup strategies. Our findings may contribute to the understanding and bioremediation of chloroethene contaminated environments containing sulfate.