Fraction of electrons consumed in electron acceptor reduction and hydrogen thresholds as indicators of halorespiratory physiology

Abstract

Measurements of the hydrogen consumption threshold and the tracking of electrons transferred to the chlorinated electron acceptor (f(e)) reliably detected chlororespiratory physiology in both mixed cultures and pure cultures capable of using tetrachloroethene, cis-1, 2-dichloroethene, vinyl chloride, 2-chlorophenol, 3-chlorobenzoate, 3-chloro-4-hydroxybenzoate, or 1,2-dichloropropane as an electron acceptor. Hydrogen was consumed to significantly lower threshold concentrations of less than 0.4 ppmv compared with the values obtained for the same cultures without a chlorinated compound as an electron acceptor. The f(e) values ranged from 0.63 to 0.7, values which are in good agreement with theoretical calculations based on the thermodynamics of reductive dechlorination as the terminal electron-accepting process. In contrast, a mixed methanogenic culture that cometabolized 3-chlorophenol exhibited a significantly lower f(e) value, 0.012.

FIG. 1

H₂ consumption by strain 2CP-C with 2-CP as the electron acceptor and H₂ as the electron donor. (A) Two freshly inoculated cultures containing 0.2 mM acetate were fed 2-CP. After the H₂ concentration was less than 0.01 ppmv, the cultures were fed 100 ppmv of H₂. 2-CP was respiked when it was depleted, except for the 14-day period indicated by the arrows. (B) Triplicate cultures were initially fed 2 mM acetate and 2-CP. After 4 mM 2-CP had been added, all of the acetate was consumed and the H₂ threshold concentrations were less than 0.01 ppmv. The cultures were then fed 2-CP and 2,500 ppmv of H₂, and H₂ consumption was monitored again. Note the logarithmic scale of the y axis. The detection limit for H₂ was 0.01 ppmv, and the open circles indicate data that could not be quantified due to the limitations of the detection method. The data were averaged, and the variability between cultures was less than 20%. No H₂ consumption was observed in cultures that did not receive 2-CP.

FIG. 2

Graphical determination of f_e values for dechlorinating cultures, in which the amounts of reducing equivalents (2[H]) generated during oxidation of the electron donor were plotted against the amounts of electron acceptor reduced. The f_e is indicated by the slope of the regression line. (A) Desulfuromonas sp. strain BB1 (acetate plus PCE). (B) Desulfitobacterium sp. strain Viet1 (lactate plus PCE). (C) OM enrichment culture (H₂ plus 2,5-DCP) and strain 2CP-C (acetate plus 2-CP). (D) Desulfitobacterium chlororespirans (lactate plus 3Cl-4-HBA) and methanogenic enrichment culture (VFA mixture and 3-CP).