The Microbial Ecology and Bioremediation of Chlorinated Ethene-Contaminated Environments

Abstract

The microbial ecology of tetrachloroethene (PCE)- and trichloroethene (TCE)-contaminated sites is complex. Fundamentally, accurate prediction of contaminant fate, the survival of dehalorespiring populations, and, thus, the performance of engineered bioremediation approaches at these sites are feasible only if the correct kinetic models are applied, and meaningful and mathematically independent parameter estimates are input into these models. A model that incorporates biomass inactivation at high chlorinated ethene concentrations, as well as the self-inhibitory and competitive inhibition effects that the elevated chlorinated ethene concentrations exert on dechlorination reactions, must be utilized to accurately predict dehalorespiring population substrate interactions and growth. The initial conditions used in batch laboratory kinetic assays, including the initial limiting substrate (S₀)-to-initial biomass concentration (X₀) ratio and the S₀-to-half-saturation constant (K_S) ratio, must be carefully selected to ensure that the parameter estimates are meaningful and independent. Kinetic assays conducted at appropriate S₀/X₀ and S₀/K_S ratios suggest that the substrate utilization kinetics of many PCE-to-dichloroethene (DCE) dehalorespirers are faster than those of Dehalococcoides mccartyi strains. Integration of mathematical simulations using appropriate dehalorespiration models and dehalorespiring co-culture experiments also showed that PCE-to-DCE dehalorespirers tend to outcompete D. mccartyi strains for higher chlorinated ethenes. Where dense nonaqueous-phase liquid (DNAPL) contamination is present, the fast substrate utilization kinetics of PCE-to-DCE dehalorespirers allow them to grow close to the DNAPL-water interface and control dissolution bioenhancement. Under excess electron donor conditions, D. mccartyi strains specialize in dehalorespiration of lesser chlorinated ethenes produced by PCE-to-DCE dehalorespirers. Maintenance of multiple dehalorespirers growing via complementary substrate interactions results in optimal utilization of electron equivalents, bioenhancement of DNAPL dissolution, and contaminant detoxification.