Anaerobic reductive dehalogenation of trichloroethylene in unacclimated freshwater sediments

Currently, “halorespiration” dominates the environmental engineering and microbiological literature as an explanation for the occurrence of microbially mediated reductive dehalogenation in the environment. However, the halorespiration theory does not account for gratuitous dehalogenation by a number of non-respiring organisms, nor the fact that reductive dehalogenation is ubiquitous in anaerobic environments, and often begins soon after contamination. Moreover, very few halorespiring microorganisms have been isolated despite this alleged widespread existence. The course of this research offers an additional explanation as to why anaerobic cultures have shown the ability to reductively dechlorinate organic pollutants. Many environments contaminated with chlorinated and non-chlorinated organic pollutants are anaerobic and have normal biological electron acceptors in short supply. The reductive dehalogenation of trichloroethylene by native fermenting organisms residing in anaerobic sediments that had never been previously exposed to trichloroethylene was examined. It was found that lactate undergoing fermentation to propionate and acetate supported dehalogenation concurrently with propionate fermentation. However, the addition of excess hydrogen, which inhibits fermentation, delayed the time to the onset of dehalogenation. A biochemical model, “halofermentation,” that is consistent with this data and environmental observations was proposed. The effects on microbially mediated reductive dehalogenation of ethanol, ethanol with lactate, and ethyl lactate, were also studied. Different substrate combinations have a significant effect on anaerobic communities. The ratio of the fermentation metabolites propionate and acetate, was found to be an excellent indicator of microbial activity and reductive dehalogenation. The monitoring of propionate to acetate concentration ratios in the field would provide insight into microbial activity that is currently not being obtained