Microbial Transformation of the Chiral Pollutants Mecoprop and Dichlorprop

Summary

Mecoprop and dichlorprop, two chiral herbicides, are enantioselectively degraded by Sphingomonas herbicidovorans MH. Experiments on the biochemistry of mecoprop and dichlorprop metabolism in this bacterium show that one reason for enantioselectivity is the existence of two distinct α-ketoglutarate-dependent dioxygenases—one that is specific for the R enantiomers and one that is specific for the S enantiomers of mecoprop and dichlorprop. Uptake experiments with radiolabeled dichlorprop provide evidence that the first step in the degradation of dichlorprop and mecoprop by strain MH is active transport and that two inducible, proton gradient-driven uptake systems exist—one uptake system for (R)-dichlorprop and (R)-mecoprop and another one for (S)-dichlorprop and (S)-mecoprop. This indicates that the enantioselective degradation of mecoprop and dichlorprop by the soil isolate S. herbicidovorans MH is not only due to enantioselective metabolism but also to enantioselective uptake. Because selective enrichment of one of the enantiomers may occur in the environment it is important to assess the environmental impact of stereoisomers separately. A field study of chiral mecoprop in a contaminated aquifer downstream of a former waste disposal site in Switzerland supports this notion as the study clearly shows that the R enantiomer enriches during groundwater passage of the landfill leachate that was polluted with racemic mecoprop. We conclude that enantioselective microbial degradation increases the enantiomeric ratio of (R)- to (S)-mecoprop and suggest that analysis of enantiomeric ratios of chiral contaminants such as mecoprop can provide information about in situ biodegradation processes.