EVOLUTION OF A PATHWAY FOR CHLOROBENZENE METABOLISM LEADS TO NATURAL ATTENUATION IN CONTAMINATED GROUNDWATER

Complete metabolism of chlorinated benzenes is not a feature that is g enerally found in aerobic bacteria but is thought to be due to a novel recombination of two separate gene clusters. Such a recombination cou ld be responsible for adaptation of a natural microbial community in r esponse to contamination with synthetic chemicals. This hypothesis was tested in a chlorobenzene (CB)-contaminated aquifer. CB-degrading bac teria from a contaminated site were characterized for a number of year s by examining a combination of growth characteristics and DNA-DNA hyb ridization, PCR, and DNA sequence data. The genetic information obtain ed for the CB pathway of the predominant microorganism, Ralstonia sp. strain JS705, revealed a unique combination of (partially duplicated) genes for chlorocatechol degradation and genes for a benzene-toluene t ype of aromatic ring dioxygenase. The organism was detected in CB-poll uted groundwater by hybridizing colonies cultivated on low-strength he terotrophic media with probes for the CB pathway. Southern hybridizati ons performed to determine the organization of the CB pathway genes an d the 16S ribosomal DNA indicated that CB-degrading organisms isolated from different wells at the site were identical to JS705. Physiologic al characterization by the Biolog test system revealed some difference s. The genes for the aromatic ring dioxygenase and dihydrodiol dehydro genase of JS705 were detected in toluene and benzene degraders from th e same site. Our results suggest that recent horizontal gene transfer and genetic recombination of existing genes between indigenous microor ganisms were the mechanisms for evolution of the catabolic pathway. Ev olution of the CB pathway seems to have created the capacity for natur al attenuation of CB at the contaminated site.