MICROBIOLOGY AND MOLECULAR IDENTIFICATION OF PERCHLORATE BIOREDUCTION

Microbial perchlorate respiration has been known for over half a century. Although originally associated with nitrate respirers it is now known that specialized perchlorate-reducing organisms (DPRB) have evolved which reduce perchlorate into innocuous chloride. Over the last decade our studies have shown the ubiquity and diversity of DPRB and investigated the environmental, biochemical, and genetic factors involved in this metabolism. DPRB were isolated from diverse environments both pristine and contaminated. The isolates utilize a broad range of energy sources both organic and inorganic. All the DPRB are facultatively anaerobic or microaerophilic and some also respire nitrate. The DPRB are phylogenetically diverse with members in four of the Proteobacteria subclasses. Several DPRB belong to previously defined microbial genera not recognized for perchlorate respiration, however, the majority are closely related to each other and to the phototrophic Rhodocyclus species. The known close relatives to the DPRB do not grow by perchlorate respiration regardless of 16S rDNA relatedness making predictions of metabolic functionality difficult. The Dechloromonas and the Azospira species represent the dominant DPRB in the environment. These species grow over a broad range of environmental conditions; but generally grow optimally at circumneutral pH values in freshwater.

Biochemical studies revealed the involvement of c-type cytochrome(s) in this metabolism. Molecular studies indicated the presence of a molybdenum-dependent chaperone gene in association with the gene encoding perchlorate reductase in Dechloromonas strain RCB. Now it is known that molybdenum is a required cofactor by DPRB. The dismutation of chlorite into chloride and O2 is a recognized central step common to all DPRB. It is mediated by a highly conserved enzyme, chlorite dismutase (CD). Phenotypic studies with D. agitata and A. suillum indicate that CD activity is only present when the organisms are grown anaerobically on perchlorate and is negatively regulated by oxygen and nitrate. As such, this gene and the protein it encodes represent ideal biomarkers unique to DPRB. Several probes based on this concept have now been developed that allow the rapid and sensitive identification of the presence and metabolic function of these organisms in the environment.