Stabilization of chromium by reductase enzyme treatment

Hexavalent chromium (Cr(VI)) is highly toxic, and a major heavy metal contaminant in the environment. An important strategy for bioremediating Cr(VI) is to microbiologically reduce it to less toxic Cr(III). One of the major routes of bacterial chromate reduction is enzymatic reduction mediated by chromate reductase after its uptake inside the cell. While the enzymatic pathway is common in aerobic bacteria, it probably also occurs anaerobically. To gain more insight about this pathway, we investigated the reduction of Cr(VI) by a highly resistant Bacillus species. Reduction depended on which of three organic substrates was used, i.e., glucose, lactate and acetate at 0.1 to 2.0 mM, and the rate of reduction decreased with increasing concentration of chromate. Because sulfhydryl sites are known to be active sites for enzyme reductase, bacterial growth and reduction of Cr(VI) by Bacillus in the presence of varying concentrations of sulfate and thiosulfate were investigated. While changes in sulfate did not affect the reduction rate, raising the thiosulfate concentration in the medium from 0.05 to 1.0 mM markedly increased the reduction rate. Thiosulfate enhanced the reduction of Cr(VI), probably by accelerating the biosynthesis of chromate reductase enzymes, although other mechanisms may be involved. Our results show that the enzymes or other substances mediating the reduction reside mostly in the cytoplasm. This reductase enzyme could be extracted and applied sites contaminated with Cr(VI), to convert it to Cr(III), which would prevent leaching of the pollutant to groundwater.