Bioreduction and immobilization of hexav

The extremely acidophilic, Fe(III)-reducing heterotrophic bacterium Acidocella aromatica strain PFBC was tested for its potential utility in bioreduction of highly toxic heavy metal, hexavalent chromium, Cr(VI). During its aerobic growth on fructose at pH 2.5, 20 µM Cr(VI) was readily reduced to Cr(III), achieving the final Cr(VI) concentration of 0.4 µM (0.02 mg/L), which meets the WHO drinking water guideline of 0.05 mg/L. Despite the highly inhibitory effect of Cr(VI) on cell growth at higher concentrations, particularly at low pH, Cr(VI) reduction was actively recorded in growth-enriched cell suspensions under micro-aerobic and anaerobic conditions. Strain PFBC was unable to grow anaerobically via dissimilatory reduction of Cr(VI). The microbial reduction of Fe(III) was only observed after complete reduction of Cr(VI) to Cr(III). Following Cr(VI) reduction, the resultant Cr(III) was partially immobilized on the cell surface through biosorption. When Cr(III) was externally supplied, it was poorly immobilized on the cell surface. This study reports the first instance of Cr(VI) reducing ability in Acidocella sp., discussing its potential role in biogeochemical cycling of Cr and its possible utility in Cr(VI) bioremediation in highly acidic environments.