Chromate reduction kinetics and chromium(VI) toxicity in Shewanella oneidensis MR-1

Microbial transformation of Cr(VI) to Cr(III) is a potential technology for remediating sites contaminated with Cr(VI) since chromium, in the trivalent form, is much less soluble, mobile and toxic compared to its hexavalent form. For the successful implementation of this technology, it is important to understand the kinetics of the biotransformation, microbial physiological conditions that enhance or decrease the reaction rate, influence of other in-situ contaminants on kinetics as well as toxicity of Cr(VI) to the microorganisms that perform Cr(VI) reduction. In the present study, using the well-known metal reducing bacterium Shewanella oneidensis MR-1 as a model microorganism, kinetic studies on Cr(VI) reduction were performed under anaerobic conditions during growth on fumarate and nitrate and at different physiological growth conditions. In addition, inhibition of Cr(VI) reduction rates was studied in the presence of nitrite, which is a potential co-contaminant present in Cr(VI) contaminated sites. The results from these studies showed that Cr(VI) reduction is carried out by multiple mechanisms working in parallel and that some of these mechanisms are induced during anaerobic growth. Taking into account these results, a novel dual-enzyme kinetic model was developed to describe Cr(VI) reduction by stationary phase cultures of MR-1. Toxicity of Cr(VI) to MR-1 under aerobic and anaerobic conditions was also studied as a function of the physiological state of growth of MR-1. It was observed that there was a significant decrease in specific growth rate and cell yields of aerobic cultures after exposure to Cr(VI) (0.04mM) and that aerobic cultures were unable to reduce all the Cr(VI) from solution. Under anaerobic conditions, it was found that at even at very low concentrations (0.02mM), Cr(VI) has acute toxic effects on MR-1 such that anaerobic growth was completely arrested immediately upon exposure to Cr(VI) and growth could only resume after reduction of Cr(VI) from solution to below detection limits (0.002mM). In addition, comparison of specific growth rates of anaerobic cultures subsequent to Cr(VI) reduction, showed that Cr(VI) toxicity mechanisms are dependent on physiological state of growth.