Physiological and ecological interactions between chromate and nitrate reduction in microbial model systems

Though microbial reduction is a promising strategy for chromium (Cr(VI)) remediation, Cr(VI) reduction in the presence of other electron acceptors has been poorly understood. The focus of this research is on Cr(VI) reduction in the presence of nitrate, a common component of groundwater and a preferred electron acceptor for bacteria. Two model systems were chosen: pure cultures of Shewanella oneidensis MR-1 and a microbial community from a contaminated aquifer in Schoolcraft, MI. The original hypothesis was that Cr(VI) and nitrate reduction would not occur concurrently due to the higher half-reaction reduction potential of nitrate. However, studies of MR-1 showed that Cr(VI) and nitrate were simultaneously reduced, birthing the idea that nitrate could be used as an electron acceptor to build Cr(Vl)-reducing biomass. However, very little is known about the Cr(Vl)-reducing capabilities of denitrifying bacteria or the potentially toxic effects of Cr(VI) on denitrification. My hypothesis was that increasing Cr(VI) concentrations would inhibit nitrate reduction, which our studies of MR-1 confirmed. However, kinetic studies showed rapid rates of Cr(VI) reduction as long as Cr(VI) was below toxic levels. To gain insight into the mechanism of Cr(VI) reduction in MR-1, whole-genome DNA microarrays were used to examine the gene expression profile during Cr(VI) reduction. Studies of the second model system were performed using slurries of groundwater and aquifer sediment, a denitrifying consortium derived from a sediment slurry, and nitrate-reducing/denitrifying pure cultures isolated from the consortium. Nitrate and Cr(VI) were concurrently reduced. Analysis of terminal restriction fragment length polymorphism (T-RFLP) and phylogenetic analyses revealed that isolates from the Cr-exposed community were related to members of the genera Enterobacter, Acidovorax, Janibacter, and Micrococcus. As observed with MR-1, increasing levels of Cr(VI) decreased rates of nitrate reduction, but the consortium tolerated high levels of Cr(VI) better than the isolates or MR-1. Results confirmed that biostimulation of denitrifiers to increase concentrations of Cr(VI)-reducing biomass is a promising remediation strategy as long as Cr(VI) is at levels low enough for denitrification. This work also highlighted the importance of electron donor choice and concentration in achieving the maximum contaminant level of 100 mug/L of total soluble Cr.