Chromate/nitrite interactions in Shewanella oneidensis MR‐1: Evidence for multiple hexavalent chromium [Cr(VI)] reduction mechanisms dependent on physiological growth conditions

Abstract

Inhibition of hexavalent chromium [Cr(VI)] reduction due to nitrate and nitrite was observed during tests with Shewanella oneidensis MR-1 (previously named Shewanella putrefaciens MR-1 and henceforth referred to as MR-1). Initial Cr(VI) reduction rates were measured at various nitrite concentrations, and a mixed inhibition kinetic model was used to determine the kinetic parameters-maximum Cr(VI) reduction rate and inhibition constant [V(max,Cr(VI)) and K(i,Cr(VI))]. Values of V(max,Cr(VI)) and K(i,Cr(VI)) obtained with MR-1 cultures grown under denitrifying conditions were observed to be significantly different from the values obtained when the cultures were grown with fumarate as the terminal electron acceptor. It was also observed that a single V(max,Cr(VI)) and K(i,Cr(VI)) did not adequately describe the inhibition kinetics of either nitrate-grown or fumarate-grown cultures. The inhibition patterns indicate that Cr(VI) reduction in MR-1 is likely not limited to a single pathway, but occurs via different mechanisms some of which are dependent on growth conditions. Inhibition of nitrite reduction due to the presence of Cr(VI) was also studied, and the kinetic parameters V(max,NO2) and K(i,NO2) were determined. It was observed that these coefficients also differed significantly between MR-1 grown under denitrifying conditions and fumarate reducing conditions. The inhibition studies suggest the involvement of nitrite reductase in Cr(VI) reduction. Because nitrite reduction is part of the anaerobic respiration process, inhibition due to Cr(VI) might be a result of interaction with the components of the anaerobic respiration pathway such as nitrite reductase. Also, differences in the degree of inhibition of nitrite reduction activity by chromate at different growth conditions suggest that the toxicity mechanism of Cr(VI) might also be dependent on the conditions of growth. Cr(VI) reduction has been shown to occur via different pathways, but to our knowledge, multiple pathways within a single organism leading to Cr(VI) reduction has not been reported previously.

Genome Sequencing Revealed Chromium and Other Heavy Metal Resistance Genes in E. cloacae B2-Dha

“…As described in the results, B2-DHA contains a number of chromate reductase genes and most of these have NAD(P) H-dependent oxidoreductase activity (Table 4). Similar kind of results has been reported previously by [56]. B2-DHA also harbors soluble quinone oxidoreductases that are expected to reduce Fe 3+ and Cr 6+ and counter oxidative stress [57].…”

Isolation of a High-Affinity Functional Protein Complex between OmcA and MtrC: Two Outer Membrane Decaheme c -Type Cytochromes of Shewanella oneidensis MR-1

“…Furthermore, to reduce insoluble metals such as Fe(III) and Mn(III/IV) at neutral pH in oxidizing environments, this network must be able to transfer electrons from the cytoplasmic membrane where electrons are generated to the extracellular surface of the outer membrane, where reduction is thought to occur (27). Identification of the proteins and their functions in mediating metal reduction is of considerable importance both from a standpoint of understanding bacterial physiology and in order to facilitate bioremediation of contaminated sites, since these metal-reducing activities may also function in the reduction and immobilization of toxic metals, including U(VI), Tc(VII), and Cr(VI) (25,(36)(37)(38)(39)(40).…”

Transcriptome Profiling of Shewanella oneidensis Gene Expression following Exposure to Acidic and Alkaline pH

“…This metabolically versatile organism is of particular interest because of its potential utility in the in situ microbial bioreduction and immobilization of various anthropogenic pollutants, such as Cr(VI) (23,38), and as a result, the complete DNA sequence of the S. oneidensis MR-1 genome has been determined (15). However, the application of such dissimilatory metal-reducing bacteria to contaminated sites can be complicated by unpredictable interactions with other microbial metabolisms and environmental stressors, such as variations in external pH, salinity, and oxidative conditions.…”