Toxicity of Cr(III) to Shewanella sp. strain MR-4 during Cr(VI) reduction

Bioremediation of chromium through the reduction of hexavalent chromium (as the chromate ion, CrO₄^2-) is based on the notion that the product, trivalent chromium (Cr(III)), is less toxic than chromate. In this study, we show that soluble Cr(III), present at pH 6-8 as the Cr³⁺ ion and/or hydroxyl complexes (henceforth referred to as uncomplexed Cr(III)), can be found transiently in significant concentrations and has a deleterious effect on Shewanella sp. MR-4. However, Cr(III) complexed to an organic ligand or precipitated as Cr(OH)₃(s) has little or no effect on cells. Similarly, during the reduction of Cr(VI) by strain MR-4, complexation of the product, Cr(III), results in increased cell survival and extended Cr(VI) reduction activity. These results and gene expression data obtained by qRT-PCR (quantitative reverse transcription-PCR) suggest that the observed toxic effect of Cr(III) formed during Cr(VI) reduction or added as an uncomplexed species is due to the interference with basic cell activities such as DNA transcription and/or replication. Important implications for the bioremediation of Cr(VI)-contaminated sites emerge from this study: Cr(VI) reduction by Shewanella sp. MR-4 is enhanced and sustained by the presence of compounds able to complex Cr(III) as it is being formed but, in turn, the complexation of Cr(III) precludes its precipitation and immobilization.