Metal Respiratory Pathway-Independent Cr Isotope Fractionation during Cr(VI) Reduction by Shewanella oneidensis MR-1

Cr isotope fractionation during microbial reduction processes is commonly recognized as a promising tool in biogeochemistry and bioremediation. However, the mechanism of Cr isotope fractionation during microbial reduction is poorly understood. In this work, the relationship between the bacterial respiratory pathway and Cr isotope fractionation was investigated in Shewanella oneidensis MR-1. For comparison with the wild type, a mutant strain (ΔomcA/ΔmtrC) with a deficiency in extracellular Cr(VI) reduction was constructed by deleting the omcA and mtrC genes, which encode the terminal reductase during extracellular reduction. The magnitudes of Cr isotope fractionation (ε) for Cr(VI) reduction by the wild type and ΔomcA/ΔmtrC were −2.42 ± 0.68‰ and −2.70 ± 0.22‰, respectively. Surprisingly, the ε values were not significantly different between the two strains. This suggests that isotope fractionation is independent of the metal respiratory pathway during Cr(VI) reduction by S. oneidensis MR-1. Moreover, a three-step Cr isotope fractionation model that includes uptake, reduction, and efflux was proposed to exist during intracellular Cr(VI) reduction by S. oneidensis MR-1. The developed model provides a better understanding of Cr isotope fractionation during microbial reduction.