Chromium isotopic fractionation during Cr(VI) reduction by Bacillus sp. under aerobic conditions

• Cr isotope fractionation induced by chromium-reducing strain Bacillus sp. QH-1 was studied.
• The affecting factors of concentration of carbon source (glucose) and temperature were investigated.
• Temperature plays an important role on the magnitude of the cell-specific reduction rate and Cr isotopic fractionation.
• The cell-specific reduction rate is not the sole control on the degree of isotopic fractionation.

This study investigated the fractionation of chromium isotopes during chromium reduction by Bacillus sp. under aerobic condition, variable carbon source (glucose) concentration (0, 0.1, 1, 2.5 and 10 mM), and incubation temperatures (4, 15, 25 and 37 °C). The results revealed that the δ53Cr values in the residual Cr(VI) increased with the degree of Cr reduction, and followed a Rayleigh fractionation model. The addition of glucose only slightly affected cell-specific Cr(VI) reduction rates (cSRR). However, the value of ε (2.00 ± 0.21‰) in the experiments with different concentrations of glucose (0.1, 1, 2.5 and 10 mM) was smaller than that from the experiment without glucose (3.74 ± 0.16‰). The results indicated that the cell-specific reduction rate is not the sole control on the degree of isotopic fractionation, and different metabolic pathways would result in differing degrees of Cr isotopic fractionation. The cSRR decreased with decreasing temperature, showing that the values of ε were 7.62 ± 0.36‰, 4.59 ± 0.28‰, 3.09 ± 0.16‰ and 1.99 ± 0.23‰ at temperatures of 4, 15, 25 and 37 °C, respectively. It shown that increasing cSRR linked to decreasing fractionations has been associated with increasing temperatures. Overall, our results revealed that temperature is a primary factor affecting Cr isotopic fractionation under microbial actions.