Effects of different metabolic pathways and environmental parameters on Cr isotope fractionation during Cr(VI) reduction by extremely thermophilic bacteria

Abstract

Abstract Chromium isotopes are promising for monitoring and quantifying the extent of microbial Cr(VI) reduction. However, the mechanisms and factors that influence Cr isotope fractionation are not well understood. In the present study, we investigated the effects of different metabolic pathways and various environmental parameters on Cr isotope fractionation during Cr(VI) reduction by the extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus. The Cr isotopic compositions were measured on unreacted Cr(VI) in the remaining solution. The reaction product is isotopically lighter than the reactant. The Cr isotope composition for remaining Cr(VI) in the solution followed a Rayleigh fractionation trend and thus became increasingly isotopically heavier as the reduction of Cr(VI) proceeded. Cr(VI) reduction mainly occurs on the cell surface by Ni–Fe hydrogenase; however, in the presence of CuCl2, the activity of Ni–Fe hydrogenase was inhibited, and Cr(VI) was reduced inside the cells. We found that despite different Cr(VI) reduction locations, Cr isotope enrichment factors (e = (α − 1) × 1000, where α is the fractionation factor) were identical within uncertainty for experiments with and without CuCl2 (−2.72 ± 0.67‰ and −2.92 ± 0.14‰, respectively). When added to the cell cultures, neutral red (NR) functioned as an electron carrier that enabled bacteria to drive the electron transfer processes. The e values were also similar for experiments with and without NR (−2.53 ± 0.40‰ and −2.92 ± 0.14‰, respectively). We compared these e values to investigate the effects of different electron transport pathways and cellular Cr(VI) transfer processes, including Cr(VI) transport across the cell envelope and binding to reacting enzymes, on Cr isotope fractionation. Furthermore, the effects of various environmental parameters on Cr isotope fractionation were tested, including temperature (50 °C and 70 °C), hydrogen concentration, and different electron donors (glucose and hydrogen). The e values were similar at both tested temperatures and stayed within the previously observed range for mesophiles; changes in hydrogen concentration also did not affect the e values. However, the e values were slightly higher with hydrogen as the electron donor (−3.64 ± 0.24‰) than those with glucose as the electron donor (−2.92 ± 0.14‰). In short, the Cr enrichment factors remained unchanged despite of changing microbial Cr (VI) reduction pathways and changing temperature conditions, but may be sensitive to the electron donor type.