Variations of stable isotope fractionation during bacterial chromium reduction processes and their implications

The Cr isotope fractionation factors ε were insensitive to pH and electron donor concentration, but dependent on the type of electron donors and redox conditions in the cultures. This indicates that isotope variations may be used to identify when different biological pathways are involved, and so to investigate metabolic processes. The ε value from all experimental conditions examined ranged between − 1.58 and − 4.93‰, with a mean value at − 3.3‰. While Cr isotopes might be used to separate the effects of abiotic and microbially mediated reduction in environmental sites, the fractionation factors from reduction by individual bacterial species overlap with those from several individual abiotic reduction processes, suggesting that site-specific data (e.g., fractionation factors associated with indigenous bacterial populations and local groundwater chemistry) are required in order to use Cr isotopes to distinguish between different reduction mechanisms.