Mass-dependent selenium isotopic fractionation during microbial reduction of seleno-oxyanions by phylogenetically diverse bacteria

Selenium (Se) isotope fractionation has been widely used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of the controls on fractionation during microbial reduction of Se-oxyanions is based on a limited number of studies. Here we complement and expand the currently available pure culture data for Se isotope fractionation by investigating for the first time six phylogenetically diverse, mesophilic, and non-respiring bacterial strains that reduce Se-oxyanions to elemental Se [Se(0)]. Experiments were performed with either selenate [Se(VI)] or selenite [Se(IV)] at lower, more environmentally-relevant Se (9–47 μM) and carbon (500 μM) concentrations than previously investigated. Enterobacter cloacae SLD1a-1, Desulfitobacterium chlororespirans Co23 and Desulfitobacterium sp. Viet-1 were incubated with Se(VI) and Se(IV). Geobacter sulfurreducens PCA, Anaeromyxobacter dehalogenans FRC-W and Shewanella sp. (NR) were examined for their ability reducing Se(IV) to Se(0). Our data confirm that microbial reduction of both Se-oxyanions is accompanied by large kinetic isotopic fractionation (reported as ^82/76ε = 1000×(^82/76α-1) in ‰). Under our experimental conditions, microbial reduction of Se(VI) shows consistently greater isotope fractionation (ε = −9.2‰ to −11.8‰) than reduction of Se(IV) (ε = −6.2 to −7.8‰) confirming the difference in metabolic pathways for the reduction of the two Se-oxyanions. For Se(VI), an inverse relationship between normalized cell specific reduction rate (cSRR) and Se isotope fractionation suggests that the kinetic isotope effect for Se(VI) reduction is governed by an enzymatically-specific pathway related to the bacterial strain-specific physiology. In contrast, the lack of correlation between normalized cSRR and isotope fractionation for Se(IV) reduction indicates a non-enzyme specific pathway which is dominantly extracellular. Our study highlights the importance to understand microbially-mediated Se isotope fractionation depending on Se species, and cell-specific reduction rates before Se isotope ratios can become a fully applicable tool to interpret Se isotopic changes in modern and ancient environments.