Collaborative Research: Antimony stable isotope systematics during bacterial and abiotic redox cycling

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Antimony (Sb) is a toxic metalloid of emerging global environmental concern that shares many chemical and toxicological similarities to arsenic (As). Oxidative or reductive ("redox") reactions that are mediated by microorganisms control the mobility and toxicity of Sb and As in the environment. Whereas As has only one stable isotope (75As), Sb has two, 121Sb and 123Sb. Kinetic isotope effects often favor faster reaction rates for lighter isotopes compared to heavier isotopes during reduction reactions (with generally weaker effects related to oxidation). This causes a fractionation effect whereby the isotopic ratio in the reaction product may become enriched in the lighter isotope compared to the reactant pool. Although isotopic fractionation effects have previously been used to indicate the occurrence and extent of redox reactions involving other toxic elements in nature, the extent to which bacterial cycling fractionates Sb isotopes is unknown. This study will establish the extent of Sb stable isotope fractionation during bacterial redox cycling. Given the geochemical and geomicrobiological similarities between Sb and As, it may also be possible to infer that redox processes detected from Sb isotopic measurements are also actively affecting As when both elements co-occur. The broader impacts of this project include an improved understanding of the geomicrobiological processes that control the behavior of Sb and As in contaminated settings. Investigators will also train two MS graduate students and 4 to 6 undergraduate students per year through programs that target underrepresented students. The scope of this project is to investigate antimony (Sb) stable isotopic fractionation during geomicrobiological cycling between the environmentally relevant Sb(V) and Sb(III) valence states. Investigators will compare the magnitude and direction of Sb isotopic fractionation during bacterial reduction and oxidation to that which occurs during chemical (abiotic) redox reactions. They will conduct experiments with cultures of known Sb(V)-reducing or Sb(III)-oxidizing bacterial strains, as well as novel strains and microcosms obtained from a range of soil, freshwater, hypersaline, and hydrothermal environments. The liquid phase and solid precipitates in Sb-amended cultures will be periodically sampled during the course of bacterial redox reactions and the isotopic fractionation factors between reactants and reaction products will be measured. The results will potentially provide a new isotopic tool by which to predict the environmental behavior of Sb or to inform new strategies for bioremediation of Sb and As. It may also be possible to use the Sb stable isotopic composition of ancient geologic materials to identify the presence of biogeochemical Sb cycling (and, by proxy, As cycling) in the geologic past. This may provide a test for the hypothesized antiquity of As-based metabolisms, or alternately a proxy by which to assess paleo-redox conditions on ancient Earth.