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

锑（Sb）是一种有毒的类金属，与砷（As）有许多化学和毒理学相似之处，是全球环境问题的重要组成部分。 由微生物介导的氧化或还原（“氧化还原”）反应控制Sb和As在环境中的移动性和毒性。 As只有一种稳定的同位素（75 As），Sb有两种，121 Sb和123 Sb。 在还原反应中，动力学同位素效应通常有利于较轻同位素比较重同位素更快的反应速率（与氧化相关的效应通常较弱）。 这引起分馏效应，由此反应产物中的同位素比与反应物池相比可以变得富集较轻的同位素。 虽然同位素分馏效应以前曾被用来指示氧化还原反应的发生和程度，涉及其他有毒元素在自然界中，细菌循环分馏锑同位素的程度是未知的。本研究将建立细菌氧化还原循环过程中Sb稳定同位素分馏的程度。 鉴于锑和砷之间的地球化学和地质微生物的相似性，它也可能推断，氧化还原过程中检测到的锑同位素测量也积极影响作为时，这两种元素共存。 该项目更广泛的影响包括更好地了解控制锑和砷在污染环境中的行为的地质微生物学过程。 调查人员还将通过针对代表性不足的学生的计划，每年培训两名MS研究生和4至6名本科生。 本项目的范围是调查在与环境有关的Sb（V）和Sb（III）价态之间的地质微生物循环过程中锑（Sb）稳定同位素分馏。 研究人员将比较细菌还原和氧化过程中Sb同位素分馏的大小和方向与化学（非生物）氧化还原反应过程中发生的情况。 他们将用已知的Sb（V）还原或Sb（III）氧化细菌菌株的培养物进行实验，以及从一系列土壤，淡水，高盐和热液环境中获得的新菌株和微观世界。 在细菌氧化还原反应过程中，将定期对Sb修正培养物中的液相和固体沉淀物进行取样，并测量反应物和反应产物之间的同位素分馏因子。 研究结果将为预测Sb的环境行为或为Sb和As的生物修复提供新的策略提供新的同位素工具。 也可能使用古代地质材料的Sb稳定同位素组成来确定地质历史中是否存在地球化学Sb循环（以及作为替代的As循环）。 这可能提供了一个测试的假设古老的As为基础的代谢，或者替代代理，以评估古地球上的古氧化还原条件。

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.