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

锑（Sb）是一种新兴的全球环境关注的有毒金属，与砷（As）具有许多化学和毒理学相似性。微生物介导的氧化或还原（“氧化还原”）反应控制着环境中锑和砷的迁移性和毒性。As只有一种稳定同位素（75As），而Sb有两种，121Sb和123Sb。在还原反应中，同位素动力学效应往往有利于较轻同位素比较重同位素更快的反应速率（通常与氧化有关的效应较弱）。这就产生了分馏效应，即反应产物中的同位素比例可能比反应物池中的同位素更轻。虽然同位素分馏效应先前已被用来表明自然界中涉及其他有毒元素的氧化还原反应的发生和程度，但细菌循环对Sb同位素分馏的程度尚不清楚。本研究将确定细菌氧化还原循环过程中Sb稳定同位素分馏的程度。考虑到锑和砷在地球化学和地球微生物学上的相似性，我们也可以推断，当这两种元素共存时，从锑同位素测量中检测到的氧化还原过程也会积极地影响砷。该项目的更广泛影响包括提高对污染环境中控制锑和砷行为的地球微生物过程的理解。研究人员还将通过针对代表性不足的学生的项目，每年培训两名硕士研究生和4至6名本科生。本项目的范围是研究锑（Sb）稳定同位素在环境相关的Sb(V)和Sb（III）价态之间的地质微生物循环过程中的分馏。研究人员将比较细菌还原和氧化过程中Sb同位素分馏的幅度和方向与化学（非生物）氧化还原反应中发生的Sb同位素分馏的幅度和方向。他们将用已知的Sb(V)还原或Sb（III）氧化菌株，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生物进行实验。在细菌氧化还原反应过程中，定期取样sb修饰培养物的液相和固相沉淀物，并测量反应物和反应产物之间的同位素分馏因子。这些结果将有可能为预测锑的环境行为或为锑和砷的生物修复提供新的同位素工具。也有可能利用古代地质物质的Sb稳定同位素组成来确定地质历史中生物地球化学Sb循环（以及替代的As循环）的存在。这可能为假设的基于砷的代谢的古代提供了一种测试，或者作为评估古代地球古氧化还原条件的替代方法。

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.