Tracing redox cycles during microbe-clay interactions using stable iron isotopes

含铁粘土矿物的氧化还原反应导致土壤和沉积物的许多物理和化学性质的变化，从而影响污染物的命运。近十年来铁同位素研究的发展表明，稳定铁同位素地球化学是研究铁生物地球化学循环的重要工具。在微生物还原含铁粘土矿物过程中，稳定铁同位素分馏将在三个系统中进行越来越复杂的研究。系统1将使用简单的间歇式反应器和模型铁还原细菌。将进行含铁亚铁与含铁粘土相互作用的非生物学实验，为解释生物学数据提供平衡分馏因子的基线。生物活性在影响黏土矿物中还原铁和氧化铁同位素分馏中的作用将通过比较生物和非生物系统进行研究。这项工作将揭示在粘土矿物中结构铁的微生物还原过程中发生的电子转移和原子交换过程。系统2将使用一个全自动生物反应器系统与模型粘土和模型微生物来研究振荡氧化还原条件对微生物-粘土相互作用过程中铁同位素分馏的影响。氧化还原条件将被严格控制，以模拟自然系统中发生的氧化还原循环，如河岸带、经历季节性地下水波动的沉积物、经常被淹没和排水的稻田以及经历干旱和降雨事件的泥炭地。用模型微生物接种的粘土悬浮液将经历多次还原和氧化半循环。在每个半周期内，将监测不同活性铁相的铁形态、浓度和同位素组成。来自井控系统的高质量实验数据将为解释从自然样品中获得的同位素数据提供一个框架。系统3将涉及一个更复杂的系统，使用天然沉积物和本地微生物群落。以往的研究表明，微生物的还原和溶解程度随着氧化还原循环次数的增加而降低。这些变化将在多个氧化还原循环中产生不同的铁同位素特征。与受控系统相比，自然土壤系统中产生的铁同位素特征有可能为土壤科学中的一个关键问题提供答案：粘土矿物在最终耗尽自然系统中的还原能力之前可以进行多少氧化还原循环？这项工作将大大提高我们对含铁粘土矿物氧化还原转化过程的分子尺度过程的理解。本研究将铁同位素作为追踪微生物-粘土相互作用过程中氧化还原循环的工具，为金属稳定同位素地球化学领域做出开创性贡献。研究结果也将填补我们对含铁粘土矿物的反应性和稳定同位素性质的理解空白，特别是在它们与铁还原菌的相互作用过程中。对微生物还原粘土矿物形成的亚铁性质的进一步了解将有助于预测它们对各种环境污染物（例如农药和重金属）的反应性。此外，振荡氧化还原条件下受控系统的铁同位素特征将极大地促进我们对土壤行为动态性质的理解，因此极大地有利于旨在最大化土壤肥力和性能的管理策略。

Redox reactions involving iron-bearing clay minerals are responsible for many changes in the physical and chemical properties of soils and sediments, and thus influence the fate of contaminants. The development of iron isotope research over the last decade has demonstrated that stable iron isotope geochemistry is a valuable tool to study biogeochemical cycling of iron. Stable iron isotope fractionations during microbial reduction of iron-bearing clay minerals will be examined with increasing complexity in three systems. System 1 will use simple batch reactors with model iron reducing bacteria. Abiological experiments for interaction between aqueous ferrous iron and iron-bearing clays will be carried out to provide a baseline of equilibrium fractionation factors for interpreting biological data. The role of biological activities in impacting iron isotope fractionation between reduced and oxidized iron in clay minerals will be investigated by comparing biological and abiological systems. This work will shed light on the electron transfer and atom exchange process that occurs during microbial reduction of structural ferric iron in clay minerals. System 2 will use a fully automated bioreactor system with model clays and model microorganisms to investigate the impact of oscillating redox conditions on iron isotope fractionations during microbe-clay interactions. Redox conditions will be tightly controlled to mimic redox cycles occurring in natural systems such as riparian zones, sediments that experience seasonal groundwater fluctuations, rice paddies which are regularly flooded and drained, and peatlands which experience drought and rain events. Clay suspensions inoculated with model microorganisms will be subjected to repeated reducing and oxidizing half-cycles. Iron speciation, concentration and isotopic compositions for different reactive iron phases will be monitored during each half-cycle. High quality experimental data from well controlled systems will provide a framework for interpretation of isotope data acquired from natural samples. System 3 will involve a more complex system using natural sediments and indigenous microbial communities. The extent of microbial reduction and dissolution has been revealed by previous studies to decrease with increasing number of redox cycles. All these changes will produce distinct iron isotope signatures during multiple redox cycles. When compared to a controlled system, the iron isotope signature produced in a natural soil system has the potential to provide answers to a key question in soil science: how many redox cycles can clay minerals be subject to before they eventually exhaust the reducing power in natural systems? The proposed work will greatly improve our understanding of molecular-scale processes during redox transformations of iron-bearing clay minerals. By applying iron isotopes as tools to trace redox cycles during microbe-clay interactions, this research will make pioneering contributions to the field of metal stable isotope geochemistry. The research findings will also fill the gap in our understanding of the reactivity and stable isotope properties of iron-containing clay minerals, particularly during their interaction with iron reducing bacteria. The improved knowledge on the nature of ferrous iron formed by microbial reduction of clay minerals will aid in predicting their reactivity towards a variety of environmental contaminants (e.g., pesticides and heavy metals). In addition, iron isotope signature from a controlled system under oscillating redox conditions will significantly advance our understanding of the dynamic nature of soil behavior, therefore greatly benefit management strategies that aim at maximizing soil fertility and performance.