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

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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.