Impact of Redox Cycling on Fe (hydr)Oxide Transformations and Organic Matter Degradation.

In soils, redox reactions drive microbial metabolism, nutrient availability and transport, inorganic and/or organic contaminant mobility, and organic matter (OM) degradation. In iron (Fe) rich environments OM degradation is influenced by Fe redox processes, linking the carbon and iron biogeochemical cycles. Under anaerobic conditions, Fe reducing bacteria use Fe (hydr)oxides as electron acceptors to produce Fe(II) and initiate secondary mineral formation. During this process OM can be used as a carbon source, sorb onto the Fe minerals, act as an electron shuttle, and form stable complexes with aqueous Fe(III). In the presence of dissolved O₂, Fe(II) is oxidized microbially by microaerophiles and/or chemically by dissolved O₂, and promotes the regeneration of Fe(III)-rich minerals. The reaction between Fe(II) and dissolved O₂ generates a series of reactive oxygen species (ROS) that may also play an important role in the degradation of OM. Previous studies have focused on studying Fe (hydr)oxide transformations and OM degradation under one oxidation or reduction period. However, most surface environments are exposed to alternating O₂ concentrations due to changes in water table and other geochemical conditions. Here, we study the formation of secondary Fe minerals and the degradation of OM under oscillating redox conditions. Both microbial metabolism and abiotic reactions play important roles in these processes. Anaerobic Fe reduction periods are stimulated by the presence of OM and a natural inoculum or the addition of aqueous Fe(II), and subsequent oxidation periods are initiated by addition of dissolved O₂. Analytical techniques track the concentrations of Fe, ROS, small organic acids, and changes in bulk OM. Mineralogical changes are studied using X-ray absorption spectroscopy, and bioinformatics tools are used to isolate genomes from metagenomes and identify gene clusters involved in OM degradation. Our findings will help elucidate the fate of C and Fe in redox active environments.