Oxic microbial ferrihydrite reduction rates of Shewanella oneidensis and the potential for Fe mobilization in oxic sediments

Microbially-mediated reduction of ferric iron, Fe(III), often found in the natural environment as ferrihydrite, plays a crucial role in Fe cycling, and hence nutrient and contaminant cycling, in subsurface environments. Traditionally, microbial ferrihydrite reduction has been considered an anaerobic process relegated to anoxic microsites within oxic subsurface environments. However, recent findings suggest that microbes can mediate Fe(III) reduction also under oxic conditions, although rates and environmental impact of this process are still unknown. Here, we quantified cell-specific rates of ferrihydrite reduction by the model organism Shewanella oneidensis MR-1 under oxic and anoxic conditions. Based on our experimental results, we assessed the relative contribution of oxic and anoxic pore spaces to Fe(II) mobilization in a laboratory analog of oxic aquifer sediments presented in the literature. Our results show that oxic Fe(III) reduction can significantly contribute to Fe(II) mobilization in oxic subsurface environments where anoxic microsites occupy a minority of the pore space, conditions that can be found in, e.g., shallow aquifers, well-drained soils, and capillary fringes. Despite the lower cell-specific rates of oxic Fe(III) reduction, it remains a persistent background process, playing a previously underestimated role in Fe cycling within oxic subsurface environments.