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

Microbially mediated reduction of ferrihydrite (Fe(III) oxyhydroxide) plays a crucial role in Fe cycling, and hence nutrient and contaminant cycling, in subsurface environments. This process is typically considered a strictly anaerobic process confined to anoxic microsites within oxic subsurface environments. However, recent findings suggest that microbes can also mediate ferrihydrite reduction under oxic conditions. Here, we quantified cell-specific rates of ferrihydrite reduction by the model organism Shewanella oneidensis MR-1 under oxic and anoxic conditions. We used these rates to assess the relative contribution of oxic and anoxic pore spaces to Fe(II) mobilization in a previously published laboratory analog of oxic aquifer sediments. Oxic reduction proceeded persistently, albeit at a per cell rate 100 times more slowly than anoxic reduction, yet still generated appreciable Fe(II). Modeling suggests that when anoxic microsites are absent or occupy a minor fraction of the pore space, oxic Fe(III) reduction can account for a significant share of total Fe(II) release. Such conditions are common in shallow aquifers, well-drained soils, and capillary fringes. We conclude that oxic Fe(III) reduction is a persistent background process that has been underestimated in current biogeochemical frameworks.