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

Le résumé fourni par la source

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.

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