Iron and manganese are the first and second most abundant redox-active elements in the Earth’s crust, respectively. Both trace metals are essential nutrients for all living organisms and their redox cycles impact numerous environmental processes. In soils, these elements often co-occur as various Fe(III) and Mn(III,IV) (hydr)oxide phases. Due to their ubiquity, large surface area, and high redox activity, these metal oxides inevitably interact through either electron transfer between Fe(II) and Mn(III, IV) oxides or surface catalyzed Mn(II) oxidation by Fe(III) (hydr)oxides. Organic acids, which can promote the dissolution of Fe and Mn (hydr)oxide phases, can provide an important link between the Fe and Mn cycles. Here we quantified Mn and Fe mobilization by desferrioxamine B (DFOB), a terrestrial bacterial siderophore, in single and mixed mineral systems, including manganite (γ-MnOOH), δ-MnO<sub>2</sub>, lepidocrocite (γ-FeOOH) and/or 2-line ferrihydrite. Desferrioxamine B mobilized Mn(III) as Mn-DFOB complexes to varying extents from both Mn(III,IV) (hydr)oxides, even in the presence of Fe(III) (hydr)oxides. However, reduction of Mn(IV) to Mn(III) was required for mobilization of Mn(III) from δ-MnO<sub>2</sub>. Additionally, the decomposition of Mn-DFOB complexes through Mn-for-Fe ligand exchange and ligand oxidation led to Mn(III) precipitation on the Fe(III) (hydr)oxides. The oxidation of the ligand was associated to a 50-80% decrease in the amount of Fe-DFOB mobilized in the minxed mineral systems compared to the single mineral systems. Our results demonstrate that siderophores, through their complexation of Mn(III), reduction of Mn(III, IV) and mobilization of Mn(II), can redistribute Mn to other soil minerals and limit the bioavailability of Fe in natural systems.

Siderophore-mediated mobilization of manganese limits iron solubility in mixed mineral systems

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