Effect of siderophores on the light-induced dissolution of colloidal iron(III) (hydr)oxides

Siderophores play an important role in biological iron acquisition in iron-limited aquatic systems. While it is widely accepted that the solubilization of iron-bearing mineral phases is a key function of siderophores, the mechanism of siderophore-promoted mineral dissolution in aquatic systems is largely unknown. In this study, we investigated the effect of siderophores (desferrioxamine B (DFOB) and aerobactin) on light-induced dissolution of goethite and lepidocrocite in the presence or absence of oxalate in aerated and deaerated suspensions at pH 6. For the irradiated two-ligand system (oxalate/siderophore), the experimental results suggest that oxalate acts as the electron donor for the formation of surface Fe(II), and the siderophore acts as an efficient shuttle for the transfer of surface Fe(II) into solution. Furthermore, even in the absence of an electron donor such as oxalate, both DFOB and aerobactin accelerated the light-induced dissolution of lepidocrocite as compared to the thermal dissolution. Experiments with dissolved Fe(III)-DFOB and Fe(III)-aerobactin complexes suggest that this enhancing effect is not due to photolysis of corresponding surface complexes but to efficient transfer of reduced surface Fe(II) into solution, where surface Fe(II) may be formed, e.g., through photolysis of surface Fe(III)-hydroxo groups. Based on this study, we conclude that the interplay of light and siderophores may play a key role in the dissolution of colloidal iron(III) (hydr)oxides in marine systems, particularly in the presence of efficient electron donors.