Siderophore-promoted dissolution of synthetic and biogenic layer-type Mn oxides

Siderophores are biogenic chelating agents exuded in terrestrial and marine environments to increase the bioavailablity of ferric iron. Recent work suggests that both solid and aqueous manganese may affect the aqueous speciation of siderophores and thus siderophore-mediated iron transport. Although the interaction of the trihydroxamate siderophore desferrioxamine B (DFOB) with several lower-valence manganese oxides has been studied, the effects of siderophores on Mn(III,IV) oxide dissolution are unknown. To remedy this situation, we measured the dissolution rates of two synthetic layer-type Mn(IV) oxides and a biogenic oxide produced by a model organism, Pseudomonas putida GB-1. For pH 5–7, we find that all minerals studied dissolve by traditional reductive (R1) dissolution, yielding Mn(II); for pH 7–9, dissolution yields Mn(III)–siderophore complexes, either by selective ligand-promoted dissolution of structural Mn(III) or by reduction of >Mn(IV) to >Mn(III) followed by complexation and solubilization of Mn(III) by DFOB. Because reductive dissolution results in siderophore oxidation, manganese oxide dissolution at acidic pH may provide a significant abiotic sink for siderophores in natural waters. At alkaline pH, Mn(III)–siderophore complexes produced may profoundly affect the aqueous speciation of siderophores as well as provide a source of reactive Mn(III) complexes.