SIDEROPHORE-PROMOTED DISSOLUTION OF HEMATITE

Siderophores are highly Fe(III)-specific bidentate ligands excreted by aerobic and facultative anaerobic microorganisms to facilitate Fe uptake in oxic environments. These compounds are thought to influence mineral weathering and the biogeochemical cycling of Fe, but quantitative information concerning this possible influence is nonexistent. Preparative quantities of a soil bacterium siderophore were extracted and purified for use in batch dissolution experiments performed with synthetic hematite particles suspended in 1 mmol dm(-3) NaNO3 at pH 3 under exclusion of light. The initial siderophore concentration used, 0.24 mmol dm(-3), was representative of microniche environments. Soluble Fe per unit mass of hematite was linear with time over an observational period between 2 and 24 h, leading to an area-based dissolution rate of 10(-8) mol m(-2) h(-1). Comparative dissolution experiments, performed with oxalate and ascorbate ligands at the 2-3 mmol dm(-3) initial concentration typical of soil environments (otherwise identical conditions to the siderophore experiments),led to dissolution rates of 5 x 10(-8) mol m(-2) h(-1), in agreement with literature values. The comparability of dissolution rates for a soil bacterium siderophore and two terrestrial organic ligands, despite an order-of-magnitude difference in their initial concentrations, suggests that siderophores may indeed figure significantly in Fe(III)-mineral weathering reactions under natural conditions.