SIDEROPHORE-PROMOTED DISSOLUTION OF HEMATITE

Siderophores are highly Fe(III)-specific bidentate ligands excreted by aerobic and facultative anaerobic microorganisms to facilitate Fe upt ake in oxic environments. These compounds are thought to influence min eral weathering and the biogeochemical cycling of Fe, but quantitative information concerning this possible influence is nonexistent. Prepar ative quantities of a soil bacterium siderophore were extracted and pu rified for use in batch dissolution experiments performed with synthet ic hematite particles suspended in 1 mmol dm(-3) NaNO3 at pH 3 under e xclusion of light. The initial siderophore concentration used, 0.24 mm ol dm(-3), was representative of microniche environments. Soluble Fe p er unit mass of hematite was linear with time over an observational pe riod 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 con centration typical of soil environments (otherwise identical condition s to the siderophore experiments),led to dissolution rates of 5 x 10(- 8) mol m(-2) h(-1), in agreement with literature values. The comparabi lity of dissolution rates for a soil bacterium siderophore and two ter restrial organic ligands, despite an order-of-magnitude difference in their initial concentrations, suggests that siderophores may indeed fi gure significantly in Fe(III)-mineral weathering reactions under natur al conditions.