Chelation modeling was used to predict the abilities of hydroxamate siderophores (HS) and other natural organic acids to chelate Fe and other ions in a modified Hoagland nutrient solution as a function of pH (4.0–10.0). Experimentally determined levels of chelated Fe (pH 5.0–10.0) agreed with our predicted results and with values previously predicted for EDTA (ethylenediaminetetraacetic acid), DTPA (diethylenetriaminepentaacetic acid), and EDDHA [ethylenediamine di(o-hydroxyphenylacetic) acid].
The natural HS, Desferrioxamine B (DFOB), and an unknown mixture of HS isolated from the ectomycorrhizal fungus Boletus edulis, remained fully chelated with Fe at all values of pH. Modeling predicted DFOB to be 99.9% chelated with Fe even in nutrient solution of pH 10.0. No significant chelation of Fe was measured or predicted above pH 6.0 for malate, malonate, oxalate, succinate, α-ketoglutarate, or pyruvate. Only 4% of citrate was chelated with Fe at pHs 6.0 and 6.4 as determined by direct measurement and computer modeling, respectively.
Solubility constants, derived for Fe(OH)3 (amorphous) (amorph.) and FePO4 (amorph.), indicated that FePO4 (amorph.) can control Fe solubility in nutrient solutions of low pH and reduce the ability of ligands to chelate Fe, relative to conditions in which Fe solubility is governed by Fe(OH)3 (amorph.).
Since our results indicate that microbially produced hydroxamate siderophores have a significant influence on Fe solubility, we discuss how such ligands could be of importance in the Fe nutrition of plants under natural soil conditions.