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Microorganisms in aerobic, circum-neutral environments are challenged to acquire sufficient nutrient Fe due to low solubilities of Fe oxides. To overcome this challenge, many aerobic microbes produce low molecular weight (MW) organic ligands, or siderophores, with extremely high Fe-binding affinities. This research expands the existing understanding of siderophore-mediated Fe acquisition from minerals by examining the effects of the siderophore desferrioxamine B (DFOB) on Fe removal from aquatic humic substances (XAD-8-isolated) and other organic matter (OM) isolates (reverse osmosis, RO; and "transphilic", XAD-4) from several rivers including the Suwannee River (GA, USA). Analysis of samples by asymmetrical flow field-flow fractionation (AsFlFFF) with in-line ICP-MS and UV-vis detectors showed that Fe was naturally abundant and primarily associated with intermediate to high MW OM. An excess of DFOB (relative to naturally present Fe) removed ∼ 75% of Fe and shifted the OM MW distribution to lower MWs, perhaps due to removal of "bridging" Fe, although additional mechanistic study of MW shifts is needed. Removal of other OM-associated metals (e.g., Al, Cu, Zn) by DFOB was minimal for all but a few samples. Fe bound to humic substances and other more "transphilic" organic components therefore should be considered readily bioavailable to aerobic, siderophore-producing microorganisms.

This research compared the bioavailability of Fe associated with two forms of the hydrous Fe oxyhydroxide nanomineral ferrihydrite (Fh)--the smaller (1-3 nm), less ordered 2-line (2L) phase and the slightly larger, (2-6 nm) more ordered 6-line (6L) phase--to the common aerobic soil bacterium Pseudomonas mendocina ymp. P. mendocina can acquire Fe from minerals using high-affinity Fe(III) binding ligands known as siderophores and a cell-associated metalloreductase that requires direct cell-mineral contact. Wild-type (WT) P. mendocina and a siderophore(-) mutant were used to monitor siderophore -related and -independent Fe acquisition from 2L and 6L Fh. Both WT and mutant strains acquired Fe from Fh, although Fe acquisition and growth were substantially greater on the 2L phase than on the 6L phase. In the absence of bacteria, copious quantities of the biofilm exopolysaccharide alginate slightly promoted dissolution of 2L and 6L Fh. In biotic experiments, added alginate slightly enhanced growth and Fe acquisition from 6L Fh but not from 2L Fh. Recent research has led to an emerging understanding that Fe-oxide nanoparticle structure, stability, and reactivity are highly sensitive to size at the nanoscale; this research emphasizes how subtle differences in nanoparticle size-related properties can also affect bioavailability.

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