Probing the fate of structurally distinct Siderophores with reactive soil components

Siderophores, iron (Fe) chelating agents produced by fungi, bacteria, and
grassy plants, play critical roles in the acquisition of Fe and other
trace metals in terrestrial systems. Siderophores are essential to proper
plant nutrition as well as the maintenance of soil quality and the
suppression of plant diseases. The ability of siderophores to solubilize
metals from minerals and the details of cellular uptake systems have been
well-explored. However, limited studies have investigated mineral surface
disruption of siderophore-promoted nutrient uptake processes, or the
variable fate of structurally distinct siderophores and their metal
complexes. Here, we examine the bacterial trishydroxamate siderophore
desferrioxamine B (DFOB), the bacterial triscatecholamide siderophore
protochelin, and the synthetic carboxylate phytosiderophore analog
proline-2’-deoxymugeneic acid (PDMA), with soil aluminosilicates and metal
(oxyhydr)oxides, including montmorillonite, kaolinite, goethite,
ferrihydrite, and δ-MnO2. Apo- and metal-siderophore complex sorption was
studied using adsorption isotherm experiments, and the fate of complexes
on the mineral surfaces was studied using gallium X-ray absorption
spectroscopy. In general, siderophore–mineral interactions were governed
by complex charge, molecular structure, and mineral surface reactivity.
DFOB and PDMA complexes exhibited similar overall trends, with swelling
clays acting as major sinks through intercalation of DFOB complexes and
inner-sphere adsorption of PDMA complexes. Both showed limited retention
on Fe (oxyhydr)oxides, with evidence for complex dissociation on
ferrihydrite and inner-sphere complexation on goethite. In contrast,
apo-PDMA showed negligible adsorption across all minerals, highlighting
the importance of metal complexation in controlling phytosiderophore
reactivity. Protochelin exhibited distinct behavior, with its anionic,
hydrophobic complexes showing limited sorption, extensive dissociation,
and susceptibility to degradation, particularly in reactions with
montmorillonite and δ-MnO2. Across all siderophores, Mn oxides promoted
reductive dissolution  with concomitant ligand degradation,
indicating that mineral-driven transformation processes can be as
important as sorption in controlling siderophore fate. Understanding these
differences in siderophore fate will provide a new angle for managing
agricultural systems so that nutrient deficiencies and crop disease
susceptibility are minimized.