Siderophores and siderophore-associated metabolites from Antarctic Pseudomonas sp. ANT_H12B contribute to iron and phosphorus mobilization in soils with contrasting properties

Abstract

Iron (Fe) oxy(hydr)oxides regulate soil Fe and phosphorus (P) cycling, but how bacterial siderophores and co-produced metabolites mobilize these nutrients across pH remains unclear. We hypothesized that siderophore-mediated Fe complexation and P mobilization depend on pH and mineral crystallinity and that siderophore-associated metabolites (SAM) are more effective than purified siderophores. A purified pyoverdine-like siderophore fraction from Antarctic Pseudomonas strain ANT_H12B was chemically and spectroscopically characterized and evaluated for Fe³ + complexation from FeCl 3 between pH 5 and 9. Iron mobilization from ferrihydrite (Fh), goethite (Gt), and hematite (Hm) was examined at pH 5.5 and 8.5 across mineral concentrations of 0.5–2 g L⁻¹; purified siderophores and SAM were compared in four P-deficient soils of contrasting pH. Iron³ + complexation increased from 6% at pH 5 to 10% at pH 9, reflecting ligand deprotonation. Ferrihydrite showed the greatest dissolution, although Fe release at pH 8.5 was 30% and 37% lower than at pH 5.5 at 1 and 1.5 g L⁻¹, respectively. Conversely, Gt dissolution at 2 g L⁻¹ was 40% higher at pH 8.5, whereas Hm showed the lowest dissolution, reaching 3 and 1.9 μM at pH 5.5 and 8.5, respectively. Purified siderophores moderately increased Olsen-P in neutral and high-Fe alkaline soils, whereas SAM increased Olsen-P approximately 1- to 26-fold across all soils, with the greatest effect in high-Fe alkaline soil. SAM also increased soluble P 11- to 40-fold and NaOH-EDTA-extractable P by 25–104% relative to controls. The results support our hypothesis, showing that siderophores contribute to Fe and P mobilization but act more effectively within SAM, where organic acids provide complementary dissolution and sorption-competition mechanisms. This finding agrees with previous ANT_H12B studies reporting improved nutrient availability and plant performance and supports using unpurified metabolite mixtures for sustainable P management. Indeed, retaining the co-produced mixture could reduce purification requirements and preserve complementary functions relevant to sustainable P management.