Phosphorus is an essential macronutrient, a structural component of DNA, membranes, and the energy currency of every living cell. Yet it is one of the most limiting factors for crop productivity on more than 40 percent of the world’s arable land. The problem is not scarcity but chemistry. Inorganic phosphate moves slowly through soil and clings tenaciously to mineral surfaces. In acidic soils, it binds tightly to iron and aluminum oxy(hydr)oxides; in alkaline soils, it precipitates as poorly soluble calcium phosphates. Soil solution concentrations of orthophosphate typically hover between 0.1 and 10 micromolar — far below what crops need. Meanwhile, vast reservoirs of so-called legacy phosphorus sit inert in soils, inaccessible to roots.

Siderophores are the microbial world’s answer to a related problem: iron starvation. These low-molecular-weight organic ligands, secreted by bacteria, fungi, and grass roots under iron-limiting conditions, bind ferric iron (Fe³⁺) with extraordinary affinity, allowing microbes to extract it from nearly insoluble minerals. Fluorescent Pseudomonas species are famous producers of pyoverdines, a class of siderophores defined by a conserved dihydroquinoline chromophore linked to a strain-specific peptide chain. More than 60 structurally distinct pyoverdines are known. Because iron oxy(hydr)oxides are also the major sinks for phosphate in soil, researchers have long suspected that siderophores might indirectly free phosphorus by stripping iron from the very minerals that hold it — but the details remained murky.

The research team, led by Xiaomeng Chen of the University of Turin together with collaborators at the University of Warsaw, set out to test three interconnected hypotheses. First, that siderophore-mediated iron complexation and mineral dissolution would depend on pH and on the crystallinity of the iron minerals involved. Second, that these mineral-scale effects would translate into soil-specific changes in iron extractability and phosphorus availability governed by soil pH and iron status. Third — and most provocatively — that the complete pool of siderophore-associated metabolites (SAM) would mobilize more iron and phosphorus than purified siderophores alone, thanks to complementary activities among the mixture’s constituents.

The strain at the heart of the work, Pseudomonas sp. ANT_H12B, was isolated from Antarctic soil on King George Island and previously shown to produce more than 500 micromolar of a pyoverdine-like siderophore. The researchers grew the bacterium in a glucose-based medium at a chilly 10 degrees Celsius, harvested the sterile cell-free supernatant as SAM, and purified the siderophore fraction from it using solid-phase extraction. Characterization by elemental analysis, FT-IR, and proton NMR spectroscopy confirmed a pyoverdine-like peptide structure rich in hydroxyl, carboxylate, and amide groups — the O- and N-donor chemistry that gives pyoverdines their grip on ferric iron. Interestingly, the fraction also contained a small amount of sulfur, hinting at structural features not typical of canonical pyoverdines. Gas chromatography-mass spectrometry of the full SAM revealed that fatty acids made up roughly 83 percent of the detectable metabolites, dominated by hexadecanoic and cis-9-hexadecenoic acids, with smaller amounts of alcohols, esters, sugars, and an indolic compound.

The laboratory experiments yielded a nuanced picture of how pH and mineralogy govern siderophore function. In solution, the purified siderophore chelated between 6 and 10 percent of added Fe³⁺, with binding increasing as pH rose from 5 to 9 — consistent with progressive deprotonation of the ligand’s functional groups, which enhances their electron-donating capacity. Notably, no saturation plateau was reached, and the binding efficiency was far below the ideal one-to-one stoichiometry seen in high-performance catecholate siderophores such as bacillibactin. Against actual minerals, the results diverged by crystal structure. Poorly crystalline ferrihydrite, with its enormous 232 square meters per gram surface area and abundant defects, dissolved most readily at both pH 5.5 and 8.5. But crystalline goethite behaved counterintuitively: at the highest mineral concentration, iron dissolution was 40 percent higher under alkaline conditions than under acidic ones. Hematite, the most stable phase, resisted dissolution almost completely, releasing at most 3 micromolar iron.

Zeta potential measurements helped explain these contrasts. Ferrihydrite flipped from positively charged at pH 5.5 (+19.0 millivolts) to strongly negative at pH 8.5 (−34.6 millivolts), a shift that would electrostatically repel negatively charged ligand groups — yet its intrinsic reactivity kept dissolution high. Goethite, by contrast, sat almost exactly at its point of zero charge at pH 8.5 (−0.1 millivolts), removing electrostatic barriers precisely when ligand deprotonation peaked, which may explain its unexpected alkaline advantage. Hematite’s persistent negative surface charge combined with low intrinsic solubility left it largely untouchable. No iron was released from any mineral in the absence of siderophore, confirming that the observed mobilization was genuinely ligand-driven.

The soil experiments, conducted on four phosphorus-deficient soils from the Piedmont region of Italy ranging from strongly acidic (pH 4.66) to strongly alkaline (pH 8.48), delivered the study’s most dramatic results. Purified siderophores produced only moderate increases in Olsen-P, the standard measure of plant-available phosphorus, chiefly in neutral and high-iron alkaline soils. The full SAM, however, increased Olsen-P roughly 1- to 26-fold across all four soils, with the largest response in the high-iron alkaline soil — where treatment lifted available phosphorus from a negligible level below 1 milligram per kilogram to a satisfactory 18.3 milligrams per kilogram. SAM also boosted readily soluble, calcium chloride-extractable phosphorus by 11- to 40-fold and NaOH-EDTA-extractable phosphorus by 25 to 104 percent relative to controls. Because the experimental design ensured identical phosphorus inputs across treatments, these differences reflect genuine soil phosphorus release rather than unequal additions.

The mechanism, the authors argue, is complementary rather than singular. In alkaline soils, where calcium phosphate precipitation dominates, fatty acids and other carboxyl-bearing metabolites in SAM may compete with phosphate for anion-binding sites on oxide surfaces — an effect especially potent in the most alkaline soil, which had the lowest iron oxide content and thus fewer binding sites to begin with. Siderophores may additionally complex calcium ions, shifting dissolution equilibria toward phosphate release, a mechanism consistent with recent findings on phosphate-solubilizing bacteria in calcareous soils. In acidic, iron-rich soils, siderophore-mediated dissolution of iron oxy(hydr)oxides may weaken iron–phosphorus associations directly. The decoupling of phosphorus release from iron release in some non-calcareous soils suggests competitive ligand adsorption can sometimes matter more than mineral dissolution itself.

Beyond the mechanistic insight, the findings carry an immediate practical message: skip the purification. Retaining the naturally co-produced metabolite mixture avoids costly downstream processing while preserving the synergistic functions that make SAM so effective. This aligns with the team’s earlier work showing that SAM from ANT_H12B enhanced soil iron, phosphorus, and potassium availability and maintained or improved radish productivity under reduced fertilization, and that a molasses-amended SAM formulation promoted basil growth while reshaping the rhizosphere microbiome. The researchers caution that dose–response relationships, metabolite persistence, plant uptake, repeated-application effects, and field-scale performance all remain to be resolved before Antarctic-derived microbial metabolite cocktails can become a mainstream tool. But the study marks a decisive step from demonstrating that these mixtures work toward understanding why they work — and in the race to exploit soil legacy phosphorus and cut fertilizer dependence, understanding why may be worth as much as the molecules themselves.

Subject of Research: Siderophore-mediated iron and phosphorus mobilization in soils by Antarctic Pseudomonas metabolites

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

Article References: Chen, X., Musiałowski, M., Dębiec-Andrzejewska, K., Stasiuk, R., Celi, L., Schiavon, M., Martin, M., & Celletti, S. (2026). Siderophores and siderophore-associated metabolites from Antarctic Pseudomonas sp. ANT_H12B contribute to iron and phosphorus mobilization in soils with contrasting properties. Frontiers in Soil Science, 6, Article 1960451. https://doi.org/10.3389/fsoil.2026.1960451

Image Credits: AI Generated

DOI: 10.3389/fsoil.2026.1960451

Keywords: siderophores, pyoverdine, phosphorus mobilization, iron oxy(hydr)oxides, Pseudomonas, Antarctica, soil biogeochemistry, fatty acids, soil pH, legacy phosphorus, sustainable agriculture, biofertilizer