Goethite Dissolution in the Presence of Phytosiderophores: Rates, Mechanisms, and the Synergistic Effect of Oxalate

Abstract

The purpose of this study was the elucidation of the chemical mechanism of an important process in iron acquisition by graminaceous plants: the dissolution of iron oxides in the presence of phytosiderophores. We were particularly interested in the effects of diurnal root exudation of phytosiderophores and of the presence of other organic ligands in the rhizosphere of graminaceous plants on the dissolution mechanism. Phytosiderophores of the type 2¢-deoxymugineic acid (DMA) were purified from the root exudates of wheat plants (Triticum aestivum L. cv. Tamaro). DMA-promoted dissolution of goethite under steady-state and non-steady-state conditions and its dependence on pH, adsorbed DMA concentration, and the presence of the organic ligand oxalate were studied. We show that dissolution of goethite by phytosiderophores follows a surface controlled ligand promoted dissolution mechanism. We also found that oxalate, an organic ligand commonly found in rhizosphere soils, has a synergistic effect on the steady-state dissolution of goethite by DMA. Under non-steady-state addition of the phytosiderophore, mimicking the diurnal exudation pattern of phytosiderophore release, a fast dissolution of iron is triggered in the presence of oxalate. To investigate the efficiency of these mechanisms in plant iron acquisition, wheat plants were grown on a substrate amended with goethite as only iron source. The chlorophyll status of these plants was similar to ironfertilized plants and significantly higher than in plants grown in iron free nutrient solutions. This demonstrates that wheat can efficiently mobilize iron, even from well crystalline goethite that is usually considered unavailable for plant nutrition.

Total Biosynthesis and Diverse Applications of the Nonribosomal Peptide-Polyketide Siderophore Yersiniabactin

“…3B). Such a result is not unexpected, since it has been reported that more complete dissolution of iron oxide deposits in siderophore-based desorption efforts involves contributions from small-molecule reductants (39)(40)(41)(42).…”

Effect of Exogenous Reductant on Growth and Iron Mobilization from Ferrihydrite by the Pseudomonas mendocina ymp Strain

“…Thermodynamically, siderophores are very capable of dissolving Fe(III) (hydr)oxides; however, the rate of in vitro dissolution is much slower than the in vivo microbial Fe uptake rate (5)(6)(7). In the presence of a reducing agent, siderophores have been shown to dissolve Fe(III) (hydr)oxides at a faster rate (1,9,12). This increased dissolution resulting from a combined effect of siderophore and reductant could potentially be the mechanism that suffices for the microbial demand for Fe (7).…”

Biogeochemistry of Metals and Metalloids at the Soil–Root Interface

“…Compared to microbial siderophores, even fewer kinetic data are available for plantborne organic ligands. However, Reichard et al (2005) studied the kinetics of dissolution of a ubiquitous Fe oxyhydroxide, goethite, in the presence of phytosiderophores and oxalate. In doing so, they were able to elucidate the role of these ligands, especially phytosiderophores that are unique to grasses, in the mobilization of Fe from goethite surfaces.…”

“…In comparison, this has been much less documented for metal cations, especially heavy metals (Rengel, 2002). A substantial proportion of recent research in this field has been dedicated to elucidating the implications of microbial and plant siderophores in the dissolution of Fe oxides (Inoue et al, 1993;Kraemer et al, 1999;Kraemer, 2004;Reichard et al, 2005). These have shown that such Fe-complexing compounds can efficiently dissolve Fe from poorly soluble Fe oxides, including goethite, which is a ubiquitous crystalline oxyhydroxide in soils.…”

“…These have shown that such Fe-complexing compounds can efficiently dissolve Fe from poorly soluble Fe oxides, including goethite, which is a ubiquitous crystalline oxyhydroxide in soils. Reichard et al (2005) have shown that the mechanism involved a ligand-promoted dissolution in the case of mugineic acid (phytosiderophore) and even reported a synergistic effect of oxalate and this phytosiderophore. There are fewer reports dealing with the potential impact of siderophores on the fate of metals other than Fe itself, although Fe oxides are known as major metal-bearing phases in soils.…”