Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate.

Naraya Carrasco,Jide Xu,Stephan M Kraemer,Ruben Kretzschmar

doi:10.1186/1467-4866-10-5

Abstract

Siderophore-promoted iron acquisition by microorganisms usually occurs in the presence of other organic molecules, including biosurfactants. We have investigated the influence of the anionic surfactant sodium dodecyl sulfate (SDS) on the adsorption of the siderophores DFOB (cationic) and DFOD (neutral) and the ligand EDTA (anionic) onto goethite (α-FeOOH) at pH 6. We also studied the adsorption of the corresponding 1:1 Fe(III)-ligand complexes, which are products of the dissolution process. Adsorption of the two free siderophores increased in a similar fashion with increasing SDS concentration, despite their difference in molecule charge. In contrast, SDS had little effect on the adsorption of EDTA. Adsorption of the Fe-DFOB and Fe-DFOD complexes also increased with increasing SDS concentrations, while adsorption of Fe-EDTA decreased. Our results suggest that hydrophobic interactions between adsorbed surfactants and siderophores are more important than electrostatic interactions. However, for strongly hydrophilic molecules, such as EDTA and its iron complex, the influence of SDS on their adsorption seems to depend on their tendency to form inner-sphere or outer-sphere surface complexes. Our results demonstrate that surfactants have a strong influence on the adsorption of siderophores to Fe oxides, which has important implications for siderophore-promoted dissolution of iron oxides and biological iron acquisition.

Highlights

The bioavailability of Fe(III) in oxic soils, sediments, and surface waters at near-neutral pH is limited by the low solubility and slow dissolution rates of iron oxides and hydroxides
In order to better understand the nature of the interactions responsible for siderophore adsorption in the presence of surfactants, we studied the influence of sodium dodecyl sulfate (SDS) on the adsorption of three complexing ligands and their respective iron complexes to goethite
Under the assumption that EDTA forms only inner-sphere complexes and that the charge of adsorbed EDTA is located at or close to the surface its adsorption does not depend on variations of the potential above the hemimicelles and admicelles

Summary

Introduction

The bioavailability of Fe(III) in oxic soils, sediments, and surface waters at near-neutral pH is limited by the low solubility and slow dissolution rates of iron oxides and hydroxides. In order to overcome this low iron solubility, many microorganisms and roots of graminaceous plants exude highly Fe(III)-specific, low-molecular weight (0.5 to 1.5 kDa) ligands, the a group of compounds called siderophores [1,2,3,4,5]. Most siderophores are hexadentate and form 1:1 Fe(III)-complexes [7,8,9,10]. Even in soils in equilibrium with calcium carbonate, where otherwise strong competition between Ca and iron for complexation would be expected, the (page number not for citation purposes)

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