Effect of pyoverdine supply on cadmium and nickel complexation and phytoavailability in hydroponics
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Siderophores are chelators with a high selectivity for Fe(III) and a good affinity for divalent metals, including Cd(II) and Ni(II). Inoculation with siderophore-producing bacteria (SPB) has thus been proposed as an alternative to chelator supply in phytoremediation. Accurate assessments of the potential of this association require a dissection of the interaction of siderophores with metals at the soil–root interface. This study focuses on pyoverdine (Pvd), the main siderophore produced by Pseudomonas aeruginosa. We first assessed the ability of Pvd to coordinate Ni(II). The stability constant of Pvd–Ni(II) (log K L′Ni = 10.9) was found to be higher than that of Pvd–Cd(II) (log K L′Cd = 8.2). We then investigated the effect of a direct supply of Pvd on the mobilization, speciation, and phytoavailability of Cd and Ni in hydroponics. When supplied at a concentration of 50 μM, Pvd selectively promoted Ni mobilization from smectite. It decreased plant Ni and Cd contents and the free ionic fractions of these two metals, consistent with the free ion activity model. Pvd had a more pronounced effect for Ni than for Cd, as predicted from its coordination properties. Inoculation with P. aeruginosa had a similar effect on Ni phytoavailability to the direct supply of Pvd.
KeywordsBacterial siderophore Divalent metals Phytoremediation Pyoverdine Smectite Speciation
We thank Sylvie Bussière, Cécile Fontaine, and Françoise Hoegy for technical assistance; Sophie Raous for providing the purified smectite; and André Schneider for his help in determining the free ionic fraction of Cd by the ion exchange method. Financial support for this work was provided by the Programme Interdisciplinaire CNRS-CEMAGREF “Ingénierie écologique” and the Alsace Region Research Network in Environmental Sciences and Engineering” (REALISE). C. Ferret held a fellowship from the Région Alsace and the DGA (Direction Générale de l’Armement).
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