Complexation of actinides with bioligands sectreted by a subsurface strain of Pseudomonas fluorescens

Microbes secrete chelating agents in the surrounding aquifer system. These bioligands, in the case of the ubiquitous fluorescent Pseudomonas spp. pyoverdin-type siderophores, possess a high potential to bind besides Fe(III) also other metals like actinides [1]. The unknown interaction of actinides (U(VI), Cm(III)) with pyoverdin-type bioligands released from the aerobic soil bacterium Pseudomonas fluorescens (CCUG 32456 A) isolated from the aquifers at the Äspö Hard Rock Laboratory, Sweden is the subject of this study. The cells synthesized a bioligand mixture which consists of ferribactin (biogenetic precursors of the pyoverdin) and pyoverdin here in a ratio of 2:1.
The complexation studies were performed at pyoverdin concentrations below 0.1 mM and at an ionic strength of 0.1 M by applying different spectroscopic techniques. Differences in the complexation behavior depending on the nature of the actinide element will be discussed on the basis of the determined complex formation constants.
Two UO22+ -P. fluorescens (CCUG 32456 A) pyoverdin species could be identified due to their individual absorption spectra and molar extinction coefficients.
The Cm(III)-borne pyoverdin species were investigated by fluorescence emission at different excitation wavelengths (360 and 395 nm), i.e. directly at the absorption maxima of Cm(III) or indirectly by energy transfer from the pyoverdin molecule. A strong red shift of the emission signal of app. 7 nm compared to the Cm(III) aquo ion was already observed at low pyoverdin concentrations of 3x10-7 M and at pH 4. Three different Cm- P. fluorescens (CCUG 32456 A) pyoverdin species could be distinguished on the basis of their individual fluorescence emission spectra and lifetimes.
This study helps to improve the understanding of the actinide coordination chemistry with natural pyoverdin-type siderophores in aqueous solution. Such investigations with selected bioligands are essential to explain the overall interaction process of actinides with microbes on a molecular level.

[1] M. Bouby, I. Billard, H.J. Maccordick, Czechoslovak J. Phys. 49, 147-150 (1999).

This work was funded by the BMWi under contract number: 02E9985.