Association of Eu(III) and Cm(III) onto an extremely halophilic archaeon

Regarding the final storage of high-level radioactive waste in a deep geological repository next to geological, geochemical and geophysical also microbial aspects have to be taken into account. Rock salt is a potential host rock formation for the repository. One in rock salt common indigenous microorganism is the halophilic archaeon Halobacterium noricense DSM15987T, which was used in our study to investigate its interactions with the trivalent actinide curium and its inactive analogue europium as function of time and concentration. Time-resolved laser-induced fluorescence spectroscopy was applied to characterize formed species in the µM europium concentration range. An extended evaluation of the data with parallel factor analysis revealed the association of Eu(III) to a phosphate compound released by the cells (F2/F1 ratio: 2.50) and a solid species (F2/F1 ratio: 1.80). The association to an aqueous phosphate species and a solid species could be proven with the site-selective TRLFS. Experiments with Cm(III) in the nM concentration range showed a time- and pCH+-dependent species distribution. These species were characterized by red shifted emission maxima, 600 – 602 nm, in comparison to the free Cm(III) aqueous ion, 593.8 nm. After 24 h 40 % of the luminescence intensity was measured on the cells corresponding to 0.18 µg Cm(III)/gDBM. Our results demonstrate that Halobacterium noricense DSM15987T interacts with Eu(III) by the formation of phosphate species, whereas for Cm(III) also a complexation with carboxylic functional groups was observed.