Multistage bioassociation of uranium(VI) on a halophilic archaeum investigated with luminescence spectroscopic and microscopic techniques

Luminescence spectroscopy and microscopy are powerful tools to study the chemistry of f-elements (actinides – An, lanthanides – Ln) in trace concentration. Manifold operating modes (e.g. steady-state, time-resolved, laser-induced, site-selective, cryogenic, etc.) can be used to investigate the environmental behaviour of An/Ln in various geological and biological systems. In this study the halophilic archaeum Halobacterium noricense DSM-15987T, which commonly occurs in rock salt [1], a potential host rock formation for the deep geological disposal of radioactive waste, was used to characterize its interaction processes with uranium(VI). The bioassociation showed differences regarding initial uranium concentration (30 and 85 µM) and incubation time (up to two weeks). The lower uranium concentration caused a multistage association behaviour with a desorption phase after 5 h of exposure to uranium. A further difference provoked by uranium concentration was the cell agglomeration. This formation was pronounced at higher uranium(VI) concentration. For lower uranium(VI) concentration agglomerate formation took longer, but still occurred. To understand these processes on a molecular level, time-resolved laser-induced fluorescence spectroscopy at low temperature of 153 K (cryo-TRLFS) was applied. Changes detected in the batch experiments were confirmed with cryo TRLFS. The spectroscopic analyses showed the involvement of a polynuclear carboxylate species and the presence of a meta-autunite like uranium(VI) mineral phase [2]. A fraction analysis revealed that the biomineralization process (formation of meta-autunite) is more pronounced at lower uranium(VI) concentration. At the higher uranium(VI) concentration of 85 µM polynuclear carboxylate species were dominating. This could be explained with the higher toxicity of uranium(VI) and the accompanying cell agglomeration, which was more pronounced at higher uranium(VI) concentration. In combination with other spectroscopic (e.g. infrared spectroscopy) and microscopic tools (e.g. scanning electron microscopy) the applied luminescence methods were essential for a better understanding of the bioassociation process of uranium(VI) to cells of the halophilic archaeon.

[1] A. Gramain, Environ. Microbiol. 2011, 13, 2105-2121. [2] M. Bader, submitted to Environ. Sci. Technol. [3] S. Fröls, Biochem. Soc. Trans. 2013, 41, 393-398.