Microscopic and spectroscopic investigations of the interactions of a Halobacterium-isolate with uranium


Microscopic and spectroscopic investigations of the interactions of a Halobacterium-isolate with uranium

Hilpmann, S.; Bader, M.; Bachran, M.; Steudtner, R.; Schmidt, M.; Stumpf, T.; Cherkouk, A.

Rock salt formations are potential host rocks for the long-term storage of high-level radioactive waste in a deep geological repository, besides clay and crystalline rock. There are multiple studies about the geological, geochemical and geophysical properties of these host rocks. However, there exists still a lack of knowledge about indigenous microorganisms and their influence on the chemical speciation. For a long-term risk assessment, it is of high interest to study how these microorganisms interact with radionuclides. Therefore, the interactions of an extremely halophilic archaeon, Halobacterium sp. GP5 1-1 with uranium, one of the major radionuclides of concern, were investigated in detail. This extremely halophilic archaeon was isolated from a German rock salt sample. Different microscopic and spectroscopic methods were combined to decipher the occurring processes on a molecular level.
To investigate the interaction kinetics of uranium(VI) onto the cells of Halobacterium sp. GP5 1-1, time-dependent association experiments with two different uranium(VI) concentrations were performed. At both concentrations the amount of bioassociated uranium(VI) increased with the incubation time. It was determined that the association process at the higher concentration (30 µM) was much slower than the kinetic at the lower uranium(VI) concentration (10 µM).
Various microscopic and spectroscopic methods were used to understand the interaction mechanisms on a molecular level. Overall, the association process is not exclusively a biosorption, which is a passive process and in general completed after a short time of incubation (0 – 2 h) [1]. The microscopic images of the live/dead staining show the formation of cell agglomerates after a certain exposition time at both concentrations. During the process, organic matter is excreted from the cells. Therefore, more functional groups are available for further uranium(VI) binding.
Electron microscopic images of the cells allowed drawing conclusions about different microbe-radionuclide interactions at different uranium(VI) concentrations. A biomineralization takes place at lower concentrations (10 µM) and uranium(VI) is bound to biofilm-like structures at higher concentrations (60 µM).
Using time-resolved laser-induced luminescence spectroscopy, different aqueous species could be extracted from the supernatant. These species differ slightly in dependence on the uranium(VI) concentration. In both cases, a uranyl-carbonate-complex is formed during the association process due to microbial released CO2. Additionally the formation of a phosphate species in the cell pellets at low uranium(VI) concentrations was observed in a uranium(VI) concentration-dependent experiment (10-60 µM). In contrast, at higher uranium(VI) concentrations a carboxylic species was formed. This is in agreement with the already mentioned excretion of organic matter from the cells during the uranium(VI) incubation.
These findings offer new insights into the microbe-actinide interactions at highly saline conditions relevant to high-level radioactive waste disposal in rock salt.[1]

[1] J. R. Lloyd, L. E. Macaskie (2002) in “Interactions of Microorganisms with Radionuclides” (Eds.: M.J. Keith-Roach, F. R. Livens), Elsevier, pp. 313-381

  • Lecture (Conference)
    Migration 2019, 15.-20.09.2019, Kyoto, Japan

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Publ.-Id: 28987