Impact of Haloarchaea on Speciation of Uranium—A Multispectroscopic Approach

Abstract

Haloarchaea represent a predominant part of the microbial community in rock salt, which can serve as host rock for the disposal of high level radioactive waste. However, knowledge is missing about how Haloarchaea interact with radionuclides. Here, we used a combination of spectroscopic and microscopic methods to study the interactions of an extremely halophilic archaeon with uranium, one of the major radionuclides in high level radioactive waste, on a molecular level. The obtained results show that Halobacterium noricense DSM 15987 influences uranium speciation as a function of uranium concentration and incubation time. X-ray absorption spectroscopy reveals the formation of U(VI) phosphate minerals, such as meta-autunite, as the major species at a lower uranium concentration of 30 μM, while U(VI) is mostly associated with carboxylate groups of the cell wall and extracellular polymeric substances at a higher uranium concentration of 85 μM. For the first time, we identified uranium biomineralization in the presence of Halobacterium noricense DSM 15987 cells. These findings highlight the potential importance of Archaea in geochemical cycling of uranium and their role in biomineralization in hypersaline environments, offering new insights into the microbe-actinide interactions in highly saline conditions relevant to the disposal of high-level radioactive waste as well as bioremediation.

Microscopic and spectroscopic bioassociation study of uranium(VI) with an archaeal Halobacterium isolate

“…Its interactions with this radionuclide have already been investigated by Bader et al . [ 17 – 19 ]. Both microorganisms differ only in one base pairing of 818 bp of the 16S rRNA gene sequence.…”

“…Afterwards, a slow association occurred, which was only completed after about two weeks. However, at higher uranium(VI) concentrations (85 μM), no desorption phase took place either [ 18 , 19 ]. It can be concluded that microorganisms which belong to the same genus or even species, do not always show the same interaction mechanisms under similar experimental conditions.…”

“…A comparison to the closely related Halobacterium noricense DSM 15987 T shows a certain agreement in the interaction mechanisms at different uranium(VI) concentrations. At 30 μM uranium(VI) a uranyl(VI) phosphate species is predominant in the spectra of the cell pellets, whereas at 85 μM the binding of uranium(VI) onto the cells occurs primarily via carboxylate groups of a lactate binding motive [ 19 ]. Nevertheless, these processes take place at higher uranium(VI) concentrations than for the studied Halobacterium species.…”

“…Subsequent measurements and data evaluation were carried out according to Bader et al . [ 19 ]. The measurements were performed at a temperature of 150 K for the supernatants and 110 K for the solid samples in order to minimize the quenching effect of the chloride anions on the uranium(VI) fluorescence [ 23 ].…”

“…The uranium(VI) concentration in the samples was 10 μM, the concentration of LPS 0.5 g/L and a background electrolyte of 3 M NaCl was used. The sample was adjusted to a pC H+ value of 6.0 and measured under the same conditions as the samples of the supernatant [ 19 ].…”