U(VI) biosorption and biomineralization by Sulfolobus acidocaldarius

Microorganisms, along with the abiotic physicochemical factors, strongly influence migration of uranium in nature. In contrast to the well investigated interactions of bacteria with uranium, the influence of archaea on the natural behaviour of this radionuclide is still not well studied. In this work the interactions of the acidothermophilic archaeon Sulfolobus acidocaldarius DSM 639 with U(VI) at acidic conditions (pH 1.5, 3, 4.5 and 6) were investigated. These pH values are relevant for some heavy metal and uranium polluted environments where S. acidocaldarius is also distributed. After the contact with the archaeal cells, U(VI) was rapidly complexed, whereby the binding capacity was strongly influenced by the pH value and the biomass concentration. A combination of X-ray absorption spectroscopy, Fourier-transform infrared spectroscopy, and time-resolved laser-induced fluorescence spectroscopy revealed that at highly acidic conditions (pH 1.5 and 3) U(VI) is predominantly bound to organic phosphate groups. At pH 4.5 carboxylic groups are involved in the U(VI) complexation as well. However, at this pH part of the added U(VI) was precipitated in inorganic uranyl phosphate mineral phases. The latter were the most predominant uranium complexes found after the treatment of the cells with U(VI) for 48 hours at pH 6. Transmission electron microscopic analysis of the cells treated at pH 4.5 showed extracellular and intracellular U(VI) accumulates. The extracellular complexes represented mainly inorganic uranyl phosphate complexes, which were formed due to the liberation of orthophosphate by the cells. The formation of the intracellular uranyl phosphate deposits is attributed to uncontrolled uptake of U(VI) as a result of the increased cell permeability due to the stress of the non-optimal pH and uranium toxicity. Our results demonstrate that at moderate acidic conditions S. acidocaldarius immobilizes U(VI) via biosorption and biomineralization.