Combined bioreduction and volatilization of SeVI by Stenotrophomonas bentonitica: Formation of trigonal selenium nanorods and methylated species.

Nowadays, metal pollution due to the huge release of toxic elements to the environment has become one of the world's biggest problems. Bioremediation is a promising tool for reducing the mobility and toxicity of these contaminants (e.g. selenium), being an efficient, environmentally friendly, and inexpensive strategy. The present study describes the capacity of Stenotrophomonas bentonitica to biotransform SeVI through enzymatic reduction and volatilization processes. HAADF-STEM analysis showed the bacterium to effectively reduce SeVI (200 mM) into intra- and extracellular crystalline Se0 nanorods, made mainly of two different Se allotropes: monoclinic (m-Se) and trigonal (t-Se). XAS analysis appears to indicate a Se crystallization process based on the biotransformation of amorphous Se0 into stable t-Se nanorods. In addition, results from headspace analysis by gas chromatography-mass spectometry (GC-MS) revealed the formation of methylated volatile Se species such as DMSe (dimethyl selenide), DMDSe (dimethyl diselenide), and DMSeS (dimethyl selenenyl sulphide). The biotransformation pathways and tolerance are remarkably different from those reported with this bacterium in the presence of SeIV. The formation of crystalline Se0 nanorods could have positive environmental implications (e.g. bioremediation) through the production of Se of lower toxicity and higher settleability with potential industrial applications.

Unlocking the bentonite microbial diversity and its implications in selenium bioreduction and biotransformation: Advances in deep geological repositories.

Selenium, 79Se, is one of the most critical radionuclides in radioactive waste disposed in future deep geological repositories (DGRs). Here, we investigate the impact of bentonite microbial communities on the allotropic transformation of Se(IV) bioreduction products under DGR relevant conditions. In addition, Se amendment-dependent shifts in the bentonite microbial populations are assessed. Microcosms of water-saturated bentonites were spiked with a bacterial consortium, treated with selenite and incubated anaerobically for six months. A combination of X-Ray Absorption Spectroscopy, Electron Microscopy, and Raman Spectroscopy was used to track the allotropic changes of the Se bioreduction products. Interestingly, the color of bentonite shifted from orange to black in the selenite-treated microcosms. In the orange layers, amorphous or monoclinic Se(0) were identified, whilst black precipitates consisted of stable trigonal Se(0) form. Illumina DNA sequencing indicated the distribution of strains with Se(IV) reducing and Se(0) allotropic biotransformation potential, like Pseudomonas, Stenotrophomonas, Desulfosporosinus, and unclassified-Desulfuromonadaceae. The archaea Methanosarcina decreased its abundance in the presence of Se(IV), probably caused by this oxyanion toxicity. These findings provide an understanding of the bentonite microbial strategies involved in the immobilization of Se(IV) by reduction processes, and prove their implication in the allotropic biotransformation from amorphous to trigonal Se(0) under DGR relevant conditions.

Effect of U(VI) aqueous speciation on the binding of uranium by the cell surface of Rhodotorula mucilaginosa, a natural yeast isolate from bentonites.

This study presents the effect of aqueous uranium speciation (U-hydroxides and U-hydroxo-carbonates) on the interaction of this radionuclide with the cells of the yeast Rhodotorula mucigilanosa BII-R8. This strain was isolated from Spanish bentonites considered as reference materials for the engineered barrier components of the future deep geological repository of radioactive waste. X-ray absorption and infrared spectroscopy showed that the aqueous uranium speciation has no effect on the uranium binding process by this yeast strain. The cells bind mobile uranium species (U-hydroxides and U-hydroxo-carbonates) from solution via a time-dependent process initiated by the adsorption of uranium species to carboxyl groups. This leads to the subsequent involvement of organic phosphate groups forming uranium complexes with a local coordination similar to that of the uranyl mineral phase meta-autunite. Scanning transmission electron microscopy with high angle annular dark field analysis showed uranium accumulations at the cell surface associated with phosphorus containing ligands. Moreover, the effect of uranium mobile species on the cell viability and metabolic activity was examined by means of flow cytometry techniques, revealing that the cell metabolism is more affected by higher concentrations of uranium than the cell viability. The results obtained in this work provide new insights on the interaction of uranium with bentonite natural yeast from genus Rhodotorula under deep geological repository relevant conditions.