Unveiling fungal diversity in uranium and glycerol-2-phosphate-amended bentonite microcosms: Implications for radionuclide immobilization within the Deep Geological Repository system

Role of growth conditions and physicochemical factors controlling the removal and biomineralization of U(VI) by Stenotrophomonas bentonitica BII-R7

Microbial U(VI)-phosphate biomineralization-based bioremediation has gained attention as a cost-effective and eco-friendly strategy. However, key environmental and biological factors controlling the process remain unclear. The present work assesses the influence of pH and physiological state of Stenotrophomonas bentonitica BII-R7 cells on U(VI) biomineralization under growing and non-growing conditions. The results showed that biomineralization was more effective at growing cells, removing up to 95 % of soluble U after 24 h, forming needle-shape accumulates on the cell surface and extracellular that avoid the entrance of U(VI) in the cells. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) analyses, along with the phosphatase activity detected under these conditions, suggested a two-stage process: first, a fast-passive biosorption of U(VI) to organic phosphate groups of the cell surface and secondly the biomineralization in form of U(VI)-phosphate precipitates by the activity of phosphatase enzymes. Furthermore, the pH seemed to influence the efficiency of the biomineralization, being more effective at pH 7 than 5.5, as it could affect the free functional groups available for biosorption. Therefore, the results highlight the key factors that need to be controlled for the long-term removal of U(VI) via biomineralization for bioremediation purposes.

Soil-dependent responses of bacterial communities, phosphorus and carbon turnover to uranium stress in different soil ecosystems

Uranium (U) can impact microbially driven soil phosphorus (P) and carbon (C) cycling. However, the response of microbial P and C turnover to U in different soils is not well understood. Through the quantitative assay of P pools and soil organic C (SOC) quantitative assay and sequencing of 16S rRNA gene amplicons and metagenomes, we investigated the effect of U on P and C biotransformation in grassland (GL), paddy soil (PY), forest soil (FT). U (60 mg kg-1) impacted the diversity, interaction and stability of soil bacterial communities, leading to a decrease in available P (AP). Under U stress, organophosphate mineralization substantially contributed to the AP in GL and FT, whereas intracellular P metabolism dominated the AP in PY. Also, the reductive citrate cycle (rTCA cycle) promoted the content of SOC in GL, while the rTCA cycle and complex organic C degradation pathways enhanced the SOC in PY and FT, respectively. Notably, functional bacteria carrying organic C degradation genes could decompose SOC to enhance soil AP. Bacteria developed various resistance strategies to cope with U stress. This study reveals soil-dependent response of microbial P and C cycling and its ecological functions under the influence of radioactive contaminants in different soil systems.

Enhancing Se(IV) bioremediation efficiency via immobilization of filamentous fungi and yeasts in eco-friendly alginate bead hydrogels

The immobilization of microorganisms in polymeric hydrogel has gained attention as a potential method for applications in various fields, offering several advantages over traditional cell free-living technologies. The present study aims to compare the efficiency of selenium (Se) bioremediation and biorecovery by two different fungal types, both in their free and immobilized forms using alginate hydrogels. Our results demonstrated an improvement in the amount of Se(IV) removed from the hydrogels of Aspergillus ochraceus (∼97%) and Rhodotorula mucilaginosa (∼43%) compared to that of the planktonic cultures (∼57% and ∼9-17%). In both cases, most of the Se(IV) is enzymatically reduced by the cells to amorphous Se(0) nanospheres, which are retained throughout the alginate hydrogels. The extensive growth, colonization and distribution of the cells throughout the highly porous hydrogel, along with their ability to maintain viability over long periods and the preservation of the structural integrity of the hydrogel, demonstrated the enormous biotechnological potential of the studied system for practical applications. The results reported show that the immobilization of fungi in alginate hydrogels is an efficient and environmentally friendly alternative for bioremediation and biorecovery of Se nanoparticles, which are of significant industrial and medical interest within the framework of a circular economy.

Insights into the Impact of Physicochemical and Microbiological Parameters on the Safety Performance of Deep Geological Repositories

Currently, the production of radioactive waste from nuclear industries is increasing, leading to the development of reliable containment strategies. The deep geological repository (DGR) concept has emerged as a suitable storage solution, involving the underground emplacement of nuclear waste within stable geological formations. Bentonite clay, known for its exceptional properties, serves as a critical artificial barrier in the DGR system. Recent studies have suggested the stability of bentonite within DGR relevant conditions, indicating its potential to enhance the long-term safety performance of the repository. On the other hand, due to its high resistance to corrosion, copper is one of the most studied reference materials for canisters. This review provides a comprehensive perspective on the influence of nuclear waste conditions on the characteristics and properties of DGR engineered barriers. This paper outlines how evolving physico-chemical parameters (e.g., temperature, radiation) in a nuclear repository may impact these barriers over the lifespan of a repository and emphasizes the significance of understanding the impact of microbial processes, especially in the event of radionuclide leakage (e.g., U, Se) or canister corrosion. Therefore, this review aims to address the long-term safety of future DGRs, which is critical given the complexity of such future systems.

Unlocking the key role of bentonite fungal isolates in tellurium and selenium bioremediation and biorecovery: Implications in the safety of radioactive waste disposal

Research on eco-friendly bioremediation strategies for mitigating the environmental impact of toxic metals has gained attention in the last years. Among all promising solutions, bentonite clays, to be used as artificial barriers to isolate radioactive wastes within the deep geological repository (DGR) concept, have emerged as effective reservoir of microorganisms with remarkable bioremediation potential. The present study aims to investigate the impact of bentonite fungi in the speciation and mobility of selenium (Se) and tellurium (Te), as natural analogues 79Se and 132Te present in radioactive waste, to screen for those strains with bioremediation potential within the context of DGR. For this purpose, a multidisciplinary approach combining microbiology, biochemistry, and microscopy was performed. Notably, Aspergillus sp. 3A demonstrated a high tolerance to Te(IV) and Se(IV), as evidenced by minimal inhibitory concentrations of >16 and >32 mM, respectively, along with high tolerance indexes. The high metalloid tolerance of Aspergillus sp. 3A is mediated by its capability to reduce these mobile and toxic elements to their elemental less soluble forms [Te(0) and Se(0)], forming nanostructures of various morphologies. Advanced electron microscopy techniques revealed intracellular Te(0) manifesting as amorphous needle-like nanoparticles and extracellular Te(0) forming substantial microspheres and irregular accumulations, characterized by a trigonal crystalline phase. Similarly, Se(0) exhibited a diverse array of morphologies, including hexagonal, irregular, and needle-shaped structures, accompanied by a monoclinic crystalline phase. The formation of less mobile Te(0) and Se(0) nanostructures through novel and environmentally friendly processes by Aspergillus sp. 3A suggests it would be an excellent candidate for bioremediation in contaminated environments, such as the vicinity of deep geological repositories. It moreover holds immense potential for the recovery and synthesis of Te and Se nanostructures for use in numerous biotechnological and biomedical applications.