Laureline Février

Influence of uranium on bacterial communities: a comparison of natural uranium-rich soils with controls.

This study investigated the influence of uranium on the indigenous bacterial community structure in natural soils with high uranium content. Radioactive soil samples exhibiting 0.26% - 25.5% U in mass were analyzed and compared with nearby control soils containing trace uranium. EXAFS and XRD analyses of soils revealed the presence of U(VI) and uranium-phosphate mineral phases, identified as sabugalite and meta-autunite. A comparative analysis of bacterial community fingerprints using denaturing gradient gel electrophoresis (DGGE) revealed the presence of a complex population in both control and uranium-rich samples. However, bacterial communities inhabiting uraniferous soils exhibited specific fingerprints that were remarkably stable over time, in contrast to populations from nearby control samples. Representatives of Acidobacteria, Proteobacteria, and seven others phyla were detected in DGGE bands specific to uraniferous samples. In particular, sequences related to iron-reducing bacteria such as Geobacter and Geothrix were identified concomitantly with iron-oxidizing species such as Gallionella and Sideroxydans. All together, our results demonstrate that uranium exerts a permanent high pressure on soil bacterial communities and suggest the existence of a uranium redox cycle mediated by bacteria in the soil.

Aqueous, solid and gaseous partitioning of selenium in an oxic sandy soil under different microbiological states

The aim of this study was to investigate the role of microorganisms on the behaviour of selenium in natural soil maintained under strictly aerobic conditions. Six-day batch experiments were performed with soils constrained to different microbiological states, either by sterilisation or by adding organic substrates. Selenium was added to the soil as selenite. The distribution of selenium in the gaseous, liquid and solid phases of the batch was measured. Selenium partitioning between the various solid phases was investigated by chemical sequential extractions. Active microorganisms played major effects on the distribution of selenium within the soil. On the one hand, microorganisms could promote selenium volatilisation (in relatively small amounts), leading to the spreading of selenium compounds outside the soil. On the other hand, microbial activities increased both amount of selenium retained by the soil and the strength of its retention (less exchangeable selenium), making selenium less susceptible to remobilisation.

Use of combined microscopic and spectroscopic techniques to reveal interactions between uranium and Microbacterium sp. A9, a strain isolated from the Chernobyl exclusion zone

Although uranium (U) is naturally found in the environment, soil remediation programs will become increasingly important in light of certain human activities. This work aimed to identify U(VI) detoxification mechanisms employed by a bacteria strain isolated from a Chernobyl soil sample, and to distinguish its active from passive mechanisms of interaction. The ability of the Microbacterium sp. A9 strain to remove U(VI) from aqueous solutions at 4 °C and 25 °C was evaluated, as well as its survival capacity upon U(VI) exposure. The subcellular localisation of U was determined by TEM/EDX microscopy, while functional groups involved in the interaction with U were further evaluated by FTIR; finally, the speciation of U was analysed by TRLFS. We have revealed, for the first time, an active mechanism promoting metal efflux from the cells, during the early steps following U(VI) exposure at 25 °C. The Microbacterium sp. A9 strain also stores U intracellularly, as needle-like structures that have been identified as an autunite group mineral. Taken together, our results demonstrate that this strain exhibits a high U(VI) tolerance based on multiple detoxification mechanisms. These findings support the potential role of the genus Microbacterium in the remediation of aqueous environments contaminated with U(VI) under aerobic conditions.

Influence of thermodynamic database on the modelisation of americium(III) speciation in a simulated biological medium

Summary An intercomparison exercise was led in order to study the impact of thermodynamic database (TDB) on the speciation of americium(III) in biochemical media. A first exercice was led, considering only the americium(III) species in solution. Even with different data sets, all the exercises are in relatively good agreement, concluding to successive complexation of the radioelement by phosphate (pH range from 0 to 3), citrate (pH range from 3 to 8) and hydroxo-carbonate (pH range from 8 to 14). Nevertheless, the speciation of americium differs for each pH range from one work to an other. These results are still observed when the speciation calculation includes the formation of solids. Nevertheless, some workers did not integrate in their data set the formation constant of Am(PO4). This solid is known to be very insoluble in typical biological media. All the calculations where Am(PO4) was not in the TDB presented a speciation where americium is solvated (mostly under citrate forms). When the formation of Am(PO4) is considered, the actinide is quantitatively present under this solid species. This work is a representative example of the impact of the thermodynamic data used in speciation exercises. Some exercices, using uncompleted TDB had wrong results and led to the conclusion of the necessity in the use of expertised and extended TDB.

Assessment of co-contaminant effects on uranium and thorium speciation in freshwater using geochemical modelling

Speciation modelling of uranium (as uranyl) and thorium, in four freshwaters impacted by mining activities, was used to evaluate (i) the influence of the co-contaminants present on the predicted speciation, and (ii) the influence of using nine different model/database combinations on the predictions. Generally, co-contaminants were found to have no significant effects on speciation, with the exception of Fe(III) in one system, where formation of hydrous ferric oxide and adsorption of uranyl to its surface impacted the predicted speciation. Model and database choice on the other hand clearly influenced speciation prediction. Complexes with dissolved organic matter, which could be simulated by three of the nine model/database combinations, were predicted to be important in a slightly acidic, soft water. Model prediction of uranyl and thorium speciation needs to take account of database comprehensiveness and cohesiveness, including the capability of the model and database to simulate interactions with dissolved organic matter. Measurement of speciation in natural waters is needed to provide data that may be used to assess and improve model capabilities and to better constrain the type of predictive modelling work presented here.

Is enough information available to derive an overall EQS for uranium in French freshwaters, according to European Guidance?

An overall criterion for protecting freshwaters against uranium chemo-toxicity should encompass all the compartments of the freshwater environment at risk (water column, sediment, predators, and humans). According to the latest European recommendations, we revised the interim Environmental Quality Standard currently applicable in France, to introduce physico-chemical parameters affecting uranium bioavailability in water and therefore toxicity, by using chemical speciation modelling in aqueous phase including both mineral ligands and organic mat-ter. Additionally, we completed the poor available data set related to sediment toxicity by implementing dedicated standardized tests and we explored the question of secondary poisoning. Human health was also briefly considered, to fulfill the methodological requirements.

Ecotoxicity of Uranium in Freshwaters: Influence of the Physico-Chemical Status of the Rivers

As uranium speciation, and then its bioavailability, is highly variable in the range of the physico-chemistry of freshwaters, determining environmental protection criteria for generic ecosystems, i.e. without considering these processes, may lead to very low values. Based on the most recent knowledge on the variation of uranium bioavailability vs. its speciation, “conditional” protection criteria have been determined for 50 main physico-chemical domains, still including some identified conservatism, due to complexation properties of uranium.

Soil prokaryotic communities in Chernobyl waste disposal trench T22 are modulated by organic matter and radionuclide contamination

&NA; After the Chernobyl nuclear power plant accident in 1986, contaminated soils, vegetation from the Red Forest and other radioactive debris were buried within trenches. In this area, trench T22 has long been a pilot site for the study of radionuclide migration in soil. Here, we used 454 pyrosequencing of 16S rRNA genes to obtain a comprehensive view of the bacterial and archaeal diversity in soils collected inside and in the vicinity of the trench T22 and to investigate the impact of radioactive waste disposal on prokaryotic communities. A remarkably high abundance of Chloroflexi and AD3 was detected in all soil samples from this area. Our statistical analysis revealed profound changes in community composition at the phylum and OTUs levels and higher diversity in the trench soils as compared to the outside. Our results demonstrate that the total absorbed dose rate by cell and, to a lesser extent the organic matter content of the trench, are the principal variables influencing prokaryotic assemblages. We identified specific phylotypes affiliated to the phyla Crenarchaeota, Acidobacteria, AD3, Chloroflexi, Proteobacteria, Verrucomicrobia and WPS‐2, which were unique for the trench soils.