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

Abstract

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.

Figure 1. Location of the study sites Vénachat and Villard and the different sampling points.

C: control samples. U: uranium-rich samples. Sampling years are labelled as: -06 (2006); -07 (2007); and -09 (2009).

Figure 2. Scanning Electron Micrographs coupled with Energy Dispersive X-ray spectra analysis of Villard ViU-09 soil particles.

For each SEM image, the corresponding EDXS map for uranium (U) and phosphate (P) is presented. (A) soil particles. (B) soil particles and cell-shaped objects. (C) cell-shaped object.

Figure 3. Uranium speciation in soils.

(A) Uranium L_III-edge XAS spectra, normalized to equal intensity at 17.176 keV. (B) k³-weighted EXAFS curves for Vénachat and Villard soils, and for yellow particles extracted from Villard soil. All spectra are compared to reference. The m-autunite spectrum was from .

Figure 4. Principal Component Analysis of DGGE profiles from Vénachat soil samples.

Individual sample projections are shown on the first two axes of the analysis. The first axis (PC1) and the second axis (PC2) account for 45.5% and 15% of the variability, respectively. C corresponds to control samples, whereas U, U1 and U2 correspond to uranium-rich samples. 06 and 07 indicate the years 2006 and 2007. a,b,c correspond to replicates, and p signifies pooled DNA.

Figure 5. Principal Component Analysis of DGGE profiles from Villard soil samples.

(A) Individual sample projections on the first two axes of the analysis. The first axis (PC1) and the second axis (PC2) account for 41.1% and 23.8% of the variability, respectively. (B) Individual DGGE band projections on the first two axes. Band_5 position (masked by band_38) is indicated with an arrow. C corresponds to control samples, whereas U corresponds to uranium-rich samples. 06, 07 and 09 indicate the years 2006, 2007 and 2009 respectively. a,b,c correspond to replicates.