Radioecological risk assessment of low selenium concentrations through genetic fingerprints and metabolic profiling of soil bacterial communities.

In a context of environmental risk assessment of nuclear (79)Se radionuclide, the impact of low Se-selenite concentrations (0.008 and 8 mg kg(-1)) on bacterial communities of two soils, a silty clay loam and a sandy soil, was investigated over a 6-month incubation time. This Se-selenite was partially labelled with (75)Se. The state of the Se-impacted bacterial communities was analyzed through total bacterial counts, DNA fingerprints (ARISA profiles) and metabolic profiling (carbon substrate utilization patterns). Furthermore, the genetic diversity of bacterial populations involved in Se volatilization was evaluated by tpm (thiopurine methyltransferase gene) profiling. Emissions of (75)Se and CaCl(2)-extractable (75)Se were measured by γ-spectrometry and scintillation analysis. Se-selenite inputs changed transiently the substrate utilization patterns of bacterial communities but did not affect the other indicators. Se volatilization was at its highest level just after adding Se-selenite and for about 1 week. This volatilization was proportional to the added Se-selenite concentrations. It was 100-fold higher in silty clay loam, even though Se bioavailability was reduced in this soil. The soils were amended with crushed grass 3 months after the addition of Se-selenite. This organic amendment affected the organization of bacterial communities and increased the Se-volatilizing activities of both soils. Original soil organic carbon and bacterial diversity and activities seemed responsible for the different levels of Se emissions observed in soils. tpm lineages, encoding Se methyltransferases, were detected in both soils, confirming the broad distribution of tpm-harbouring bacteria and their probable role in the emissions of volatile Se. Five distinct groups of tpm were recorded per soil, with tpmI lineage being detected throughout the incubation period. This study demonstrates the ability of bacterial communities at volatilizing Se concentrations inferior to geochemical backgrounds and suggests that a probable transfer of nuclear Se will occur through volatilization after an environmental spill.