Bioavailability and microbial adaptation to elevated levels of uranium in an acid, organic topsoil forming on an old mine spoil

An old mine spoil at a 19th‐century mining site with considerable residues of uranium (400–800 mg U/kg) was investigated with respect to U concentrations in soil and plants and tolerance to U in the soil microbial community in order to describe the bioavailability of U. Measurements of soil fractions representing water‐soluble U, easily exchangeable U, and U bound to humified organic matter showed that all fractions contained elevated concentrations of U. Plant U concentrations were only 10 times higher at the mine spoil site compared to the reference site (3 mg U/kg vs 0.3 mg U/kg), while the most easily available soil fractions contained 0.18 to 0.86 mg U/kg soil at the mine spoil. An ecotoxicity bioassay using incorporation of [ 3 H]thymidine into the indigenous microbial communities of the two soils in the presence of increasing U concentrations showed that microorganisms at the mining site were sensitive to U but also that they had acquired a substantial tolerance toward U (EC50, the effective concentration reducing activity by 50% of UO 2 ‐citrate was ≈ 120 μM as compared to 30 μM in the reference soil). In the assay, more than 40% of the microbial activity was maintained in the presence of 1 μM UO2‐citrate versus 3% in the reference soil. We conclude that U‐enriched mining waste can contain sufficiently elevated concentrations of bioavailable U to affect indigenous microorganisms and that bioavailable U imposes a selection pressure that favors the development of a highly uranium‐tolerant microbial community, while plant uptake of U remains low.