Calorimetrically Determined U(VI) Toxicity in Brassica napus Correlates with Oxidoreductase Activity and U(VI) Speciation.

Radioecological studies depend on the quantitative toxicity assessment of environmental radionuclides. At low dose exposure, the life span of affected organisms is barely shortened, enabling the transfer of radionuclides through an almost-intact food chain. Lethality-based toxicity estimates are not adequate in this regime because they require higher concentrations. However, increased radionuclide concentration alters its speciation, rendering the extrapolation to the low dose exposure chemically inconsistent. Here, we demonstrate that microcalorimetry provides a sensitive real-time monitor of toxicity of uranium (in the U(VI) oxidation state) in a plant cell model of Brassica napus. We introduce the calorimetric descriptor "metabolic capacity" and show that it correlates with enzymatically determined cell viability. It is independent of physiological models and robust against the naturally occurring fluctuations in the metabolic response to U(VI) of plant cell cultures. In combination with time-resolved laser-induced fluorescence spectroscopy and thermodynamic modeling, we show that the plant cell metabolism is affected predominantly by hydroxo-species of U(VI) with an IC50 threshold of ∼90 μM. The data emphasize the yet-little-exploited potential of microcalorimetry for the speciation-sensitive ecotoxicology of radionuclides.