12-35: Thorium uptake by microalgal cells: Toxicity and bioaccumulation

The toxicity and bioaccumulation of thorium by two green microalgae, Monoraphidium sp. and Scenedesmus sp. were studied. During the toxicity tests, the microalgal cultures were inoculated in ASM-1 culture medium in the presence and absence of thorium. Its effect was monitored by direct counting (Fuchs- Rosenthal chamber) and with the help of software developed by the group. The toxicity of the metal over the species was observed just for thorium concentrations over 50.0 mg/L. A Monoraphidium sp. culture containing 6.25x10⁵ microorganisms/mL reached a final concentration of 5.52x10⁷ microorganisms/mL in the presence of thorium in the concentration of 10.0 mg/L. If we consider the 100.0 mg/L thorium solution reached a final concentration of 8.57x10⁶ microorganisms/mL. Control tests indicated a final concentration of 2.51x10⁷ microorganisms/mL. Scenedesmus sp. cells proved to be more resistant to the presence of thorium in solution. Low concentrations of the radionuclide favored the growth of these microalgae. A culture containing 7.65x10⁵ microorganisms/mL reached a final concentration of 2.25x10⁶ microorganisms/mL, in the absence of thorium. Toxicological tests indicated a final culture concentration of 5.87x10⁶ microorganisms/mL in the presence of 0.5 mg/L thorium. The software used for comparison of direct count method proved to be very useful for the improvement of accuracy of the results obtained and a decrease in the uncertainty in counting. From the present results one can conclude that the presence of thorium in the medium suggests that low thorium concentrations have a positive effect on this phytoplankton, indicating its potential for ecotoxicological studies.