Toxic effects of Cd++, Cu++, and Zn++ on Selenastrum capricornutum were evaluated in various experimental conditions in order to determine the influence of the composition of the test medium and of the concentration of the algal biomass on the toxicity of metals to algae. Four media recommended in different standard methods (International Standards Organisation, U.S. Environmental Protection Agency, Organisation for Economic Cooperation and Development, AFNOR) were compared. Results of algal bioassays showed that the sensitivity of algae during the exponential growth phase was not influenced by the concentration of macronutrients in the medium.
On the contrary, the numeration of the algal suspensions tested appeared determinant as the toxicity decreased when the quantity of algal inoculum was increased: with inocula of nearly 3 × 104, 2.5 × 105, 6 × 105, and 3 × 106 cells/mL at t = 0, the corresponding EC50 were 46, 80, 110, and up to 300 μg/L for cadmium, 10, 65, 105, and 280 μg/L for copper, and 90, 163, 225, and 365 μg/L for zinc. Modifications in the speciation forms of the metal induced by a rapid increase of the pH could be responsible for part of this decreased toxicity.
A method is described for continuous monitoring of the photosynthetic oxygen evolution of the microalga Chlorella uulgaris, which was immobilized between alumina filters on the surface of a teflon-covered carbon electrode. Sensitivity of the algal biosensor to herbicides was tested using an electrochemical flow cell with illumination provided by light-emitting diodes. This configuration also allows the biosensor to be used in real-time environmental monitoring. The herbicides atrazine, isoproturon, and propanil were detected within 30 min at concentrations below 100 Fg dm-3, while the herbicide bromoxynil was only detected after several hours at mg dm-3 concentrations. Good sensor stability was obtained in the absence of pollutants, with more than 24 h of continuous monitoring being achieved.
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