The importance of light intensity in algal tests with coloured substances

Current algal test protocols do not distinguish between chemical toxicity and growth inhibition due to physical shading when coloured substances like dyes are evaluated. To eliminate the interfering physical effect, we investigated the importance of light intensity and culture volume in the algal growth inhibition tests with Desmodesmus subspicatus and three selected dyestuffs. A photon flux density above light saturation is needed to prevent inhibition of algal growth solely due to the light absorption by dyes. Furthermore, a smaller culture volume and moderate shaking of the test flasks is advantageous in that these conditions decrease the influence of the light absorption. At 120 μE s−1 m−2, the shading effect of three tested dyes could be eliminated, up to a concentration of 100 mg l−1. In a mixture with a blue dyestuff, the model toxicant potassium dichromate showed nearly the same EC50 as if applied alone, indicating that the impact of the dye was negligible. Also a small culture volume decreases the inhibition of the average growth rate of Desmodesmus. Thus, by increasing the light intensity and optionally also reducing the culture volume, this method allows a more direct measure of a potential inherent chemical toxicity of coloured substances to algae. Additionally, high light intensities and lower culture volumes result in lower coefficients of variation, which improves the sensitivity of the test for the statistical calculation of toxicity data.