Toxic effect of heavy metals on the activated sludge protozoan community

The acute toxicity of five heavy metals to the protozoan community inhabiting the activated sludge of a waste treatment plant, was determined on the basis of reduction in both cell density and species richness. The activated sludge mixed-liquor was treated with different concentrations of cadmium, copper, chromium (VI), lead, and zinc for a period of 24-h. The experimental results enabled to determine the relative toxicity of the tested metals, and indicated that the order of toxicity of the five metals to the studied microbial community was generally: Cd, Cu > Pb > Zn > Cr. For nine out of sixteen protozoan species constituting the studied community was possible to determine the 24-h median lethal concentration (24-h LC(50)) for each tested metal. Large differences appeared in sensitivities of the nine species to the metals. Ciliated protozoa such as Chilodonella uncinata and Trochilia minuta showed the highest sensitivity to all the studied metals, while Opercularia coarctata and O. minima, were the most tolerant species.