Effects of inoculum's mean cell volume on algal toxicity tests

The aim of the present study is to compare the effects of the mean cell vol ume (MCV) of algal inoculum to the sensitivity and reproducibility of the t oxicity test. Two sets of experiments were conducted using inoculums with t he MCV equal to 60 mum(3)/cell and 40 mum(3)/cell, respectively. The initia l cell densities, however, were kept the same at 10,000 cells/ml. The resul ts show that larger MCV of the inoculum results in greater EC50 values. Sim ilar observations and conclusions were obtained from comparisons based on N OEC values. Thus, MCV of the inoculum significantly influences the observed toxic response in terms of EC50 and NOEC values. It is also evident that, there is a significant increase in the test variability if the MCV is not c ontrolled properly. This study reports that standardizing the initial mean cell volume of algal culture can improve inter-laboratory and intra-laborat ory precision. Furthermore, replacing the EC50 values by the calculated exp osure values (EE50) can reduce the variability in data due to differences i n the MCV of the inoculum.