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 volume (MCV) of algal inoculum to the sensitivity and reproducibility of the toxicity test. Two sets of experiments were conducted using inoculums with the MCV equal to 60 μm3/cell and 40 μm3/cell, respectively. The initial cell densities, however, were kept the same at 10,000 cells/mL. The results show that larger MCV of the inoculum results in greater EC50 values. Similar observations and conclusions were obtained from comparisons based on NOEC 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 controlled properly. This study reports that standardizing the initial mean cell volume of algal culture can improve inter-laboratory and intra-laboratory precision. Furthermore, replacing the EC50 values by the calculated exposure values (EE50) can reduce the variability in data due to differences in the MCV of the inoculum.