Influence of Cellular Density on Determination of EC50 in Microalgal Growth Inhibition Tests

Growth inhibition tests for copper were carried out on four marine microalgal species: Chlorella autotrophyca, Nannochloris atomus (Chlorophyceae), Phaeodactylum tricornutum (Bacillariophyceae), and Isochrysis aff. galbana (Primnesiophyceae). The test initial cellular densities were reduced to 50 and 10% from the recommended initial cellular density in most of standardized assays. OECD test protocol (originally described for freshwater) was adapted for seawater. The EC50 values were reduced when initial cellular density decreased. The green algae used in this study exhibited lower sensitivity than P. tricornutum and quite lower than I. aff. galbana. The latter species was found to be very sensitive to copper. The concept of cellular toxic quote (amount of toxic per cell) is defined in order to improve the results of toxicity tests.