Round Robin testing with the Selenastrum capricornutum microplate toxicity assay

Abstract

Three Quebec-based ecotoxicological laboratories participated in an intercalibration exercise to assess the performance of a recently-published cost-efficient algal microplate toxicity assay. Three test series were carried out with six operators (2 from each laboratory) and two reference toxicants (Cd²⁺ as CdCl₂ and phenol). Variables included algal cultivation technique (series 1), presence or absence of Na₂EDTA in the growth medium (series 2), and passive or active gas exchange during incubation (series 3). Control growth variability conferred an overall test precision of 8.7% (coefficient of variation obtained for 204 microplate tests). Cadmium (96 h EC50 = 56 µg · l⁻¹) and phenol (96 h EC50 = 69.7 mg · l⁻¹) toxicity test reproducibility was reflected by coefficients of variation of 24.3% and 34.9%, respectively. Algal cultivation technique, whether standardized or ‘in house’, had no effect on toxicity results. Na₂EDTA, as part of the growth medium, significantly ameliorated algal growth and toxicity. While active gas exchange during microplate incubation significantly improved growth, toxicity results were unaffected. Phenol volatility was found to have a marked influence on algal growth. This effect can be offset, however, by providing appropriate modifications to better seal individual wells and to improve experimental design.