Development and Application of a Microplate Assay for Toxicity Testing on Aquatic Cyanobacteria

Regulatory environmental risk assessment, applied to establish a protection limit for all bacterial diversity in surface waters, relies on a growth inhibition test performed on a single species of cyanobacteria and the activated sludge respiration inhibition test. Recently, the ability of this approach to protect adequately for bacteria that provide important ecosystem services has been questioned, and empirical data on additional species to further investigate the effectiveness of the environmental risk assessment are urgently required. We present the development and validation of a cost-effective and time-efficient microplate assay that is comparable to the traditional shake flask test for measurement of cyanobacteria growth rate after chemical exposure. The assay has been optimized to ensure that comparisons of cyanobacteria sensitivity under exponential growth are assessed across equivalent experimental conditions using phycocyanin fluorescence as a surrogate for cell density. The test system is validated using potassium dichromate, and the results are compared with those obtained in an Organisation for Economic Co-operation and Development (2011) test guideline 201 shake flask test system. This assay is suitable for the screening of new and legacy chemicals (including antibiotics) for which ecotoxicology data are lacking across a wide range of cyanobacteria, with the aim of developing more comprehensive environmental risk assessment. Environ Toxicol Chem 2020;39:705-720. © 2019 SETAC.