Validation of rapid algal bioassay using delayed fluorescence in an interlaboratory ring study.

Algal growth inhibition tests are generally used to determine the toxic effects of chemical substances on algae growth. In this report, we describe a rapid and simple test procedure using delayed fluorescence (DF) to determine chemical toxicities more rapidly than the conventional 72h or 96h growth inhibition tests. We assess the suitability of DF to serve as an alternative endpoint for biomass production and determine the variability by an interlaboratory ring study using a typical reference toxicant 3,5-dichlorophenol (DCP). The results suggest that DF has the potential to be used as a surrogate measure of photosynthetically-active biomass in the algal growth inhibition tests. The half maximal effective concentration (EC50) values of DCP determined from the DF inhibition test in 6h and 24h (1.2±0.3mg/L and 2.7±0.5mg/L respectively) are in reasonable agreement with the EC50 value of DCP determined by the 72h conventional method (1.8mg/L). In the interlaboratory ring study, the intralaboratory and interlaboratory variabilities of the EC50 of the DF inhibition test for a 24h exposure period are 12% and 28% respectively. DF intensity can be considered as a surrogate of living biomass with active photosynthesis, and we conclude that a 24h exposure duration better estimates the toxic effects measured using conventional surrogate measures for dry weight such as cell counts, volume, optical density or fluorescence.