The use of delayed fluorescence intensity as an endpoint for rapid estimation of the effective concentration (ECx) has been reported as an alternative to standard growth inhibition (at 72 h after exposure) in some algal species including Pseudokirchneriella subcapitata. In marine algae, although an approach of bioassaying using delayed fluorescence measurements has not been performed yet, its development would provide many benefits for marine environmental risk assessment. In this study, we selected marine cyanobacterium Cyanobium sp. (NIES-981) as our test algal species and demonstrated that this species is valid for the standard growth inhibition test based on criteria provide by Organization for Economic Co-operation and Development guidelines. Furthermore, standard inhibition tests and shorter period test using DF were performed in NIES-981 using five chemicals (3,5-DCP, simazine, diflufenican, K2Cr2O7, and CuSO4), and their EC50 and low-toxic-effect values (EC10, EC5, and NOEC) were determined from two dose-response curves. Based on comparisons of the two dose-response curves and the EC50 values, we conclude that DF intensity is useful as an endpoint for rapid estimation of EC50 in NIES-981.
