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The current study presents the toxicity data of 90 organic compounds with various modes of actions to Pseudokirchneriella subcapitata. The assessment was conducted using a closed-system technique, and a biomass-type end point based on the cell density was employed. The above toxicity data were compared with test results from ciliate (Tetrahymena pyriformis), water flea (Daphnia magna), fish (Pimephales promelas), and luminescent bacteria (Photobacterium phosphoreum). Satisfactory correlation relationships between toxicity data from algae and other aquatic organisms were found (r2 = 0.66-0.82). Pseudokirchneriella subcapitata revealed considerably higher sensitivity to organic toxicants compared with other organisms. Benzenes, aldehydes, and alkanes also were highly selective to the test alga. In addition, the results show that conventional algal batch tests tend to underestimate the toxicity of organic compounds, except in the case of 4-chlorophenol. Toxicity observed from the closed-system test is approximately 2- to 380-fold higher than that estimated by conventional batch tests. Such a phenomenon can be found in nearly all organic compounds, regardless of the chemical's Henry's law constant. In the risk assessment of chemicals, following the European Union's practice, approximately 30% (7 of 23) of the cases may result in a more strict classification when the batch test is replaced by the closed-system test. More effort therefore is needed to revise the algal toxicity database using the closed-system test method.

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