Linking algal growth inhibition to chemical activity: Excess toxicity below 0.1% of saturation.

Chemical activity quantifies the energetic level of an organic compound relative to its pure liquid [0-1], and several studies have reported that baseline toxicity generally requires chemical activities of 0.01-0.1. The first aim was to challenge this chemical activity range for baseline toxicity. Algal growth inhibition data (median effective concentrations, EC ₅₀ ) were compiled from two recent studies and included 108 compounds categorised as non-polar (mode of toxic action, MOA1) and polar (MOA2) narcotics. These data were linked to chemical activity by (1) plotting them relative to a regression for (subcooled) liquid solubility (S _L ), which served as visual reference for chemical activity of unity and (2) determining EC ₅₀ /S _L ratios that essentially equal median effective chemical activity (Ea ₅₀ ). Growth inhibition required chemical activity >0.01 for MOA1 and >0.001 for MOA2 compounds. The second aim was to identify compounds exerting excess toxicity, i.e., when growth inhibition occurred at chemical activity <0.001. From a recent review with 2323 data entries, 315 EC ₅₀ values passed our selection criteria. 280 of these EC ₅₀ values were within or near the baseline toxicity range (Ea ₅₀ >0.001), and 25 compounds were found to exert excess toxicity (Ea ₅₀ <0.001). Of these compounds, 16 are pesticides or precursors. Methodologically, this study includes two methods for translating EC ₅₀ values into the chemical activity framework, each having advantages and limitations. Scientifically, this study confirms that baseline toxicity generally requires chemical activities of 0.01-0.1 and extends the application of the chemical activity approach beyond baseline toxicity, by demonstrating its utility to identify compounds that exert excess toxicity.
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