Correlation of Tetrahymena and Pimephales toxicity: evaluation of 100 additional compounds

In the summary/recommendations for the Ecotoxicology Session of TestSmart--A Humane and Efficient Approach to Screening Information Data Sets (SIDS) Data Workshop, it was recommended that more population growth impairment data using Tetrahymena be generated and compared with available lethality data for the fathead minnow. To comply with this recommendation, 100 additional chemicals were tested in the ciliate assay. Toxicity values for the 96-h Pimephales promelas mortality assay (log[LC50(-1)]) and the 2-d Tetrahymena pyriformis growth assay (log[IGC50(-1)]) were compared. Each chemical was a priori assigned a mode of action. The majority of compounds were classified as either narcotics (n = 46) or direct-acting electro(nucleo)philes (n = 43), while 11 chemicals were listed as carboxylic acids, diesters, proelectrophiles, or weak acid respiratory uncouplers. Toxicities for narcotics showed an excellent relationship between endpoints with the coefficient of determination (r2) being 0.93. A weaker relationship, r2 = 0.78, was observed for the electro(nucleo)philes. The poorer fit for the covalent-reacting electro(nucleo)philes is attributed to differences in protocol, in particular, to test-medium composition and exposure scheme. Those chemicals whose potency is mediated by metabolism in fish (diesters and proelectrophiles) as well as the acids exhibited poor correlation between endpoints, with toxicity in the fish assay being greater than that predicted from the ciliate data. The regression analysis between endpoints, regardless of mode or mechanism of toxic action, yielded the model log(LC50(-1)) = 1.12(log[IGC50(-1)]) + 0.46, with n = 92, r2 = 0.82, s (root of the mean square error) = 0.87, F = 399, and p > F = 0.0001. A result for the present investigation supports earlier findings that, with noted exceptions, there is a strong relationship between toxicity potency as quantified by P. promelas mortality and T. pyriformis growth impairment.