Structure-activity relationships in aquatic toxicology

Relationships among chemical structure, aquatic toxicity, and bioconcentration potential were examined for several classes of organic compounds. Structure-toxicity correlations were based largely on median lethal concentrations (LC50) and toxicant threshold concentrations (LC1) determined in mini-chronic tests with early life stages of fish and amphibians. Exposure was initiated at fertilization and maintained through 4 days posthatching. Bioconcentration potential was assessed using n-octanol/water partition coefficients (log P). In tests with polychlorinated biphenyls (PCB), acute and chronic toxicity generally increased with percent chlorination. In addition, toxicity of specific PCB's appeared to be affected by the ratio of less chlorinated to more highly chlorinated isomers. The toxicity of chlorinated methanes (i.e., methylene chloride, chloroform, carbon tetrachloride) also increased with chlorination. Concerning single ring aromatic compounds, pyridine was much less toxic than benzene and benzene was less toxic than its mono-substituted derivatives, including chlorobenzene, nitrobenzene, toluene, and phenol. However, no consistent order of toxicity was observed for the substituted compounds. Acute toxicity also increased with the number of aromatic rings in a series of nitrogen heterocyclic compounds, and the latter were less toxic than corresponding alicyclic compounds. Within most classes of compounds, a direct correlation was observed between acute toxicity and bioconcentration potential. As observed with PCB compounds, the mini-chronic test described in this study permitted evaluations of structure-activity relationships using both LC50 and LC1 values determined with early life stages. The LC1's compared well with results obtained in life-cycle studies, thus providing an economical and reliable means of estimating chronic values for reproductive impairment.