Toxicity of three binary mixtures to Daphnia magna: Comparing chemical modes of action and deviations from conceptual models

Complex mixtures makes the assessment of environmental hazards difficult due to possible antagonistic or synergistic interactions that can occur between chemicals, or even more complex effect patterns like dose-level or dose-ratio-dependent responses. The aim of the present work was to investigate the acute and sublethal responses of Daphnia magna Straus exposed to four single chemical compounds (imidacloprid, thiacloprid, nickel chloride, and chlorpyrifos) and three binary chemical mixtures. In the immobilization and feeding inhibition bioassays, chlorpyrifos was the most toxic to D. magna, followed by nickel chloride, and imidacloprid and thiacloprid, which showed similar levels of toxicity. The MIXTOX was used to evaluate mixture toxicity. Observed data was compared with the expected mixture effects predicted by concentration addition (CA) and independent action (IA) models; deviations for synergistic/antagonistic interactions, dose-level and dose-ratio dependency were also used. In the mixture toxicity assessment, several patterns of response were obtained depending on the mixture but also on the endpoint tested. For imidacloprid and thiacloprid, deviations for synergism were observed in acute exposures (immobilization), and antagonism for feeding rates at sublethal concentrations. For imidacloprid and chlorpyrifos, antagonism was found in both exposures. In the nickel and chlorpyrifos case study, deviations for synergism were observed in the acute exposure; a dose-ratio deviation was observed in the feeding inhibition test, with a pattern for antagonism, except for where nickel exerts more than 60% of the mixture toxicity.