Toxicogenomic signatures and behavioral effects for mechanistic differentiation of the effects of prochloraz and endosulfan on Daphnia magna

Chemical contamination of aquatic ecosystems presents a major environmental challenge, with pesticides constituting a significant portion of these pollutants. This study investigates the acute and sublethal effects of the fungicide prochloraz and the insecticide endosulfan as model substances with known modes of action (MoA) on aquatic invertebrates, utilizing Daphnia magna as a model organism. Acute toxicity tests were conducted in accordance with OECD test guideline 202 and complemented by monitoring of swimming activity as well as transcriptomic analyses as sublethal endpoints to elucidate and differentiate the MoA of both compounds. The acute toxicity assays yielded EC50 values consistent with previous literature as well as the registration dossiers of the substances. In subsequent behavioral assays employing sublethal nominal test concentrations, prochloraz induced a significant reduction in swimming activity, whereas endosulfan increased swimming activity with increasing test concentrations, providing preliminary evidence of substance-specific MoA. The toxicogenomic analysis revealed significant alterations in gene expression for both pesticides. While some secondary downstream cellular processes were affected by both substances, functional transcriptome analysis underscored clear MoA distinctions: prochloraz primarily regulated genes involved in lipid, sterol, and steroid biosynthesis, whereas endosulfan predominantly influenced ion transport-related genes. In summary, our study demonstrates distinct MoA-specific behavioral and gene expression responses provoked by prochloraz and endosulfan in D. magna, offering valuable mechanistic insights for environmental risk assessment.