Locomotor activity and physiological responses of parasite-infected Gammarus fossarum exposed to the herbicide metazachlor

Herbicides are among the most commonly found contaminants in freshwater ecosystems. Standard tests are frequently employed to assess their ecotoxicological impacts, but sublethal endpoints in non-target species are often not considered. In addition, ecotoxicological investigations rarely take into account that many species from field populations are naturally infected with parasites. To overcome these gaps, our study aimed to investigate how environmentally relevant concentrations of the herbicide metazachlor affect the locomotor activity and selected physiological responses of Gammarus fossarum infected with the acanthocephalan Polymorphus minutus and microsporidians. Prior to the study of sublethal effects, acute immobility, and lethality (EC50 and LC50) tests were conducted. Polymorphusminutus, but not microsporidians, slightly enhanced chemical stress tolerance in infected G. fossarum in the acute immobility and lethality test. Infections with P. minutus significantly increased the host's locomotory activity in comparison to uninfected individuals when exposed to environmentally relevant concentrations of metazachlor, while metazachlor exposure alone had no apparent impact on locomotion. In contrast, the effects of metazachlor on physiological responses (glutathione S-transferase, glycogen, and phenoloxidase) of G. fossarum were significant, while parasite infection alone did not exhibit any significant impact on these biomarkers. The findings of our study indicate that the locomotor activity of G. fossarum in the conducted exposure tests was mostly influenced by P. minutus infections. Conversely, physiological responses were predominantly associated with exposure to metazachlor at environmentally relevant concentrations. We recommend future ecotoxicological studies involving non-target, field-collected species to consider the potential bias introduced by parasitic infections to ensure accurate evaluations of the effects of environmental contaminants.