Parasitic interactions modulate the physiological responses of Gammarus fossarum to the herbicide metazachlor

Parasites are ubiquitous in natural environments, yet their role in shaping host responses to chemical stressors remains largely underexplored in ecotoxicology. We investigated how natural infections by cystacanths of the acanthocephalan Polymorphus minutus and microsporidians modulate the toxicity of the chloroacetamide herbicide metazachlor (MTZ) in Gammarus fossarum during a high-exposure scenario. We exposed P. minutus- and/or microsporidia-infected and uninfected G. fossarum to 1 and 10 mg/L MTZ, assessing behavioral and physiological responses. We observed significantly higher mortality at 10 mg/L, with slightly lower values for P. minutus-infected individuals. Although locomotion was unaffected by MTZ or infection, both MTZ and parasites significantly influenced biochemical responses with significant interactions. We found distinct physiological response patterns associated with parasite infection and MTZ exposure. Two major infection-dependent response clusters were (i) coinfected (with P. minutus and microsporidians) and P. minutus-infected individuals, and (ii) uninfected and microsporidia-infected individuals, suggesting distinct physiological strategies for coping with parasites and MTZ. Coinfection and P. minutus appeared to elevate glutathione S-transferase (GST) and phenoloxidase (PO) activities compared to the other cluster at high MTZ and control, while catalase activity decreased. In contrast, microsporidia infection and the absence of parasites were associated with increased catalase activity at the highest MTZ treatment. Decreased GST activity across infection groups at high MTZ suggested reduced detoxification capacity, while MTZ induced depletion of glycogen and lipids indicated metabolic costs. The complex interactions observed between MTZ and parasites highlight the importance of considering multiple stressors when assessing the ecological impacts of environmental contaminants like MTZ.