Physiological and transcriptomic responses in brown trout, Salmo trutta, to multiple stressors: pesticide mixtures, elevated water temperature and the proliferative kidney disease
Contamination with pesticides, increased temperature and the presence of pathogens is a multiple stressor scenario relevant to surface water ecosystems globally. This study investigated the effects of these three environmental stressors and their combinations on juvenile brown trout, Salmo trutta. After acclimation to 12 or 15°C, fish were exposed for 14 days to one (at 12°C) or two (at 15°C) sublethal concentrations of a pesticide mixture (fluopyram, epoxiconazole, diuron, chlorpyrifos, ?-cyhalothrin). Following exposure, the fish were transferred for three months into clean water, with half of the individuals being exposed to spores of T. bryosalmonae, the causative agent of  proliferative kidney disease (PKD).  After 14 days of exposure, pesticides showed no impact on survival, growth, hematocrit, or organ indices but significantly altered the transcriptome (RNA-Seq) and gene expression in the brain and liver. A significant decrease in basal oxygen consumption was observed 2.5 months after fish were exposed to the high concentration of pesticides at 15°C, raising concerns that sublethal effects of pesticides may be limiting ecological fitness longer than expected. Susceptibility to PKD, assessed from infection prevalence, parasite intensity and kidney somatic index, was not affected by pesticide pre-exposure. No interactions between temperature and pesticide exposure were observed on apical, physiological, or qPCR endpoints. However, transcriptomic analysis revealed that the number of differentially expressed genes (DEGs) induced by pesticides was greater at 15°C. Interestingly, the number of DEGs affected by temperature in the brain was strongly reduced in the presence of pesticides. Overall design: RNA-Seq profiling of liver (30 samples) and brain (30 samples) tissues of 6-month old brown trout exposed 14 days to 1 pesticide mixture at 12°C or 2 different pesticide mixtures at  15 °. RNA-Seq profiling of liver tissues of 6-month old brown trout exposed 4 days  to spores of Tetracapsuloides bryosalmonae, inducing the proliferative kidney disease and collected after 44 days exposure in clean water (12 samples).