Individual and combined effects of heat stress and aqueous or dietary copper exposure in fathead minnows (Pimephales promelas).

Despite its role as an essential micronutrient, copper (Cu) can be present in aquatic ecosystems at concentrations able to cause adverse health effects on aquatic organisms. Although Cu is acquired by fish by either water or diet, studies that have investigated Cu impacts in fish have mainly focused on the toxicity of waterborne Cu. Moreover, as the majority of experiments were carried out under simplified conditions, little is known about the effects of natural factors other than competitive ions on Cu toxicity in fish. As temperature is a primary factor that affects the physiological state of poikilotherm organisms, we investigated the individual and combined effects of temperature and waterborne or dietary Cu on fathead minnows (Pimephales promelas). Fish were exposed to environmentally realistic concentrations of waterborne or dietary Cu at 20 °C and 32 °C. Transcriptional and enzymatic responses of various indicators of metabolic capacities as well as indicators of heat, oxidative and metal stresses were measured in fish muscle. Under our experimental conditions, temperature was the most important factor affecting the general condition of fish. Although no significant Cu accumulation was observed in the muscle of Cu-exposed fish, at 20 °C, waterborne and dietary Cu triggered significant changes in the transcription level of genes encoding for proteins involved in energy metabolism, metal detoxification and protein protection. Moreover, the response was quantitatively more important for dietary Cu than for waterborne Cu. Combined exposure to heat and Cu triggered the most significant changes in gene transcription levels and enzyme activities. During combined exposure to heat and Cu, in addition to synergistic effects of the two factors, both waterborne and dietary Cu impaired the adaptive response developed by fish to curb heat stress. Reciprocally, temperature impaired the adaptive response developed by fish to combat Cu toxicity. These results suggest that wild fish populations subjected to elevated temperatures due to seasonal warming or global climate change may become more susceptible to Cu pollution, and vice versa.