Temperature-dependent enhancement of methylmercury toxicity in the water flea Daphnia magna: Linking physiological and molecular stress responses

Global warming alters the toxicity and bioavailability of environmental pollutants in aquatic ecosystems. Methylmercury (MeHg), a highly toxic form of mercury, poses significant risks, yet its interaction with temperature remains understudied. Thus, this study aims to understand how elevated temperature affects the physiological and molecular toxicity of MeHg exposure in aquatic invertebrates. We investigated the combined effects of elevated temperature (23 and 28 °C) and MeHg (10 and 50 ng/L) on the freshwater invertebrate Daphnia magna. Acute toxicity was significantly enhanced at 28 °C. Chronic exposure reduced survival, reproduction, and growth, particularly under combined elevated temperature and MeHg conditions. While mercury accumulation increased with MeHg concentration, temperature did not influence internal Hg levels. Biochemical analyses showed that elevated temperature reduced ROS levels but increased antioxidant enzyme activity, whereas antioxidant gene expression was suppressed. MeHg exposure inhibited acetylcholinesterase activity, with greater inhibition observed under combined exposure. Detoxification responses were temperature-specific. Glutathione-mediated system was activated at 28 °C. ABCC activity increased with temperature and MeHg, whereas ABCB activity was suppressed at 28 °C, but partially recovered with MeHg. These findings demonstrate that elevated temperature amplifies MeHg toxicity through physiological and molecular disruptions and emphasize the value of considering temperature–pollutant interactions in ecological risk assessments under climate change scenarios.