Keywords: 
• Cd /
• temperature /
• Daphnia magna /
• 21-day survival /
• energy reserves /
• hsp70 /
• MT

Abstract: Although aquatic organisms are mostly ectotherms and temperature has considerable impact on chemical toxicity, toxicity studies are mainly performed at the laboratory standard (20 ℃) or species' optimal temperature. We investigated the effects of temperature acclimation (20 ℃, 24 ℃ and 28 ℃) on physiological and sub-cellular effects in Daphnia magna during cadmium (Cd) exposure (0, 20 and 40 μg·L^-1). Endpoints included acute Cd toxicity (48 h-EC₅₀), 21-day survival, 21-day cumulative reproduction (21-d CR), time to first brood (TFB), energy reserves, electron transport system activity (ETS), stress protein hsp70 and metallothionein (MT). The present results demonstrate that the tolerance of D. magna to Cd elevated as temperature decreased. The 48 h-EC₅₀s at 20, 24 and 28 ℃ were (40.2±9.9) μg·L^-1, (15.6±2.7) μg·L^-1 and (10.7±1.9) μg·L^-1. At 28 ℃, the 21 days survival of D. magna decreased by 23% (20 μg·L^-1 Cd exposure) and 87% (40 μg·L^-1 Cd exposure) compared to control (20 ℃, 0 μg·L^-1 Cd exposure) and cumulative reproduction (21-d CR) significantly decreased. Energy reserves decreased with increasing temperature and were 46% and 62% (0 μg·L^-1 Cd exposure), 48% and 60% (20 μg·L^-1 Cd exposure), 80% and 91% (40 μg·L^-1 Cd exposure) lower. Only when concentrations higher than 40 μg·L^-1, Cd exposure can affect the energy reserves. The acclimated temperature and Cd concentration did not affect electron transport system activity (ETS) of D. magna. Both acclimation temperature at 28 ℃ and Cd concentration at 40 μg·L^-1 can alone induce significant rises of hsp70 and MT. Under the combination of temperature and Cd stress, the hsp70 levels increased significantly, then decreased and the MT levels increased significantly all the time. In short, as for Cd for which a lot of toxicity data is available, inclusion of toxicity data at different acclimated temperatures to ecological risk assessment would ensure more comprehensive protection. Moreover, hsp70 and MT maybe an integral part of the general stress response in aquatic ectotherms and may not be simply viewed as specific biomarkers without consideration of environmental factors.