摘要:
虽然水生生物大多是变温动物并且温度对化合物的毒性有非常重要的作用，但生态毒理学实验一般在实验室标准温度(20 ℃)或者生物最适温度下进行。我们调查了适应温度(20 ℃、24 ℃和28 ℃)对镉暴露(0，20和40 μg·L-1)中大型溞生理和亚细胞水平的作用，实验终点包括镉的急性毒性(48 h-EC50)、大型溞的21 d存活率、21 d累积繁殖数(21-d CR)、第一次生育时间(time to first brood, TFB)、能量储存、电子转移活性(electron transport system activity, ETS)、热应激蛋白hsp70和金属硫蛋白(MT)。结果显示：适应温度升高时大型溞的镉耐性降低，20 ℃、24 ℃和28 ℃下的48 h-EC50分别为(40.2±9.9) μg·L-1、(15.6±2.7) μg·L-1和(10.7±1.9) μg·L-1；28 ℃下，大型溞21 d存活率分别比对照(20 ℃，0 μg·L-1镉暴露)降低23%(20 μg·L-1镉暴露)和87%(40 μg·L-1镉暴露)并且大型溞的21 d累积繁殖数(21-d CR)显著降低。大型溞的能量储存随适应温度的升高而降低，24 ℃和28 ℃时分别降低了46%和62%(0 μg·L-1镉暴露)，48%和60%(20 μg·L-1镉暴露)、80%和91%(40 μg·L-1镉暴露)。而镉浓度只有达到40 μg·L-1时，对大型溞的能量储存有明显影响。适应温度和镉暴露对电子转移系统活性(ETS)没有显著影响。28 ℃的适应温度和40 μg·L-1的镉暴露都能诱导大型溞热应激蛋白hsp70和金属硫蛋白(MT)显著上升，但适应温度(20 ℃~28 ℃)和镉暴露(0~40 μg·L-1)的结合对hsp70先诱导后抑制, MT则一直被诱导上升。总之，生物在不同温度下对毒物的反应存在差异，将不同适应温度下的毒性数据整合到生态危险性评价能够保证生物获得更充分的保护；此外，hsp70和MT的变化是环境压力的综合反应，作为某种污染物的生物标志物时需要综合考虑环境因素。
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.
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