The simultaneous presence of natural and anthropogenic stressors in aquatic ecosystems can challenge the identification of factors causing decline in fish populations. These stressors include chemical mixtures and natural abiotic and biotic factors such as water temperature and parasitism. Effects of cumulative stressors may vary from antagonism to synergism at the organismal or population levels and may not be predicted from exposure to individual stressors. This study aimed to evaluate the combined effects of chronic exposure to cadmium (Cd) and elevated water temperature (23 °C) or parasite infection in juvenile rainbow trout (Oncorhynchus mykiss) using a multi-level biological approach, including RNA-sequencing. Fish were exposed to diet-borne Cd (6 μg Cd/g wet feed), individually and in combination with thermal (23 °C) or parasitic stressors, for 28 days. The parasite challenge consisted of a single exposure to glochidia (larvae) of the freshwater mussel (Strophitus undulatus), which encysts in fish gills, fins and skin. Results indicated lower fish length, weight, and relative growth rate in fish exposed to a higher water temperature (23 °C). Body condition and hepatosomatic index of trout were, however, higher in the 23 °C temperature treatment compared to the control fish kept at 15 °C. Exposure to thermal stress or parasitism did not influence tissue Cd bioaccumulation. More than 700 genes were differentially transcribed in fish exposed to the individual thermal stress treatment. However, neither Cd exposure nor parasite infection affected the number of differentially transcribed genes, compared to controls. The highest number of differentially transcribed genes (969 genes) was observed in trout exposed to combined Cd and high temperature stressors; these genes were mainly related to stress response, protein folding, calcium metabolism, bone growth, energy metabolism, and immune system; functions overlapped with responses found in fish solely exposed to higher water temperature. Only 40 genes were differentially transcribed when fish were exposed to Cd and glochidia and were related to the immune system, apoptosis process, energy metabolism and malignant tumor. These results suggest that dietary Cd may exacerbate the temperature stress and, to a lesser extent, parasitic infection stress on trout transcriptomic responses. Changes in the concentrations of liver ethoxyresorufin-o-deethylase, heat shock protein 70 and thiobarbituric acid reactive substances coupled to changes in the activities of cellular glutathione S-transferase and glucose-6-phosphate dehydrogenase were also observed at the cellular level. This study may help understand effects of freshwater fish exposure to cumulative stressors in a changing environment.

中文翻译：

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镉和自然胁迫（温度和寄生虫感染）对少年虹鳟鱼分子和生化反应的累积影响。

在水生生态系统中同时存在自然和人为压力源可能会挑战确定导致鱼类种群减少的因素。这些压力源包括化学混合物以及天然非生物和生物因素，例如水温和寄生温度。累积应激源的作用在机体或种群水平上可能因拮抗作用而有所不同，并且可能无法通过暴露于单个应激源来预测。这项研究旨在使用包括RNA测序在内的多级生物学方法评估慢性虹鳟（Oncorhynchus mykiss）中长期暴露于镉（Cd）和升高的水温（23°C）或寄生虫感染的综合影响。将鱼单独或与热（23°C）或寄生应激源一起暴露于饲料中的镉（6μgCd / g湿饲料）中，持续28天。寄生虫的挑战包括一次接触淡水贻贝（Strophitus undulatus）的圆头虫（幼虫），该贻贝包裹在鱼g，鳍和皮肤中。结果表明，暴露于较高水温（23°C）的鱼中鱼的长度，体重和相对生长率较低。然而，与保持在15°C的对照鱼相比，在23°C的温度处理中，鳟鱼的身体状况和肝体指数更高。暴露于热应激或寄生状态不会影响组织Cd的生物蓄积。暴露于单独热应激处理的鱼中有700多个基因被差异转录。然而，与对照相比，镉暴露和寄生虫感染均不会影响差异转录基因的数量。在暴露于镉和高温胁迫共同作用的鳟鱼中，观察到差异转录的基因数量最多（969个基因）。这些基因主要与应激反应，蛋白质折叠，钙代谢，骨骼生长，能量代谢和免疫系统有关。其功能与仅暴露于较高水温的鱼类的响应重叠。当鱼类暴露于Cd和glochidia时，只有40个基因被差异转录，并且与免疫系统，细胞凋亡过程，能量代谢和恶性肿瘤有关。这些结果表明，饮食中的镉可能会加剧温度应激，并在较小程度上加剧鳟鱼转录组反应的寄生虫感染压力。肝脏乙氧基试卤灵-o-脱乙基酶浓度的变化，在细胞水平上还观察到热休克蛋白70和硫代巴比妥酸反应性物质与细胞内谷胱甘肽S-转移酶和葡萄糖-6-磷酸脱氢酶活性的变化有关。这项研究可能有助于了解在变化的环境中淡水鱼暴露于累积应激源的影响。