Slow heating rates increase thermal tolerance and alter mRNA HSP expression in juvenile white sturgeon (Acipenser transmontanus)

<b>Abstract</b><br/><p><span lang="EN-US">Fresh</span><span lang="EN-US">water fish such as white sturgeon (<em>Acipenser transmontanus</em>) are particularly vulnerable to the effects of anthropogenically induced global warming. Critical thermal maximum tests (CT<sub>max</sub>) are often conducted to provide insight of the impacts of changing temperatures; however, little is known about how the rate of temperature increase in these assays affects thermal tolerance. To assess the effect of heating rate (0.3<span style="color:black;">°C/min, 0.03°C/min, 0.003°C/min) we measured thermal tolerance, somatic indices, and Hsp mRNA expression. </span>Contrary to what has been observed in most other fish species,<span style="color:black;"> white sturgeon thermal tolerance</span> was highest at the slowest heating rate of 0.003°C/min, suggesting an ability to rapidly acclimate to slowly increasing temperatures. Hepatosomatic index decreased in all heating rates relative to control fish, indicative of the metabolic costs of thermal stress. <span style="background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial;">At the transcriptional level</span>, slower heating rates resulted in higher mRNA expression of<em> Hsp90a,</em> <em>Hsp90b, </em>and <em>Hsp70. Hsp70 </em>mRNA expression was increased in all heating rates relative to controls, whereas<em> </em>expression of<em> Hsp90a </em>and <em>Hsp90b</em> mRNA only increased in the two slower trials. Together these data indicate that white sturgeon have a very plastic thermal response, which is likely energetically costly to induce. <span style="background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial;">Acute temperature changes may be more detrimental to sturgeon as they struggle to acclimate to rapid changes in their environment, however under slower warming rates they demonstrate strong thermal plasticity to warming.</span></span></p>

摘要
淡水鱼类如美洲白鲟（*Acipenser transmontanus*，white sturgeon）极易受到人为活动驱动的全球变暖的影响。临界热最大值试验（Critical thermal maximum, CTₘₐₓ）常被用于解析温度变化的生态效应，但目前学界对该试验中的升温速率如何影响鱼类热耐受性仍知之甚少。为评估升温速率（0.3℃/min、0.03℃/min、0.003℃/min）对热耐受性的影响，本研究测定了受试个体的热耐受性、躯体指数以及热休克蛋白（Heat Shock Protein, Hsp）mRNA的表达水平。与多数其他鱼类的已有研究结果相悖，美洲白鲟的热耐受性在最慢升温速率0.003℃/min下达到峰值，提示其具备快速适应缓慢升温环境的能力。与对照组相比，各升温速率组的肝体指数均出现显著下降，这一结果反映了热应激所带来的代谢代价。在转录层面，较慢的升温速率下*Hsp90a*、*Hsp90b*与*Hsp70*的mRNA表达水平显著升高。相较于对照组，所有升温速率组的*Hsp70* mRNA表达均有所上调，而*Hsp90a*与*Hsp90b*的mRNA表达仅在两个较慢升温速率组中出现上调。综上，本研究数据表明美洲白鲟具备极强的热响应可塑性，而诱导该可塑性可能需要付出高昂的能量成本。剧烈的温度变化可能对白鲟更为有害，因为它们难以快速适应环境的快速波动；但在缓慢升温的条件下，它们展现出优异的热适应可塑性。