Impact of long-term moderate hypercapnia and elevated temperature on the energy budget of isolated gills and branchial in vivo and in vitro enzyme capacities of Atlantic cod (Gadus morhua)

Effects of severe hypercapnia have been extensively studied in marine fishes, while knowledge on the impacts of moderately elevated CO2 levels and their combination with warming is scarce. Here we investigate ion regulation mechanisms and energy budget in gills from Atlantic cod acclimated long-term to elevated PCO2 levels (2500 µatm) and temperature (18 °C). Isolated perfused gill preparations established to determine gill thermal plasticity during acute exposures (10-22 °C) and in vivo costs of Na+/K+-ATPase activity, protein and RNA synthesis. Maximum enzyme capacities of F1Fo-ATPase, H+-ATPase and Na+/K+-ATPase were measured in vitro in crude gill homogenates. After whole animal acclimation to elevated PCO2 and/or warming, branchial oxygen consumption responded more strongly to acute temperature change. The fractions of gill respiration allocated to protein and RNA synthesis remained unchanged. In gills of fish CO2-exposed at both temperatures, energy turnover associated with Na+/K+-ATPase activity was reduced by 30% below rates of control fish. This contrasted in vitro capacities of Na+/K+-ATPase, which remained unchanged under elevated CO2 at 10 °C, and earlier studies which had found a strong upregulation under severe hypercapnia. F1Fo-ATPase capacities increased in hypercapnic gills at both temperatures, whereas Na+/K+ATPase and H+-ATPase capacities only increased in response to elevated CO2 and warming indicating the absence of thermal compensation under CO2. We conclude that in vivo ion regulatory energy demand is lowered under moderately elevated CO2 levels despite the stronger thermal response of total gill respiration and the upregulation of F1Fo-ATPase. This effect is maintained at elevated temperature.严重高碳酸血症对海洋鱼类的影响已得到广泛研究，但目前关于中度升高的二氧化碳水平及其与升温协同作用的影响的认知仍较为匮乏。本研究以长期暴露于升高的二氧化碳分压（PCO2，2500 µatm）与温度（18℃）下的大西洋鳕为研究对象，探究其鳃组织的离子调控机制与能量收支。本研究建立离体灌流鳃标本，以测定急性温度胁迫（10~22℃）下的鳃热可塑性，以及钠钾腺苷三磷酸酶（Na+/K+-ATPase）活性、蛋白质与RNA合成的体内能量消耗；同时通过体外实验，测定了鳃粗匀浆中F1Fo腺苷三磷酸酶（F1Fo-ATPase）、氢质子腺苷三磷酸酶（H+-ATPase）与钠钾腺苷三磷酸酶的最大酶活性。经整体动物驯化至升高的二氧化碳分压和/或升温条件后，鳃耗氧量对急性温度变化的响应更为显著，且鳃呼吸能耗中分配至蛋白质与RNA合成的占比未发生变化。在两种温度下经二氧化碳暴露的大西洋鳕鳃组织中，与钠钾腺苷三磷酸酶活性相关的能量代谢速率较对照组降低了30%；这与体外测定的钠钾腺苷三磷酸酶活性形成鲜明对比：在10℃的升高二氧化碳分压条件下，该酶活性未发生改变，而此前的研究发现，严重高碳酸血症下该酶活性会显著上调。两种温度下，高碳酸血症暴露的鳃组织中F1Fo腺苷三磷酸酶活性均有所升高；而钠钾腺苷三磷酸酶与氢质子腺苷三磷酸酶活性仅在升高的二氧化碳分压与升温协同作用下才会上升，这表明在二氧化碳胁迫下不存在热补偿效应。本研究结论为：尽管总鳃耗氧量的热响应更为显著，且F1Fo腺苷三磷酸酶活性上调，但中度升高的二氧化碳分压条件下，体内离子调控的能量需求仍有所降低；该效应在升温条件下仍得以维持。