ALBERT

Description: Anthropogenic CO2 emissions threaten marine ecosystems by increasing water temperature and acidification. Fish generally seem to be less sensitive to elevated CO2 concentrations due to their efficient ion regulatory capacities. As temperature affects all ion regulatory processes, an increased sensitivity to elevated CO2 levels at the edges of the thermal window can be postulated. In this study, we are acclimating Atlantic cod to a combined setup of elevated PCO2 (390, 1120 and 3000 µatm) and temperatures (10, 15 and 18 °C). In isolated, perfused gill arches we determine the fractional costs of ion regulation, protein and RNA synthesis in relation to the global energy budget after acclimation and under acute exposure by the application of specific inhibitors. Acute warming increased overall gill oxygen consumption rates and fractional costs of the processes investigated, the latter most obvious when combined with hypercapnia. Furthermore, apical Na+/H+ exchangers seem to be essential for the response towards acidification. In isolated gill cells we fluorometrically track the changes of intracellular pH in gill cells after acute exposure to acidification to quantify cellular ion regulative processes and performance. Additionally, ion transport proteins are going to be inhibited to quantify their involvement in the entire regulation process. For the characterisation of the ion regulatory transcriptome and proteome we combine genetic, immunohistological and functional approaches. So far, genes of essential transporters involved in ion regulation have been isolated, and the expression of relevant transporters (NBC1, AE1, H+-ATPase, Na+/K+-ATPase) and the capacity of the Na+/K+-ATPase were found seasonally and population specific regulated. Measuring a portfolio of haematological and immunological endpoints the hypothesis is tested whether increased energy expenditure e. g. for ion regulation at elevated CO2 levels will suppress the immunocompetence at the edges of the cods thermal window. Together, all data will be integrated into a mechanistic model to mathematically formulate ion regulative capacities and its limitations in response to predicted climate scenarios.

Description: Effects of severe hypercapnia have been extensively studied in marine fishes, while the 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 (2,500 μatm) and temperature (18°C). Isolated perfused gill preparations were established to determine gill thermal plasticity during acute exposures (10-22°C) and in vivo costs of Na(+)/K(+)-ATPase activity and of 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 percent below rates of the control group. 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 more 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.