Adjustments of molecular key components of branchial ion and pH regulation in Atlantic cod (Gadus morhua) in response to ocean acidification and warming

Marine teleost fish sustain compensation of extracellular pH after exposure to hypercapniaby means of efficient ion and acid-base regulation. Elevated rates of ion and acid-base regulation under hypercapniamay be stimulated further by elevated temperature. Here, we characterized the regulation of transepithelial ion transporters(NKCC1, NBC1, SLC26A6, NHE1 and 2) and ATPases(Na(+)/K(+) ATPaseand V-type H(+) ATPase) in gills of Atlantic cod( Gadusmorhua) after 4 weeks of exposure to ambient and future PCO2levels (550 μatm, 1200 μatm, 2200 μatm) at optimum (10 °C) and summer maximum temperature (18 °C), respectively. Gene expression of most branchial ion transportersrevealed temperature- and dose-dependent responses to elevated PCO2. Transcriptional regulation resulted in stable protein expression at 10 °C, whereas expression of most transport proteinsincreased at medium PCO2and 18 °C. mRNA and protein expression of distinct ion transportproteins were closely co-regulated, substantiating cellular functional relationships. Na(+)/K(+) ATPasecapacities were PCO2independent, but increased with acclimation temperature, whereas H(+) ATPasecapacities were thermally compensated but decreased at medium PCO2and 10 °C. When functional capacities of branchial ATPaseswere compared with mitochondrial F1Fo ATP-synthasestrong correlations of F1Fo ATP-synthaseand ATPasecapacities generally indicate close coordination of branchial aerobic ATP demand and supply. Our data indicate physiological plasticity in the gills of cod to adjust to a warming, acidifying ocean within limits. In light of the interacting and non-linear, dose-dependent effects of both climate factors the role of these mechanisms in shaping resilience under climate change remains to be explored.