Rapid blood acid-base regulation by European sea bass (Dicentrarchus labrax) in response to sudden exposure to high environmental CO2

Fish in coastal ecosystems can be exposed to acute variations in CO 2 that can approach 1 kPa CO 2 (10,000 μatm). Coping with this environmental challenge will depend on the ability to rapidly compensate the internal acid-base disturbance caused by sudden exposure to high environmental CO 2 (blood and tissue acidosis); however, studies about the speed of acid-base regulatory responses in marine fish are scarce. We observed that upon exposure to ~1 kPa CO 2 , European sea bass ( Dicentrarchus labrax ) completely regulate erythrocyte intracellular pH within ~40 minutes, thus restoring haemoglobin-O 2 affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ~2 hours, which is one of the fastest acid-base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO 3 − in blood, which increased from ~4 to ~22 mM. While the abundance and intracellular localisation of gill Na + /K + -ATPase (NKA) and Na + /H + exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid-base regulation was completely prevented when the same high CO 2 exposure occurred in seawater with experimentally reduced HCO 3 − and pH, likely because reduced environmental pH inhibited gill H + excretion via NHE3. The rapid and robust acid-base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO 2 fluctuations that naturally occur in coastal environments. Summary statement European sea bass exposed to 1 kPa (10,000 μatm) CO 2 regulate blood and red cell pH within 2 hours and 40 minutes, respectively, protecting O 2 transport capacity, via enhanced gill acid excretion.