Impacts of ocean acidification on respiratory gas exchange and acid–base balance in a marine teleost, Opsanus beta
The oceanic carbonate system is changing rapidly due to rising atmospheric CO2, with current levels expected to rise to between 750 and 1,000 μatm by 2100, and over 1,900 μatm by year 2300. The effects of elevated CO2 on marine calcifying organisms have been extensively studied; however, effects of imminent CO2 levels on teleost acid–base and respiratory physiology have yet to be examined. Examination of these physiological processes, using a paired experimental design, showed that 24 h exposure to 1,000 and 1,900 μatm CO2 resulted in a characteristic compensated respiratory acidosis response in the gulf toadfish (Opsanus beta). Time course experiments showed the onset of acidosis occurred after 15 min of exposure to 1,900 and 1,000 μatm CO2, with full compensation by 2 and 4 h, respectively. 1,900-μatm exposure also resulted in significantly increased intracellular white muscle pH after 24 h. No effect of 1,900 μatm was observed on branchial acid flux; however, exposure to hypercapnia and HCO3− free seawater compromised compensation. This suggests branchial HCO3− uptake rather than acid extrusion is part of the compensatory response to low-level hypercapnia. Exposure to 1,900 μatm resulted in downregulation in branchial carbonic anhydrase and slc4a2 expression, as well as decreased Na+/K+ ATPase activity after 24 h of exposure. Infusion of bovine carbonic anhydrase had no effect on blood acid–base status during 1,900 μatm exposures, but eliminated the respiratory impacts of 1,000 μatm CO2. The results of the current study clearly show that predicted near-future CO2 levels impact respiratory gas transport and acid–base balance. While the full physiological impacts of increased blood HCO3− are not known, it seems likely that chronically elevated blood HCO3− levels could compromise several physiological systems and furthermore may explain recent reports of increased otolith growth during exposure to elevated CO2.