Water pH limits extracellular but not intracellular pH compensation in the CO 2 -tolerant freshwater fish Pangasianodon hypophthalmus

Abstract

Preferentially regulating intracellular pH (pH i ) confers exceptional CO 2 tolerance on fish, but is often associated with reductions in extracellular pH (pH e ) compensation. It is unknown whether these reductions are due to intrinsically lower capacities for pH e compensation, hypercarbia-induced reductions in water pH or other factors. To test how water pH affects capacities and strategies for pH compensation, we exposed the CO 2 -tolerant fish Pangasianodon hypophthalmus to 3 kPa P CO2 for 20 h at an ecologically relevant water pH of 4.5 or 5.8. Brain, heart and liver pH i was preferentially regulated in both treatments. However, blood pH e compensation was severely reduced at water pH 4.5 but not 5.8. This suggests that low water pH limits acute pH e but not pH i compensation in fishes preferentially regulating pH i . Hypercarbia-induced reductions in water pH might therefore underlie the unexplained reductions to pH e compensation in fishes preferentially regulating pH i , and may increase selection for preferential pH i regulation.