Water pH limits extracellular but not intracellular pH compensation in the CO2 tolerant freshwater fish, Pangasianodon hypophthalmus

Abstract

Preferentially regulating intracellular pH (pHi) confers exceptional CO2 tolerance on fishes, but is often associated with reductions in extracellular pH (pHe) compensation. It is unknown if these reductions are due to intrinsically lower capacities for pHe 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 CO2 tolerant fish, Pangasianodon hypophthalmus to 3 kPa PCO2 for 20 h at ecologically relevant water pH's of 4.5 or 5.8. Brain, heart and liver pHi was preferentially regulated in both treatments. However, blood pHe compensation was severely reduced at water pH 4.5 but not 5.8. This suggests low water pH limits acute pHe but not pHi compensation in fishes preferentially regulating pHi. Hypercarbia-induced reductions in water pH might therefore underlie the unexplained reductions to pHe compensation in fishes preferentially regulating pHi, and may increase selection for preferential pHi regulation.