Water pH limits extracellular but not intracellular pH compensation in the CO2 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.

Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)

“…This phenomenon has been demonstrated with low pH-induced metabolic depression in isolated gill cells [82]. Given the large knowledge gaps concerning the mechanisms underpinning our results, we argue—as have others—that more multi-stressor studies are needed [12,13,83,84,85].…”

Preferential intracellular pH regulation is a common trait amongst fishes exposed to high environmental CO2

“…He also observed a similar pattern in salamanders exposed to aquatic hypercarbia (Heisler et al, 1982). Later, this strategy of preferential pH i regulation was observed in armored catfish (Pterygoplichthys pardalis; Brauner et al, 2004), white sturgeon (Acipenser transmontanus; Baker et al, 2009a) and striped catfish (Pangasianodon hypophthalmus; Sackville et al, 2018), and was hypothesized to be associated with acute hypercarbia tolerance (Brauner and Baker, 2009). However, it is not known whether preferential pH i regulation is restricted to a few specialized fishes, or whether it represents a more common pattern among fishes.…”

“…This is similar to the response observed in the striped catfish of the Mekong, which compensate pH e by 48 h at ca. 4 kPa P CO2 (Damsgaard et al, 2015) while preferentially regulating pH i (Sackville et al, 2018). In the striped catfish, the degree of pH e regulation was significantly reduced at lower water pH despite complete preferential pH i regulation (Sackville et al, 2018), indicating that water pH also has a direct effect on the ability to compensate pH e .…”

“…4 kPa P CO2 (Damsgaard et al, 2015) while preferentially regulating pH i (Sackville et al, 2018). In the striped catfish, the degree of pH e regulation was significantly reduced at lower water pH despite complete preferential pH i regulation (Sackville et al, 2018), indicating that water pH also has a direct effect on the ability to compensate pH e . Thus, preferential pH i regulation does not preclude pH e regulation, but results in a far more rapid and robust pH i regulatory response at times when pH e is depressed during initial exposure to hypercarbia.…”

“…The capacity for pH e compensation is influenced by exposure time and water quality, as fishes chronically exposed to gradual hypercarbia can compensate pH e beyond their capacity during acute exposure (McKenzie et al, 2003;Smart et al, 1979). Furthermore, the rate and degree of pH e compensation during acute hypercarbia exposure is also affected by water salinity (Tovey and Brauner, 2018), ion composition and pH (Sackville et al, 2018). However, survival during severe acute hypercarbia may depend on protecting pH i (Shartau et al, 2017a;Vandenberg et al, 1994;Wood et al, 1983;Yoshikawa et al, 1994) more so than pH e .…”