Hyperoxia acclimation and the aerobic response in red drum (Sciaenops ocellatus)

Hyperoxia is supersaturation of oxygen beyond 100 % air saturation (∼21 kPa) in aquatic environments. Hyperoxia promotes an oxygen diffusion gradient that greatly increases arterial and venous oxygen in the circulatory system of fish. Under these conditions fish can potentially upregulate mechanisms that benefit from supersaturation by increasing oxygen use and delivery. To test this, we acclimated the estuarine red drum (Sciaenops ocellatus) to three weeks of normoxic control (100 % air saturation, ∼20.7 kPa) or hyperoxic treatment (150 % air saturation, ∼32.5 kPa). We measured whole-animal metabolic rate in a cross design to assess aerobic scope and observed oxygen supply capacity, α_o. Following recovery, fish were sampled for magnitude of the Root effect and mitochondrial respiration of cardiac tissue. Red drum acclimated to hyperoxia showed lower mean cell hemoglobin content, decreased methemoglobin, and a modest decrease in red blood cell pH. The Root effect, the pH sensitivity that limits carrying capacity, was measured to ascertain differences in O₂ delivery in the heart; however, we found no differences due to acclimation. Cardiac mitochondria respiration showed no difference between acclimations. Acclimation did not impact whole-animal metabolic rate. However, fish from both acclimations increased maximum metabolic rate when exhaustively exercised in hyperoxia, which subsequently increased aerobic scope. Measures of α calculated at maximum metabolic rate and at the critical oxygen tension (P_crit) showed that α was lower when calculated in hyperoxia than normoxia or at P_crit. These data suggest that fish can benefit from hyperoxia in acute exposures, with subtle adjustments made in chronic hyperoxia.