Respiratory plasticity improves aerobic performance in hypoxia in a marine teleost

Ocean deoxygenation is a pressing concern in the face of climate change. In response to prolonged hypoxia, fishes have demonstrated the ability to dynamically regulate hemoglobin (Hb) expression to enhance oxygen (O2) uptake. Here, we examined hypoxia-inducible Hb expression in red drum (Sciaenops ocellatus) and the subsequent implications on Hb-O2 binding affinity and aerobic scope. Fish were acclimated to 30 % air saturation for 1 d, 4 d, 8 d, 2 w, or 6 w, and red blood cells were collected for gene expression and biochemical profiling. Hypoxia acclimation induced significant up-regulation of one Hb subunit isoform (hb alpha 2) relative to control by 4 d with consistent upregulation thereafter. Hematocrit increased in hypoxia, with no changes in the allosteric modulator [NTP] at any time point. Changes in Hb expression co-occurred with a reduced Root effect (-26 % in normoxia, -14 % in hypoxia) at a physiologically relevant pH while increasing O2 binding affinity (i.e., lower P50). These changes correlated with increased maximum metabolic rate and aerobic scope relative to controls when fish were tested in hypoxia. These results demonstrate an important role for Hb multiplicity in improving O2 affinity and maximizing respiratory performance in hypoxia.

Automated summary

This study investigates the expression of hemoglobin (Hb) and its effects on oxygen (O2) uptake in red drum fish under hypoxic conditions. The findings reveal that fish are able to up-regulate a specific Hb isoform in response to prolonged hypoxia, leading to increased oxygen affinity and improved respiratory performance. These adaptive responses are crucial for surviving in low-oxygen environments caused by climate change.