Time and Tissue-Specific Responses of Mitochondrial Metabolism to Hypoxia in Fish

AbstractPeriods of hypoxia are extremely common in aquatic systems and are predicted to have enduring impacts on aquatic life. Mitochondrial metabolic responses are important for animal performance during hypoxia, but tissue-specific responses and time needed for mitochondria to adjust remain unclear. Here, we investigate how mitochondrial metabolism responds to hypoxia (50% air saturation) over a prolonged period (15-21 wk) in sea bass (Dicentrarchus labrax). We used a longitudinal assessment of mitochondria from three repeated, but nonlethal, samplings of red blood cells (RBCs) at 3-wk intervals (15, 18, and 21 wk of hypoxia) alongside a terminal sampling of two other tissues (liver and heart). We found that hypoxic fish increased their RBC oxidative phosphorylation between weeks 15 and 18 but did not change it between weeks 18 and 21. We also show that mitochondrial respiratory capacities were depressed in the heart but not in the liver or RBCs of sea bass held for 21 wk in hypoxia compared with those of sea bass maintained in normoxia. The time and tissue-specific responses to hypoxia likely have consequences for how organisms adjust their different organ functions under the constraints of oxygen availability. As the occurrence of hypoxia is expected to increase in marine ecosystems, our data also indicate that understanding temporal changes in mitochondrial metabolism is crucial to predict organismal responses in the face of ongoing environmental change.