Tissue- and substrate-dependent mitochondrial responses to acute hypoxia–reoxygenation stress in a marine bivalve (Crassostrea gigas)

Abstract

Hypoxia is a major stressor for aquatic organisms, yet intertidal organisms such as the oyster Crassostrea gigas are adapted to frequent oxygen fluctuations by metabolically adjusting to shifts in oxygen and substrate availability during hypoxia–reoxygenation (H/R). We investigated the effects of acute H/R stress (15 min at ∼0% O2 and 10 min reoxygenation) on isolated mitochondria from the gill and the digestive gland of C. gigas respiring on different substrates (pyruvate, glutamate, succinate, palmitate and their mixtures). Gill mitochondria showed better capacity for amino acid and fatty acid oxidation compared with mitochondria from the digestive gland. Mitochondrial responses to H/R stress strongly depended on the substrate and the activity state of mitochondria. In mitochondria oxidizing NADH-linked substrates, exposure to H/R stress suppressed oxygen consumption and generation of reactive oxygen species (ROS) in the resting state, whereas in the ADP-stimulated state, ROS production increased despite little change in respiration. As a result, electron leak (measured as H2O2 to O2 ratio) increased after H/R stress in the ADP-stimulated mitochondria with NADH-linked substrates. In contrast, H/R exposure stimulated succinate-driven respiration without an increase in electron leak. Reverse electron transport (RET) did not significantly contribute to succinate-driven ROS production in oyster mitochondria except for a slight increase in the OXPHOS state during post-hypoxic recovery. A decrease in NADH-driven respiration and ROS production, enhanced capacity for succinate oxidation and resistance to RET might assist in post-hypoxic recovery of oysters mitigating oxidative stress and supporting rapid ATP re-synthesis during oxygen fluctuations, as is commonly observed in estuaries and intertidal zones.