Effects of temperature and cadmium exposure on the mitochondria of oysters (Crassostrea virginica) exposed to hypoxia and subsequent reoxygenation

Intertidal bivalves are commonly exposed to multiple stressors including periodic hypoxia, temperature fluctuations and pollution, which can strongly affect energy metabolism. We used top-down control and elasticity analyses to determine the interactive effects of intermittent hypoxia, cadmium (Cd) exposure and acute temperature stress on mitochondria of the eastern oyster Crassostrea virginica. Oysters were acclimated at 20 degrees C for 30 days in the absence or presence of 50 mu g l(-1) Cd and then subjected to a long-term hypoxia (6 days at <0.5% O-2 in seawater) followed by normoxic recovery. Mitochondrial function was assessed at the acclimation temperature (20 degrees C), or at elevated temperature (30 degrees C) mimicking acute temperature stress in the intertidal zone. In the absence of Cd or temperature stress, mitochondria of oysters showed high resilience to transient hypoxia. In control oysters at 20 degrees C, hypoxia/reoxygenation induced elevated flux capacity of all three studied mitochondrial subsystems (substrate oxidation, phosphorylation and proton leak) and resulted in a mild depolarization of resting mitochondria. Elevated proton conductance and enhanced capacity of phosphorylation and substrate oxidation subsystems may confer resistance to hypoxia/reoxygenation stress in oyster mitochondria by alleviating production of reactive oxygen species and maintaining high aerobic capacity and ATP synthesis rates during recovery. Exposure to environmental stressors such as Cd and elevated temperatures abolished the putative adaptive responses of the substrate oxidation and phosphorylation subsystems, and strongly enhanced proton leak in mitochondria of oysters subjected to hypoxia/reoxygenation stress. Our findings suggest that Cd exposure and acute temperature stress may lead to the loss of mitochondrial resistance to hypoxia and reoxygenation and thus potentially affect the ability of oysters to survive periodic oxygen deprivation in coastal and estuarine habitats.