Effects of variable oxygen regimes on mitochondrial bioenergetics and reactive oxygen species production in a marine bivalve, .

Estuarine and coastal benthic organisms often experience fluctuations in oxygen levels that can negatively impact their mitochondrial function and aerobic metabolism. To study these impacts, we exposed a common sediment-dwelling bivalve, the soft-shell clam , for 21 days to chronic hypoxia (  ∼4.1 kPa), cyclic hypoxia (  ∼12.7-1.9 kPa, mean 5.7 kPa) or normoxia (  ∼21.1 kPa). pH was manipulated to mimic the covariation in CO/pH and oxygen levels in coastal hypoxic zones. Mitochondrial respiration, including proton leak, the capacity for oxidative phosphorylation (OXPHOS), the maximum activity of the electron transport system (ETS), reactive oxygen species (ROS) production, and activity and oxygen affinity of cytochrome oxidase (CCO) were assessed. Acclimation to constant hypoxia did not affect the studied mitochondrial traits except for a modest decrease in the OXPHOS coupling efficiency. Cyclic hypoxia had no effect on OXPHOS or ETS capacity, but increased proton leak and lowered mitochondrial OXPHOS coupling efficiency. Furthermore, mitochondria of clams acclimated to cyclic hypoxia had higher rates of ROS generation compared with the clams acclimated to normoxia or chronic hypoxia. CCO activity was upregulated under cyclic hypoxia, but oxygen affinity of CCO did not change. These findings indicate that long-term cyclic hypoxia has a stronger impact on the mitochondria of than chronic hypoxia and might lead to impaired ATP synthesis, higher costs of mitochondrial maintenance and oxidative stress. These changes might negatively affect populations of in the coastal Baltic Sea under increasing hypoxia pressure.