Hypoxia modulates spawning-induced changes in mitochondrial function and energy reserves in blue mussels (Mytilus edulis).
Supervisor: Ina Sokolova (Univ. Rostock)
Hypoxia (oxygen deficiency) is a frequent event in coastal area acting as a major stressor for marine benthic organisms. Mitochondrial function and energy metabolism are sensitive to hypoxia in benthic animals as low oxygen levels limit ATP production and increase generation of potentially toxic reactive oxygen species (ROS). In broadcast spawners such as the blue mussels Mytilus edulis, spawning is associated with a major energy loss through the release of gametes that might need to be compensated by elevated mitochondrial ATP production. However, the mitochondrial response to spawning events and its potential modulation by the oxygen regime are not well understood. We studied the effect of short-term hypoxia (7 days) and spawning (pre-spawning, and 3 h and 72 h post-spawning) on mitochondrial respiration, reactive oxygen species (ROS) production, and energy budget of the mussels. The mussels’ mitochondria showed increased oxidative phosphorylation (OXPHOS) rate at 72 h post spawning in normoxia but continuous suppression in hypoxia exposure. Mitochondrial ROS production decreased in both the resting (LEAK) and actively phosphorylating (OXPHOS) states indicating that M. edulis was able to prevent oxidative stress and mitochondrial damage during spawning. Total energy reserves declined in the digestive gland of the mussels upon hypoxia exposure. Notably, energy demands of spawning were mostly fueled by the protein breakdown and not by the glycogen or lipids. The aerobic capacity of mussels (ETS activity) and cellular energy allocation changed in response to hypoxia and spawning indicating a moderate stress level that likely falls into the pejus (suboptimal) range for this species. This may reflect the stress tolerance of M. edulis which has evolved as an intertidal species and adapted to large and rapid fluctuations of the abiotic stressors in intertidal habitats.

Keywords: Mitochondria, Hypoxic stress, Spawning activity, Bioenergetics, Bivalve.