Phenotypic plasticity during diel cycling hypoxia in Arctic char (Salvelinus alpinus)

Oxygen concentration naturally fluctuates in aquatic environments. Due to increased eutrophication caused by anthropic activities, this phenomenon could be amplified and result in a daily cycle of alternating normoxic and hypoxic conditions. At the metabolic level, lack of oxygen and reoxygenation can both have serious repercussions on fish due to fluctuations in ATP supply and demand and an elevated risk of oxidative burst. Thus, fish must adjust their phenotype to survive and equilibrate their energetic budget. However, their energy allocation strategy could imply a reduction in growth which could be deleterious for their fitness. Although the impact of cyclic hypoxia is a major issue for ecosystems and fisheries worldwide, our knowledge remains however limited. Our objective was to characterise the effects of cyclic hypoxia on growth and metabolism in fish. We monitored growth parameters (specific growth rate, condition factor), hepatosomatic and visceral indexes, relative heart mass and hematocrit of Arctic char (Salvelinus alpinus) exposed to thirty days of cyclic hypoxia. We also measured the hepatic protein synthesis rate, hepatic triglycerides as well as muscle glucose, glycogen and lactate, and quantified hepatic metabolites during this treatment. Arctic char appeared to acclimate well to oxygen fluctuations. The first days of cyclic hypoxia induced a profound metabolome reorganisation in the liver. However, fish rebalanced their metabolic activities and successfully maintained their growth and energetic reserves after one month of cyclic hypoxia. These results demonstrate the impressive ability of fish to cope with their changing environment. Summary statement This study characterizes the metabolic adjustments performed by Arctic char when coping with one month of cyclic hypoxia. Fish reached a new phenotype by defending their growth and energy stores.