Molecular responses and mechanisms of the sea cucumber Apostichopus japonicus under prolonged hypoxic conditions

Abstract

In recent decades, hypoxia has become widespread in coastal waters. Research on the molecular response mechanisms of sea cucumbers (Apostichopus japonicus) under long-term hypoxic stress is limited. Consequently, an 18-day hypoxia experiment was conducted to examine the extreme tolerance. We simulated realistic conditions by establishing dissolved oxygen (DO) levels of 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0 mg/L. The results showed that chronic hypoxia had detrimental effects on respiration, survival, and weight gain of sea cucumbers. The oxygen consumption rate of A. japonicus decreased as the DO level declined (P < 0.05). Survival rates were lowest at DO levels of 2.0 and 3.0 mg/L, with survival rates of 20 ± 5 % and 45.0 ± 6.8 %, respectively. Compared to the control group, sea cucumbers at 2.0 mg/L not only exhibited a higher mortality rate, but also experienced a significant reduction in body weight (P < 0.05), with a weight gain rate of −8.75 ± 0.61 %. At the molecular level, sea cucumbers downregulate the expression of PFK and PDH in long-term hypoxia, thereby constraining energy generation via glycolysis and the tricarboxylic acid (TCA) cycle, and prompting A. japonicus to increasingly depend on augmented fatty acid β-oxidation to meet its energy requirements. Furthermore, cortisol secretion enhances carbohydrate metabolism, which in turn helps alleviate oxidative stress. Concurrently, A. japonicus increased the expression of GCL and MGST, thereby enhancing their cellular antioxidant and immune functions by regulating glutathione metabolism and other amino acids. This study enhances the understanding of the molecular mechanisms by which A. japonicus responds to prolonged hypoxia.