Effects of temperature and salinity on CO2 fluxes dynamics and respiration metabolism in the sea cucumber Apostichopus japonicus (Selenka).

Global warming has led to higher temperatures and extreme precipitation events, significantly affecting the physiological status of sea cucumbers and causing substantial economic losses in the aquaculture industry. This study explored the effects of high temperature (HT), low salinity (LS), and their combination (TS) on CO2 fluxes at the water-air interface, energy allocation, and the expression of respiratory metabolism-related genes in sea cucumbers. The results showed that HT and/or LS treatments reduced growth performance by 8.37-21.64 % and increased respiratory metabolic carbon content by 17.85-57.68 %. Respiratory metabolizable energy increased by 22.59 % (HT) and 25.87 % (TS). Enzyme activities and gene expression of pyruvate dehydrogenase (PDH), pyruvate kinase (PK), and hexokinase (HK) in respiratory tree were significantly higher than in body wall in response to high temperature stress. Conversely, activities and gene expression of α-ketoglutarate dehydrogenase (OGDH), lactate dehydrogenase (LDH), and malate dehydrogenase (MDH) were significantly higher in body wall in response to low salinity stress. These results indicate that the respiratory tree is more responsive to high temperature, while low salinity primarily affects the body wall. CO₂ fluxes positively correlated with respiratory metabolizable energy, respiratory metabolic carbon content, and LDH activities (R > 0.51, p < 0.05), but negatively correlated with OGDH activity and specific growth rate (R > 0.50, p < 0.05). The respiratory tree's specific functions resulted in higher CO2 fluxes in HT group compared to LS group. Furthermore, the combined stress resulted in highest CO2 fluxes, suggesting an additive effect on the physiological metabolism of sea cucumber. This study provides insights into the adaptation mechanisms of aquatic animals to environmental stresses.