Effects of Different Dietary Lipid Levels Under Low Salinity Stresses on the Growth Performance and Expression of Lipid Metabolism-Related Genes in Juvenile Turbot (Scophthalmus maximus)

Turbot (Scophthalmus maximus) is one of the most economically important fish in the northern coastal areas of China. Recently, low salinity stress is an important factor influencing the development of the turbot industry. Lately, the industry has faced challenges posed by low salinity stress, a key factor influencing turbot development. Salinity fluctuations represent a crucial environmental stressor in aquaculture, impacting osmotic pressure and inducing abnormal energy metabolism. Our preliminary research had identified that low salinity stress contributes to lipid metabolism disorders in turbot. This study aims to investigate the impact of varying lipid concentration levels on the lipid metabolism disorders induced by low salinity stress. To achieve this, feeding experiments were conducted with different lipid concentration levels under low salinity stress conditions, determining the optimal lipid requirement for turbot to cope with reduced salinity. Three salinity gradients (10, 20, and 30) were employed, with four lipid concentration gradients (8%, 12%, 16%, and 20%) for each salinity group. The results revealed that at a lipid concentration of 16%, no significant difference in the growth performance of juvenile turbot was observed between the salinities of 20 and 30. However, the growth performance under a salinity of 10 was lower than that under salinities of 20 and 30. Notably, under a salinity of 10, the growth performance of juvenile turbot increased with higher lipid concentrations, reaching its peak at 20%, surpassing values under the salinities of 20 and 30. Regarding the expression of genes related to lipid metabolism, under the salinities of 10 and 20, the expression levels of genes related to lipid synthesis, lxrα, cyp7a1, and srebp-1, showed a trend of first increasing and then decreasing with increasing lipid concentration, particularly under the salinity of 10. The expression level of the 12% lipid group was higher than that of other groups and the expression level of the 20% lipid group was significantly lower than that of other groups (P < 0.05). In addition, under a salinity of 10, the expression levels of acc and fas also showed a similar situation. They were both suppressed in the 20% lipid group, and the expression levels were higher in the 12% lipid group. The expression level of the gene apoa-IV related to lipid absorption in the 20% lipid group under a salinity of 10 was markedly higher than that in other groups under the same s