Physiological, transcriptomic, and metabolomic analyses reveal the adaptation mechanism of Betaphycus gelatinus under different salinity conditions

Betaphycus gelatinus is an essential raw material for the industrial extraction of carrageenan, and its growth and reproduction are significantly influenced by changes in salinity. To investigate the adaptation mechanisms of B. gelatinus under different salinity conditions, B. gelatinus was cultured for 7 d under 15, 20, 25, 30, 35, 40, and 45 psu and subjected to physiological, transcriptomic, and metabolomic analyses. The growth rate, photosynthetic performance and pigment content of B. gelatinus decreased significantly, while the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) and contents of malondialdehyde, soluble sugar, and glutathione (GSH) increased significantly under salt stress. Transcriptomic and metabolomic analyses indicated that photosynthesis, antioxidant systems, nitrogen metabolism, carbohydrate metabolism, lipid synthesis, and ABC transporters may be involved in the salt stress response of B. gelatinus. Under low-salinity stress, differentially expressed genes (DEGs) associated with photosynthesis, Calvin cycle, ascorbate (AsA) − GSH cycle, glycolysis/gluconeogenesis, and the pentose phosphate pathway were downregulated, while those associated with nitrogen metabolism were upregulated. In contrast, under high-salinity stress, DEGs associated with photosynthesis, chlorophyll degradation, Calvin cycle, AsA − GSH cycle, lipid synthesis, and ABC transporters were upregulated, while those associated with chlorophyll synthesis and nitrogen metabolism were downregulated. The amino acid content was increased under low-salinity stress but decreased under high-salinity stress, while the contents of saturated fatty acids and unsaturated fatty acids were increased under both low and high salinity conditions. These findings help elucidate the adaptation mechanisms of B. gelatinus under varying salinity conditions and provide theoretical guidance for efficient industrial breeding of B. gelatinus.