As a key environmental driver, temperature shapes the growth, metabolism, and stress tolerance of marine macroalgae. Gracilaria salicornia is broadly distributed along the Pacific coast and persists in tropical-subtropical coastal waters year-round, withstanding summer temperatures exceeding 30°C. Despite this remarkable thermal tolerance, the underlying physiological and molecular mechanisms remain poorly understood. To uncover the molecular and physiological basis of temperature-induced responses in G. salicornia, we exposed G. salicornia thalli to low (15℃), control (25℃), and high (35℃) temperatures, we performed an integrated analyses of growth, pigments, antioxidant physiological traits , and transcriptomic profiles. The results revealed that G. salicornia exhibited maximal growth (RGR = 2.67%) at 25℃. Low temperature significantly suppressed growth (RGR = 0.36%) and induced broad transcriptional reprogramming. Compared with the control, low-temperature treatment upregulated 58,623 DEGs, which were significantly enriched in pathways related to endocytosis, phagosome, hormone signaling, and plant hormone signal transduction, suggesting extensive regulation of cellular maintenance and stress-acclimation processes. In contrast, high temperature reduced growth (RGR = 1.84%), but enhanced stress-related physiological responses, including a significant increase in soluble protein content relative to the control (p = 0.012), carotenoid accumulation (p < 0.05), and a threefold increase in SOD activity (p < 0.01). MDA content remained statistically unchanged among treatments (p > 0.05), indicating no significant increase in membrane lipid peroxidation under high-temperature treatment. Transcriptomic analysis revealed a more focused functional response at high temperature, characterized by upregulated DEGs mainly associated with oxidative phosphorylation, photosynthesis, carbon metabolism, and the biosynthesis of amino acids and cofactors, whereas downregulated DEGs were primarily related to ribosome functions, protein processing in the endoplasmic reticulum, and central carbon and amino acid metabolism. This pattern suggests a reallocation of metabolic resources toward stress maintenance and cellular protection, accompanied by reduced investment in growth-related processes. These findings suggest that G. salicornia exhibits distinct temperature-dependent response strategies: low temperature induces extensive regulation of cellular maintenance and stress-acclimation processes, whereas high temperature is associated with metabolic reorganization and enhanced antioxidant-related protection alongside reduced growth. This study provides insights into thermal tolerance in marine macroalgae and contributes to understanding their potential responses to climate change.

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