Effects of Photoperiod and Light Quality on Phlorotannin Accumulation and Multi-Omics Responses in Sargassum muticum

Brown algae contain phlorotannins with recognized biological and ecological functions, but the responses of phenolic metabolism to different light environments remain incompletely understood. In this study, Sargassum muticum was cultivated for 15 days under different photoperiods (12L:12D, 16L:8D, and 20L:4D) and light qualities (white, blue, and red light). Growth, photosynthetic performance, oxidative stress indicators, and phlorotannin content were examined, together with transcriptomic and untargeted metabolomic responses. Extended photoperiods were associated with increased growth and a transient increase in phlorotannin content, whereas blue light reduced growth but maintained relatively high chlorophyll a content and photosynthetic efficiency. Transcriptomic analysis identified 10,322, 3790, and 9301 differentially expressed genes under long-photoperiod, blue-light, and red-light treatments, respectively, including 388 genes shared among the three comparisons. Seven transcripts were putatively annotated as polyketide synthases, including four candidate type III polyketide synthase transcripts, and 26 transcripts were annotated as candidate acetyl-CoA carboxylase-related sequences. Untargeted LC–MS analysis yielded 949 putatively annotated metabolites, among which features annotated as salicylic acid and γ-glutamylcysteine showed increased relative abundance under the three light treatments. A feature putatively annotated as phloroglucinol did not differ significantly among treatments. Correlation analysis indicated coordinated, but non-causal, associations between light-responsive transcripts and changes in amino acid, carbohydrate, and lipid metabolism. These results characterize the physiological and multi-omics responses of S. muticum to altered light regimes and provide candidate molecular targets for future functional and targeted chemical validation.