High light alters biomacromolecule composition and enhances exopolysaccharide production in Porphyridium aerugineum.

Microalgae of the genus Porphyridium are valuable bioresources for producing sulfated exopolysaccharides (EPS) with industrial potential, yet their productivity under high light (HL) stress is underexplored. Here, the HL acclimation of the red alga Porphyridium aerugineum (550 μmol/m2/s) was investigated through time-course physiological assays, ultrastructural imaging, and multi-omics profiling. HL significantly enhanced biomass accumulation and EPS yield (3.5-fold) despite reduced photosynthetic efficiency. These data indicate altered biomass composition, with proteins and intracellular carbohydrates declining (by 40.7% and 36.5% on day 20 relative to LL, respectively), neutral lipids reduced, and cell walls more compact. Moderate ROS elevation and upregulated proline and ascorbate peroxidase under HL suggest a well‑regulated oxidative stress response, while monosaccharide analysis revealed a slight increase in glucuronic acid content, potentially benefiting EPS bioactivity. Multi-omics revealed coordinated downregulation of photosynthesis and nitrogen assimilation (NRT2, NR), enrichment of the ribosome pathway (indicating accelerated protein turnover for nitrogen recycling), and post-transcriptional buffering of carbon fixation. Based on these multi-omics data, UGPase, GBSS1, and a bZIP transcription factor are proposed as candidate targets for future strain improvement. These findings suggest that HL can be used to enhance EPS production in P. aerugineum, providing a potential basis and candidate targets for microalgal biorefineries.