Light stress regulates pigment concentrations, growth, and gene expression in hatchery‐cultivated <i>Kappaphycus alvarezii</i> (Solieriaceae, Rhodophyta)

Kappaphycus alvarezii is a commercially valuable red seaweed cultivated for biomass and phycocolloid production. Enhancing its productivity, stress tolerance, and phycocolloid yield requires genetic diversification and optimized cultivation techniques. This study investigated the effects of different light intensities on growth, pigment composition, and gene expression in wild K. alvarezii to inform hatchery cultivation strategies. Eight wild-type strains were clonally cultivated under low, medium (control), and high light conditions for 28 days. High light resulted in the highest daily growth rate (6.03% ± 1.86%), whereas phycoerythrin (PE) levels were significantly higher under low light conditions. Gene expression analysis revealed that high light upregulated antioxidant-related genes (e.g., carotenoid-associated genes) and genes involved in DNA synthesis and repair, while repressing photosynthesis-related genes, likely as a protective response against oxidative stress. Low light reduced the expression of light-harvesting-related genes and intensified the expression of polysaccharide-degradation genes, potentially compromising carrageenan biosynthesis. Notably, the lectin KAA protein was highly induced under low light, suggesting a potential protective role against mannose-dependent pathogens. These results highlight the acclimative mechanisms of K. alvarezii under light stress and provide insights into metabolic regulation and energy allocation. Understanding these responses can help inform seaweed farmers about optimal cultivation depth and light exposure to maximize growth and enhance carrageenan yield, thereby contributing to sustainable aquaculture practices. Furthermore, transcriptome profiling identified key genes associated with carrageenan biosynthesis; together with the identification of light-regulated processes, these results provide candidate molecular targets for future genotype-based improvement and cultivation strategies aimed at enhancing carrageenan production.