Combined Effects of Marine Heatwaves and Light Intensity on the Physiological, Transcriptomic, and Metabolomic Profiles of Undaria pinnatifida

Marine heatwaves (MHWs) are spreading across global oceanic regions with unprecedented intensity, frequency, and duration, and are often accompanied by changes in underwater light, thereby imposing multiple stressors on coastal macroalgae. In this study, the effects of MHW intensities (moderate: +3 °C; severe: +6 °C) and light intensities (normal: 90 μmol photons m^-2 s^-1; high: 270 μmol photons m^-2 s^-1) on cultivated <i>Undaria pinnatifida</i> were investigated through an integrated analysis of physiological, transcriptomic, and metabolomic responses. Under moderate MHW conditions, <i>U. pinnatifida</i> exhibited enhanced growth and photosynthetic performance, with increased pigment content, improved electron transport, and the early activation of antioxidant defenses. Following severe MHW exposure, the partial recovery of some physiological traits was observed, while photosynthetic capacity, membrane integrity, and energy metabolism remained impaired, and oxidative damage was not fully resolved. High light stress further aggravated stress responses under both MHW intensities by disrupting photoprotection and weakening antioxidant defense systems. These results suggest that <i>U. pinnatifida</i> exhibits adaptive capacity under moderate MHWs and delayed physiological damage and incomplete recovery under severe MHWs. High light stress further exacerbates both responses, ultimately affecting yield and quality.