Physiological and molecular responses of Pyropia yezoensis thallus to high-light stress

Pyropia yezoensis, a typical intertidal macroalga, is frequently exposed to various abiotic stresses in its natural habitat. However, the strategies of Pyropia to tolerate high light stress are poorly understood. This study characterized the morphological changes, physiological responses, and transcriptomic reprogramming of P. yezoensis thalli under high-light stress to explore its underlying adaptive mechanisms. High‑light stress slightly induced thallus elongation and increased cell division. However, it significantly reduced blade thickness and, as a result, decreased overall biomass accumulation. Further, thylakoid lamellae became thinner and more loosely arranged, with increased plastoglobuli accumulation. Photosynthetic physiology, pigment composition, and metabolite contents were decreased, while reactive oxygen species were induced. By isolating high-light stress from heat stress, we conducted a genome-wide transcriptomic analysis to delineate photo-stress responses, revealing that P. yezoensis prioritizes energy allocation towards emergency responses such as DNA repair by reducing high-energy-consuming biological processes (ribosome biogenesis) and overall metabolic activity (tricarboxylic acid cycle, pentose phosphate pathway, glycolysis/gluconeogenesis, and floridean starch metabolism), thereby enhancing survival capacity under environmental stress. Notably, NADPH-dehydrogenase-dependent cyclic electron transport (CET) played a more critical role in the overall CET contribution under high-light stress. Furthermore, the mitochondrial alternative oxidase (AOX) pathway, responsible for dissipating excess reducing equivalents, was not significantly activated, which may be a significant reason why P. yezoensis thalli does not display extreme tolerance to high-light stress. Our findings elucidate the survival strategies of P. yezoensis in response to high-light stress, providing new insights into the ancient adaptations of intertidal red algae to harsh environmental conditions. Furthermore, the results of this study offer a theoretical foundation for the genetic improvement of stress-resistance traits and contribute to the sustainable development of the P. yezoensis industry.