Floating transition unlocks a photosynthesis-centered physiological advantage enabling dominance of Ulva prolifera in Yellow Sea green tides

In Yellow Sea green tides (YSGTs), the macroalga Ulva prolifera exhibits negligible competitive superiority in the attached growth stage but rapidly outcompetes co-occurring Blidingia sp. after shifting to floating lifestyles. While the physiological basis of this transition remains unclear. Here, we reveal that the attached-to-floating transition triggers comprehensive physiological remodeling in U. prolifera , characterized by stabilized photosynthesis, activated nitrogen assimilation and maintained antioxidant homeostasis. Structural equation modeling quantifies that photosynthesis and nitrogen assimilation contribute 52.14% and 45.88% to algal growth, respectively. Further inhibitor experiments confirm photosynthesis as the dominant regulatory driver: disruption of photosynthetic electron transport concurrently impair carbon and nitrogen metabolism and suppress algal growth, whereas nitrate reductase inhibition has little effect on photosynthetic performance. Collectively, these photosynthesis-dominated synergistic advantages, specifically formed during the early floating stage, determine the early dominance and large-scale bloom expansion of U. prolifera , elucidating the intrinsic mechanism driving recurrent Yellow Sea green tide outbreaks.