Physiological and Optical traits and Nutrients shape the dominance of pelagic <i>Sargassum</i> variants in the Atlantic

Abstract Pelagic Sargassum forms extensive floating populations, which expand across the tropical North Atlantic without spatial restrictions. Its ecological drivers and the dominance of three genetic variants remain poorly understood. Here, we characterized their optical (absorptance and specific absorption coefficient) and photosynthetic traits under variable light and temperature regimes, revealing distinct light absorption and photosynthetic efficiencies, and a metabolic adaptation to temperature. Two variants, S. natans-VIII and S. fluitans-III, showed reduced thallus respiration rates, enhancing photosynthetic production at elevated temperature, and explaining their dominance in warmer tropical regions. Conversely, the smallest variant, S. natans I, exhibited superior light-harvesting and photosynthetic efficiencies and lower nutrient demands, aligning with its previous dominance in the Sargasso Sea oligotrophic conditions. These functional differences, combined with increased nutrient availability from ocean fertilization and rising temperatures due to global warming, can explain the massive proliferation of Sargassum in the tropical Atlantic. Our findings stress the role of physiological and optical traits in explaining variant dominance within algal populations and provide a foundation for predictive models and management strategies addressing large-scale algal blooms. They also highlight the potential of pelagic Sargassum as a potential model for studying adaptive traits in asexual lineages through metabolic tuning.