Adaptation to heat and ocean fertilization, two keys for understanding the massive Sargassum growth in the Atlantic

Summary A floating ecosystem constituted by three genetic variants of holopelagic Sargassum has extended since 2011 throughout the tropical North Atlantic without spatial restrictions. We characterized the differential capacity and efficiency of each variant to collect light and fix this energy in photosynthesis under variable light and temperature regimes, focusing on the description of key physiological and optical traits the differential response to light and temperature. Our results revealed metabolic adaptations of two genetic variants to the warmer conditions of the tropical Atlantic and contrasting efficiencies in light absorption and use in photosynthesis, indicative of distinct competitive abilities under growth limitations. We concluded that the increased fertility of a warmer ocean is the most plausible explanation for the massive presence of holopelagic Sargassum in the tropical Atlantic, which also may explain the current ecological success of the opportunistic strategy of a previously rare variant. The optical and physiological descriptors documented can assist in developing quantitative models for predicting Sargassum biomass in the Atlantic.