Collaborative Research: Deciphering Nutrient Sources and Microbial Processes Fueling the Great Atlantic Sargassum Belt

Massive floating mats of Sargassum seaweed now form every year across the tropical Atlantic, creating the Great Atlantic Sargassum Belt. These blooms provide habitat for marine life but also threaten coastal ecosystems, tourism, fisheries, and public health when they wash ashore in large quantities. It is still not understood why these blooms have become so extensive or what nutrient sources allow them to persist as they drift across the Atlantic Ocean. This project determines how nutrients and beneficial microorganisms support the growth and persistence of Sargassum by combining ocean expeditions with chemical, biological, and genetic analyses. The results will improve our understanding of the processes that fuel these recurring blooms, which could lead to better forecasting and management of Sargassum inundation in US coastal regions. This project will also train students and early-career scientists and engage the public through educational and outreach activities. Since 2011, recurring blooms of holopelagic Sargassum have formed the Great Atlantic Sargassum Belt, an 8000-km expanse spanning the tropical Atlantic that has profound ecological, biogeochemical, and socioeconomic impacts. Despite extensive observations and modelling, the nutrient sources and microbial mechanisms sustaining these basin-scale blooms remain poorly constrained. Atmospheric deposition, vertical nutrient supply, nitrogen fixation, and microbial nutrient regeneration have all been implicated, but their relative contributions and interactions have not been quantified because of limited in-situ process measurements. This project addresses this critical knowledge gap by integrating biogeochemical tracers, microbial ecology, and nutrient budgets to test two central hypotheses: (1) Sargassum growth is primarily supported by vertically supplied nutrients and Sargassum-associated nitrogen fixation, with their relative importance varying across the Atlantic basin, and (2) microbially-mediated nutrient regeneration within the Sargassum holobiont sustains nutrient availability and bloom persistence during trans-Atlantic transport. Guided by preliminary evidence of basin-scale shifts in diazotroph communities, this project evaluates the Dynamic Consortium Hypothesis, which proposes that transient nutrient inputs initiate bloom development while microbial consortia progressively enhance nitrogen fixation and internal nutrient recycling as Sargassum drifts westward. A trans-Atlantic research cruise combines measurements of dissolved nutrients, aerosol deposition, stable isotopes, elemental composition, process rate incubations, metagenomics, and functional gene analyses to directly quantify nutrient sources and microbial functions across the Great Atlantic Sargassum Belt. The resulting mechanistic understanding advances predictions of Sargassum bloom dynamics, improves representation of nutrient cycling in marine biogeochemical models, and provides a framework for understanding large-scale floating macroalgal ecosystems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.