By early June of this year, the Caribbean shores, along with the Gulf of Mexico and the northern coasts of South America, witnessed an unprecedented influx of approximately 38 million tons of Sargassum seaweed. This extensive accumulation has reached a historical peak, disrupting both ecosystem balance and local economies reliant on tourism. Decomposing masses of this brown algae emit an intense and unpleasant odor, creating an inhospitable environment on affected beaches and threatening fragile coastal ecosystems. Despite its problematic nature near shorelines, Sargassum floating freely in open ocean waters serves as a vital habitat and nourishment source for a myriad of marine life forms.
Sargassum algae originate from the Sargasso Sea, situated east of Florida. Since 2011, the emergence of the Great Atlantic Sargassum Belt—a vast expanse of this gulfweed stretching from equatorial regions towards the Caribbean—has caught the attention of researchers worldwide. Prevailing easterly winds facilitate this drift. For years, scientists speculated that nutrient influxes from terrestrial sources like overfertilization in agricultural areas and deforestation of rainforests played a role in fueling these algae flourishes. Yet, these hypotheses could not comprehensively account for the drastic surge in Sargassum biomass observed over recent years.
A multinational research consortium, spearheaded by experts at the Max Planck Institute for Chemistry, has now elucidated the principal biochemical and climatic drivers behind these vast seaweed blooms. Their investigation provides critical insights into the sustaining mechanisms of Sargassum proliferation and sets the foundation for predictive tools to forecast future stranding events, offering hope for mitigation strategies tailored to affected coastal regions.
Central to the team’s findings is the revelation that cyanobacteria colonizing the surface of Sargassum are pivotal in supporting its explosive growth. These microorganisms engage in a symbiotic relationship with the algae, harnessing atmospheric nitrogen gas (N₂) and converting it into biologically accessible forms through nitrogen fixation. The nitrogen thus fixed acts as a supplemental nutrient, supplementing the algae’s needs and conferring a competitive advantage over other marine flora in the Equatorial Atlantic waters. This symbiotic nitrogen acquisition was identified as a critical driver behind the recent surges in Sargassum biomass.
The researchers identified that the availability of phosphorus, an essential macronutrient, catalyzes nitrogen fixation rates by the cyanobacteria. They detailed a process wherein intense wind-driven upwelling near the equator transports phosphorus from nutrient-rich deep waters to the sunlit ocean surface. This nutrient-enriched surface water subsequently moves northwards into Caribbean waters, facilitating the proliferation of cyanobacteria and thus indirectly promoting Sargassum blooms. This process was only recently understood and offers a compelling explanation for earlier enigmatic patterns in algal bloom occurrences.
To establish this connection and peer into the historical trends of these events, scientists analyzed the chemical composition of coral skeletal deposits collected from diverse Caribbean locations. Coral growth layers, akin to terrestrial tree rings, serve as natural archives of oceanic chemistry, embedding signatures that reflect changes in environmental conditions over centuries. Through meticulous isotopic analysis of nitrogen within these coral layers, researchers quantified shifts in nitrogen fixation rates spanning the last 120 years, demonstrating an intensification of this process in recent decades correlating closely with Sargassum proliferation records.
Detailed nitrogen isotopic studies revealed that when microbial nitrogen fixation is high, corals register a decrease in the ratio of nitrogen-15 (¹⁵N) to nitrogen-14 (¹⁴N). This signature served as a proxy to reconstruct historical nitrogen fixation events. Notably, significant spikes aligning with record Sargassum bloom years of 2015 and 2018 reinforced the temporal coupling of nitrogen fixation and algal major biomass expansions. These findings delivered the first robust empirical evidence directly linking nitrogen fixation by cyanobacteria to the magnitude of Sargassum blooms.
Further comparative data analyses have demonstrated that since 2011, the fluctuations in Sargassum biomass closely track variations in nitrogen fixation rates. This synchrony gained significance given that the year 2010 marked the first major displacement of brown algae from the stable confines of the Sargasso Sea to the broader tropical Atlantic, pointing to climatic influences driving distribution changes. This finding underscores the intertwined nature of oceanographic and atmospheric processes shaping bloom dynamics.
Crucially, the study ruled out alternative nutrient sources previously suggested as primary contributors. For instance, iron delivery through Saharan dust storms, while important in other marine nutrient cycles, showed no meaningful correlation with bloom intensity. Likewise, nutrient runoff from the Amazon and Orinoco river systems failed to match the temporal and spatial bloom patterns documented. These eliminations sharpen focus on phosphorus upwelling and nitrogen fixation as the dominant bloom-stimulating mechanisms.
The research team proposes a comprehensive mechanistic model wherein phosphorus-rich waters from equatorial upwelling fuel cyanobacterial nitrogen fixation on Sargassum surfaces, driving the dramatic algal growth observed over recent decades. Variations in atmospheric pressure, influenced by antipodal temperature contrasts between the tropical North and South Atlantic, produce wind anomalies that induce this upwelling. Sea surface temperature fluctuations thereby exert indirect yet potent control over nutrient dynamics and consequently bloom formations.
Understanding these mechanistic pathways allows for the prospect of predictive models incorporating real-time monitoring of sea temperatures, wind patterns, and subsurface nutrient levels. Such integrative forecasting efforts could significantly aid coastal managers and policymakers by anticipating bloom intensities and timings, thus improving response strategies to mitigate environmental and socio-economic impacts.
Looking ahead, the Max Planck Institute’s team aims to expand their historical reconstructions by sampling additional coral records distributed across the Caribbean basin. The accumulation of comprehensive datasets will facilitate refining predictive capabilities and deepen insight into how global climatic shifts may alter phosphorus supply and nitrogen fixation dynamics. Ultimately, understanding the interplay between warming oceanic conditions and nutrient cycling will be key to managing the future prevalence and impact of Sargassum blooms in tropical Atlantic waters.
Alfredo Martínez-García, the senior author of the study, emphasized the vital necessity of integrating climatological and oceanographic variables to grasp the trajectory of Sargassum proliferation under ongoing global warming. While the phosphorus-nitrogen fixation mechanism currently stands as the leading explanation for bloom phenomena, the researchers acknowledge remaining uncertainties regarding other potential contributing factors. Nonetheless, these breakthroughs mark a pivotal advance in marine biogeochemistry and highlight nature’s intricate feedback systems affecting marine ecosystems and human livelihoods.
In sum, this research not only sheds light on the biogeochemical processes propelling a growing environmental challenge in the Atlantic but also underscores the urgency of interdisciplinary science to unravel and address complex ecological crises. As climate change accelerates, such knowledge will be indispensable for developing adaptive measures to preserve both natural marine habitats and the economic vitality of coastal communities dependent on them.
Subject of Research: Not applicable
Article Title: Equatorial upwelling of phosphorus drives Atlantic N2 fixation and Sargassum blooms
News Publication Date: 5-Nov-2025
Web References: DOI:10.1038/s41561-025-01812-2
References: Max Planck Institute for Chemistry study published in Nature Geoscience
Image Credits: Not provided
Keywords: Sargassum blooms, nitrogen fixation, phosphorus upwelling, cyanobacteria symbiosis, Great Atlantic Sargassum Belt, coral isotope analysis, nutrient cycling, marine biogeochemistry, Caribbean ecosystems, oceanographic processes, climate variability, global warming impacts
