New research recently published in Nature Communications has finally highlighted what is driving the huge Sargassum blooms that have taken the Atlantic Ocean by storm. A team of their scientists has recently figured out why these blooms thrive primarily on the upwelling of cool, nutrient-rich deep water. They depend on nitrogen fixation processes carried out by diazotrophic bacteria.
Sargassum, a species of free-floating seaweed, comes from the Sargasso Sea east of Florida. This changed dramatically in 2025. Nearly 38 million tons of Sargassum started washing up on the shores of Caribbean islands, the Gulf of Mexico, and northern South America. This massive influx of seaweed has made its way onshore across the region, decomposing and releasing foul odors into the air— especially during the hot summer months.
Growing since 2011, scientists have been monitoring the Great Atlantic Sargassum Belt. This huge mat of gulfweed is moving north from the tropical equator along with the warmer waters of the Caribbean. The finger of prevailing easterly winds helps to complicate the drift. The true culprits behind these algal blooms are phosphorus released from upwelling deep water and nitrogen derived from nitrogen fixation.
Ecologists have recently discovered an equally amazing symbiotic relationship between Sargassum and nitrogen-fixing bacteria. This partnership increases Sargassum’s competitive advantage over other algae species in the Equatorial Atlantic. For the study, scientists sampled coral cores from 25 diverse locations throughout the Caribbean. They mapped the effects of ocean acidification over centuries and found a connection between these changes and Sargassum growth.
Even beyond that, the research team, led by Chief Scientists Dr. Using the annual growth layers found in coral, like tree rings, they reconstructed these shifts. Seawater samples were instrumental in calibrating the nitrogen isotopic composition of modern corals. This research indicates that corals can be useful recorders of nitrogen fixation activities.
Prior hypotheses thought that iron-rich Saharan dust was the engine that powered this Sargassum growth. Researchers have ruled out this hypothesis as the primary culprit. Researchers have confirmed that lower than normal sea surface temperatures in the tropical North Atlantic are very important. At the same time, rising sea surface temperatures in the south Atlantic supply the phosphorus needed to fuel the Sargassum blooms.
Jonathan Jung, a member of the research team, emphasized the findings:
“In the first set of measurements we noticed two significant increases in nitrogen fixation in 2015 and 2018, two years of record Sargassum blooms. So we compared our coral reconstruction with annual Sargassum biomass data, and the two records aligned perfectly! At that time, however, it was not at all clear whether there was a causal link.”
Further insights from Alfredo Martínez-García highlighted the ongoing uncertainty surrounding future trends:
“Ultimately, the future of Sargassum in the tropical Atlantic will depend upon how global warming affects the processes that drive the supply of excess phosphorus to the equatorial Atlantic.”
Jung concluded with a note on their findings’ implications:
“Our mechanism explains the variability of Sargassum growth better than any previous approaches. However, there is still uncertainty as to whether and to what extent other factors also play a role.”