What’s Causing the Sargassum Seaweed Invasion?

The upwelling of deep waters rich in phosphorus favors the presence of a nitrogen-fixing symbiont in Sargassum algae, giving them a competitive advantage.

A Quick Summary:
– Each year, extensive masses of sargassum spread across the tropical Atlantic, contaminating Caribbean coastlines. Analyses of coral cores extracted through drilling help explain the mechanism driving these brown algae blooms.
– Phosphorus-rich deep waters, driven to the surface by wind, favor the development of nitrogen-fixing cyanobacteria that live in symbiosis with Sargassum algae, providing them with essential nutrients in this nitrogen-poor region.
– Understanding how the blooms occur can improve predictions of sargassum strandings.

In early June of this year, approximately 38 million tons of sargassum arrived on the coasts of the Caribbean islands, the Gulf of Mexico, and northern South America, setting a negative record. Especially during the summer months, these brown algae accumulate on beaches, decompose, and emit a foul odor. This not only deters tourists but also threatens coastal ecosystems. In the open ocean, the sargassum floating on the surface serves as food and habitat for numerous marine species.

The algae originally come from the Sargasso Sea, located east of Florida. However, since 2011, researchers have repeatedly observed the so-called Great Atlantic Sargassum Belt, a gigantic carpet of algae that moves from the equator toward the Caribbean when easterly winds predominate. Until now, the sources of the phosphorus (P) and nitrogen (N) nutrients driving its rapid growth were unknown. It was hypothesized that nutrient runoff from over-fertilization and deforestation of the rainforest could be responsible. However, these processes do not explain the increase in Sargassum biomass observed in recent years.

An international research team, led by the Max Planck Institute of Chemistry, has discovered the primary mechanism causing the proliferation of these algae. The researchers have also identified the climatic conditions that facilitate this phenomenon, allowing them to develop a predictive system for future sargassum strandings.

Additional Nitrogen Provided by Cyanobacteria Growing on the Algae

In the latest issue of the journal Nature Geoscience, researchers from Mainz explain how strong wind-driven upwelling near the equator transports phosphorus to the ocean surface and shifts it northward to the Caribbean. This increase in phosphorus availability benefits cyanobacteria that grow on the brown algae. These bacteria can capture atmospheric gaseous nitrogen (N₂) and convert it into a form usable by the algae, a process termed nitrogen fixation.

Cyanobacteria are known to colonize algae of the genus Sargassum, forming a symbiotic relationship in which the Sargassum benefits from an additional source of nitrogen. According to the new study, this symbiotic relationship offers a competitive advantage over other algae in the equatorial Atlantic and can explain the observed changes in Sargassum biomass.

Nitrogen Isotopes in Coral Cores Have Revealed Nitrogen Fixation Rates Over the Last 120 Years

Researchers have identified the connection between algae proliferation, increased nitrogen fixation, and the upwelling of cold, nutrient-rich deep waters by analyzing coral cores from various zones in the Caribbean. Corals constitute vital archives for reconstructing past oceanic changes, as during their growth they incorporate the chemical signatures of the water into their calcareous skeletons.

By analyzing the annual growth layers of corals, similar to tree rings, researchers can reveal changes in the ocean's chemical composition over the last centuries. In this study, researchers from the Max Planck Institute analyzed the isotopic composition of nitrogen in corals to reconstruct the amount of nitrogen fixed in the ocean by microorganisms over the last 120 years.

During nitrogen fixation, bacteria reduce the ratio of the stable nitrogen isotopes 15N to 14N in the ocean. Therefore, periods with a low ratio of 15N to 14N analyzed in the coral layers indicate times of high nitrogen fixation rates. Seawater samples collected by the research vessel Eugen Seibold were used to calibrate the nitrogen isotopic composition in modern corals, demonstrating that they record nitrogen fixation.

Since 2011, Algae Growth and Nitrogen Fixation Have Remained Coupled

Jonathan Jung, a doctoral student at the Max Planck Institute of Chemistry and the study's first author, explained: "In the first set of measurements we observed two significant increases in nitrogen fixation in 2015 and 2018, two years of record sargassum blooms. So we compared our coral reconstruction with the annual sargassum biomass data, and the two records matched perfectly! However, at that time it was not at all clear if there was a causal relationship."

The researchers identified a connection after examining both sets of measurements. It turned out that not only the peak values, but the entire data series on algae growth and nitrogen fixation, including the minimum values, have been correlated since 2011. This temporal coincidence is important because, in 2010, strong winds displaced brown algae from the Sargasso Sea to the tropical Atlantic for the first time.

The research team concluded that excess phosphorus is the key factor in the proliferation of sargassum, ruling out other possibilities. A previous theory suggested that iron-rich Saharan dust, which frequently moves from Africa toward the Atlantic, favors algae growth. However, the dust input did not correlate with the biomass. Similarly, nutrient inputs from the Amazon or Orinoco rivers did not correlate with the observations of sargassum proliferation.

The New Mechanism Can Be Used to Improve Predictions of Future Sargassum Blooms

In their publication, the researchers describe a mechanism by which phosphorus from deep-water upwelling and nitrogen from nitrogen fixation drive the algae blooms observed during the last decades. The geochemist Jung added: "Our mechanism explains the variability of Sargassum growth better than any previous approach. However, uncertainty still exists about whether other factors also influence it and to what extent."

The phosphorus input occurs due to cooler sea surface temperatures in the tropical North Atlantic and warmer ones in the South Atlantic. These temperature variations cause changes in atmospheric pressure, generating wind anomalies that displace surface water and allow phosphorus-rich water from the deep sea to flow into the zone.

According to researchers from Mainz, observing winds, sea temperatures, and the resulting changes in upwelling in the equatorial Atlantic can improve predictions of sargassum growth. Alfredo Martínez-García, group head at the Max Planck Institute of Chemistry and senior author of the study, explained: "Ultimately, the future of sargassum in the tropical Atlantic will depend on how global warming affects the processes that drive the input of excess phosphorus to the equatorial Atlantic."

His team plans to offer a more detailed view of these processes by analyzing new coral records from different locations in the Caribbean. The researchers hope that these new findings can guide efforts to mitigate the impacts of the blooms on Caribbean reef ecosystems and coastal communities.