Upwelling of phosphorus-rich deep water promotes an N-fixing symbiont of the Sargassum algae giving it a competitive advantage.
Playa del Carmen, a popular vacation destination on Mexico's Yucatán Peninsula, faces significant Sargassum strandings during summer months, as do other Caribbean coastlines. To maintain beach access for swimmers, the brown algae must be regularly cleared using machinery. Researchers at the Max Planck Institute for Chemistry have now been able to use coral drill cores to uncover the mechanism driving these algal blooms.
© Arkadij Schell
To the point:
- Each year, vast mats of Sargassum spread across the tropical Atlantic, fouling Caribbean coastlines. Analyses of coral drill cores help explain the mechanism that drives these brown algal blooms.
- Phosphorus-rich deep water, driven to the surface by winds, promotes nitrogen-fixing cyanobacteria that live in symbiosis with Sargassum algae and supply them with essential nutrients in this nitrogen-poor region.
- Understanding how the blooms are caused can improve predictions of Sargassum stranding events
By the beginning of June this year, approximately 38 million tons of Sargassum drifted towards the coasts of the Caribbean islands, the Gulf of Mexico, and northern South America, marking a negative record. Especially during the summer months, the brown algae accumulate on beaches, decomposing and emitting a foul odor. This not only repels tourists but also threatens coastal ecosystems. In the open ocean, Sargassum seaweed floating on the surface serves as nourishment 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 gulfweed that drifts from the equator towards the Caribbean when easterly winds prevail. Until now, the sources of nutrients phosphorus (P) and nitrogen (N) fueling their rapid growth was unclear. It was hypothesized that nutrient runoff from overfertilization and rainforest deforestation might be responsible. However, these processes cannot explain the increase in Sargassum biomass observed during the past years.
An international research team led by the Max Planck Institute for Chemistry has now uncovered the main mechanism behind these algae blooms. The researchers have also identified the climatic conditions that facilitate this phenomenon, enabling them to develop a predictive system for future stranding events of Sargassum.
Extra nitrogen provided by cyanobacteria growing on the algae
In the latest issue of the journal Nature Geoscience, the researchers from Mainz explain how strong wind-driven upwelling near the equator transports phosphorus to the ocean's surface and moves it northward into 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 called nitrogen fixation. Cyanobacteria are known to colonize Sargassum algae, forming a symbiotic relationship where Sargassum benefits from an additional nitrogen source. According to the new study this symbiotic relationship offers a competitive advantage with respect to other algae in the Equatorial Atlantic and can explain past changes in Sargassum biomass.
Nitrogen isotopes bound in coral cores have unveiled nitrogen fixation rates over the past 120 years
The researchers have identified the connection between algal blooms, increased nitrogen fixation, and the upwelling of cool, nutrient-rich deep water by analyzing coral cores from diverse Caribbean locations. Corals are vital archives to reconstruct past changes in the ocean because during their growth they incorporate chemical signatures from the water in their calcareous skeletons. By analyzing coral annual growth layers, which are akin to tree rings, researchers can reveal changes in the chemical composition of the ocean over the past centuries.
In this study the Max Planck researchers have analyzed the nitrogen isotopic composition of corals to reconstruct the amount of nitrogen fixed in the ocean by microorganisms over the last 120 years. During nitrogen fixation bacteria lower the ratio of stable nitrogen isotopes 15N to 14N in the ocean. Thus, periods of low 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 of modern corals demonstrating that they record nitrogen fixation.
Since 2011, algae growth and nitrogen fixation have remained coupled
Jonathan Jung, a PhD student at the Max Planck Institute for Chemistry and first author of the study, explains, "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."
The researchers identified a connection after examining both sets of measurements. It turned out that not only the maximum values but the entire data series for algae growth and nitrogen fixation, including minimum values, have been coupled since 2011. This timing is important because, in 2010, strong winds displaced brown algae for the first time from the Sargasso Sea to the tropical Atlantic.
The research team concluded that the excess of phosphorus is the key factor in Sargassum blooms by eliminating other possibilities. One previous theory suggested that iron-rich Saharan dust, which frequently blows from Africa to the Atlantic, promotes the growth of the algae. However, the dust input did not correlate with biomass. Similarly, nutrient inputs from the Amazon or Orinoco rivers did not correlate with observations of Sargassum blooms.
In Soubise, Grenada, two boys fight through thick Sargassum carpet to beach their boat. Such scenes are increasingly common since 2011, when summer algae blooms exploded across the Atlantic. The cause is that winds drive phosphorus-rich deep water to the surface. This promotes nitrogen-fixing cyanobacteria living symbiotically with the Sargassum, providing the crucial nutrient in the nitrogen-poor ocean.
© Jonathan Jung /MPIC
The new mechanism can be used to improve predictions of future Sargassum blooms
In their publication, the researchers therefore describe a mechanism in which phosphorus from upwelling deep water and nitrogen from nitrogen fixation drive algal blooms observed during the past decades. Geochemist Jung adds: "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."
The supply of phosphorus occurs by cooler sea surface temperatures in the tropical North Atlantic and warmer temperatures in the southern Atlantic. These temperature variations cause changes in air pressure, leading to wind anomalies that displace surface water and allow phosphorus-rich water from the deep sea to flow in.
According to Mainz researchers, observing winds, sea temperatures, and the resulting upwelling changes in the equatorial Atlantic can improve predictions of Sargassum growth. Alfredo Martínez-García, group leader at the Max Planck Institute for Chemistry and senior author of the study, explains "Ultimately, the future of Sargassum in the tropical Atlantic will depend upon how global warming affects the processes that drive the supply of excess phosphorous to the equatorial Atlantic". His team plans to provide a more detailed view of these processes by measuring new coral records from different locations across the Caribbean. The researchers expect that these new findings can guide efforts to mitigate the impacts of the blooms on Caribbean reef ecosystems and coastal communities.
