In an article in The Conversation, CNRS Research Director (Roscoff Biological Station, Sorbonne University), provides an update on the proliferation of green algae on the Breton coast, which has an abundance of nitrogen in common with sargassum, which proliferates in the Caribbean. Research carried out at the MIO has clarified the role of diazotrophs, nitrogen-fixing bacteria, in the proliferation of sargassum.
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What do strandings of green algae in Brittany and sargassum in the Caribbean have in common? An abundance of nitrogen, which the algae can take advantage of through different mechanisms. While it is difficult to prevent the proliferation of sargassum, we can act at source to starve the green algae.
A recurring and all-too-predictable occurrence every summer, strandings of brown seaweed (sargassum) have once again been reported. in the Caribbean islands in 2024. 6,000 km away, the Breton bays were once again covered in their thick coat of green algae.
In these areas, the nauseating smell of hydrogen sulphide emanating from the decomposition of these algae has become unbearable, and has even led to closing off access to beaches that were once paradise. The accumulation of decomposing algae will also deplete the environment of oxygen, leading to a decline in biodiversity and even episodes of mass mortality in ecosystems.
However, recent research has revealed that the origin of these proliferations is paradoxically different between temperate waters, which are saturated with nitrates, and tropical waters, which are very poor in nutrients and rich in organisms that fix atmospheric nitrogen.
Well-established mechanisms
The mechanisms that trigger Breton green tides have been well established for over twenty years, thanks to the work of all the teams involved. by Ifremer in the 1990s.
These algae benefit from the presence of excess nitrates in coastal waters. Opportunistic, they are able to grow much faster than other species of algae as soon as light and current conditions are right.
This is because watercourses, even those with modest flows, carry nitrogen from leakage caused by over-fertilisation of the land with mineral fertilisers and animal waste. Anaerobic bacteria (which can develop in the absence of oxygen) then carry on the process of fertilisation. degradation of organic matterwhich generates gaseous fumes with a rotten egg smell, in particular hydrogen sulphide, an lethal gas in high doses for humans and land animals.
In the tropical Atlantic, paradoxically, bacteria play a vital role in the proliferation of sargassum brown algae. As you move further from the coast, the ocean becomes depleted of dissolved nitrogen. To thrive, brown algae need to take advantage of all the resources available, whether they come from animals (fish, crustaceans, etc.) or from the sea, hydraires) but also micro-organisms capable of fixing nitrogen from the air, which are found in these drifting ecosystems.
From very recent results indicate that these microbial symbioses are essential for maintaining the growth of Sargassum offshore, and that they contribute much more than large rivers to supplying Sargassum blooms with nitrogen.
Without nitrogen, algae growth remains limited
Understanding the conditions under which algae proliferate is crucial, as it enables us to develop strategies to limit the environmental and health impacts and to better manage accumulations of algae on beaches.
- In natural systems, nitrogen is the mineral element that limits the growth of algae.
- Phosphorus is the second most common limiting nutrient, but it is now stored in the atmosphere. massively in coastal sediments.
There are many mechanisms by which nitrogen influences the growth of algae. Nitrogen can exist in several forms: two inorganic forms, nitrate and ammonium, and one organic form, urea. Algae can therefore grow by mobilising several different sources of nitrogen.
For example, thanks to external inputs of nitrates via river water in catchment areas, water transfers via thermal stratification or by upwelling of nutrient-rich deep cold water (known as "upwellings). Algae can also "recycle" nitrogen from ammonium and urea produced by invertebrates and fish in the ecosystem.
Finally, certain bacteria associated with algae, the diazotrophsare capable of fixing nitrogen from the air and converting it into ammonia, which is then transformed into amino acids that can be used by algae.
Proliferation remains a mystery
Sargassum has traditionally been found in the Sargasso Sea, the area of the Atlantic Ocean where the seaweed is concentrated. But since 2011, it has also been found between West Africa, the Caribbean and Brazil. This area is known as the Great Atlantic Sargassum Belt, some 8,000 km long.
In 2023, an American team reported that the mineral nutrition capacity of sargassum residing in the GASB must have differed from those in the Sargasso Sea, since they contained higher levels of nitrogen and arsenic, the latter being inversely correlated with the abundance of phosphorus.
The sources of the nutrients feeding the GASB are not yet very clear. The nitrogen (and in particular its isotopic composition) and phosphorus content of this sargassum can be used to determine whether these nutrients are of atmospheric, oceanic or fluvial origin.
In fact, the results of the Origins and FORESEA research projectsThese studies, carried out between 2019 and 2023 with the support of ADEME and ANR, have ruled out the hypothesis that the planet's three largest rivers - the Amazon, the Congo and the Orinoco - are responsible for the proliferation of sargassum.
Satellite detections have shown that only 10 % of the total annual biomass of sargassum is found in regions under the influence of the Amazon river plumes, whereas the river represents 20 % of the volume of fresh water discharged into all the world's oceans.
The causes of these proliferations have yet to be determined. But the hypothesis of a bacterial source is increasingly credible.
Bacteria, the keystone of the Sargasso ecosystem
Bacteria and other microbes associated with living organisms form a specific biofilm on the surface of these organisms. This is also the case for sargassum. Scientists at the Marseille Institute of Oceanography (MIO) have been looking at how sargassum is formed. genetic diversity of this biofilm and the microbes in the surrounding waters.
Certain bacteria, in particular, are excellent biological tracers for detecting the passage of Sargassum rafts in the Atlantic. This is the case for diazotrophs, bacteria capable of fixing nitrogen from the air, which were predominant in both the GASB Sargassum biofilm and that of the Sargassum Sea.
Analysis of the diversity and nitrogen assimilation genes of bacterial communities also revealed, for the first time, the predominance of bacteria belonging to the phylum Proteobacteria. While diazotrophs of the Cyanobacteria are much more abundant in ocean plankton.
Finally, analysis of the nitrogen isotope ratio supports the hypothesis that the nitrogen consumed by the GASB sargassum is of atmospheric origin. Diazotrophs, nitrogen-fixing bacteria, appear to be involved in the proliferation of these brown algae in tropical areas.
Green and brown tides threaten coastal areas in particular
Sargassum, like green algae, poses no danger as long as it is at sea. Sargassum rafts are even considered by fishermen to be excellent "nurseries", as they attract larger fish to feed under the floating rafts.
It is when seaweed arrives on our coasts, trapped by mangroves or stranded on the sand, that it threatens biodiversity. The degradation of algae produces hydrogen sulphide, which is harmful not only to humans, but also to many ecosystems.
The nocturnal respiration of these algae and their decomposition by microbes also consumes a lot of oxygen, which often leads to more or less extensive areas of anoxia, i.e. a deprivation of oxygen for all the organisms whose respiration is essential.
Episodes of increased fish mortality have been described in the Caribbean, as have the effects of strandings on sea turtle reproduction and coral health.
The consequences for biodiversity also extend to beach ecosystems, as shown by the work by Nolwenn Quillien and Jacques Grall on Breton beaches affected by green tides.
Learning to live with sargassum...
In reality, even if inputs from catchment areas and other anthropogenic sources can be controlled, it seems impossible to limit Sargassum blooms by reducing nitrogen inputs. Each winter, this phenomenon is replenished by stocks of Sargassum dispersed by autumn cyclones, which drift from the Sargasso Sea to the tropical Atlantic each year. It is this mechanism that determines where the sargassum washes up, depending on weather conditions and currents.
Thierry Thibaut, a MIO researcher and coordinator of the ANR-ORIGINS project, sums up:
"The GASB is a well-established natural ecosystem with no shoreline. We won't be able to prevent them from proliferating or running aground. From now on, we have to learn to live with it".
Strandings are inevitable, so they need to be anticipated, either by monitoring from space, from the sea or from the air. The biggest challenge, of course, is logistical: how can we intervene quickly on Sargasso rafts detected in coastal areas, or limit the ecological impact of strandings before the Sargasso putrefies?
Several research projects in different parts of the Caribbean and West Africa are being coordinated to bringing solutions to fruition which should also enable this biomass to be used for local economies.
... But the possibility of taking action against green tides
However, in Brittany and in many green tide sites, action can be taken to limit nitrogen inputs from catchment areas. Studies led by Luc Aquilina from the University of Rennes have shown that in just a few years, we can see the virtuous effects of reducing nitrogen leakage into aquifers to limit the development of green algae.
This modelling work also shows us that to continue to improve the quality of the water in our rivers and combat eutrophication in coastal areas, it is essential to continue to reduce nitrate leaks into the groundwater.
In the short term, all solutions capable of reducing these blooms must be explored, including early harvests or the cultivation of other algae in competition with green algae, for example in the shellfish beds affected by eutrophication.
