Sheet 1: The origin of Sargassum Algae


What is Sargassum seaweed ?

Sargassum is a pelagic brown seaweed, meaning that they form in the open sea. The clusters of algae drift for several kilometers before washing up on the coastlines, decomposing, invading the beaches inch by inch, and releasing gas which can be dangerous for health (hydrogen sulphide or H2S, a smell of rotten eggs characterizes this gas). There’s been numerous studies, mainly since 2011, when the first invasions in large quantities started in the Caribbean.

However, Sargassum was not discovered in 2011: in 1493, a certain Christopher Columbus opted for a course further south on his way to the Antilles and Latin America, to avoid the Sargassum Sea. Scientists have established that since 2011, Sargassum no longer comes only from the small Sargassum Sea, located near Bermuda and Florida, but also and mostly from the Amazon delta in Brazil and the Congo delta. New independent Sargassum Seas are developing.

A recurring phenomenon, now visible from space

Each year, the Sargassum events last longer and the amount of algae increases considerably. In 2018, for example, a 8850 km length belt (from West Africa to Florida) of nearly 20 million tonnes was observed, forming a plant continent linking the African continent to the American continent.


A recognizable appearance

The two main species observed (out of the 350 known in the world) during invasion events are Sargassum natans and Sargassum fluitans. Sargassum natans has a plant body shaped like a pod, usually with small leaves or spikes, long and narrow leaves with toothed edges, and gaseous bladders (floaters) topped with a spike. Fluitan Sargassum has wider toothed-edged leaves, lateral branches with thorns, and gaseous vesicles without spikes, but edged with a fin (like a fish fin). These two species are the best known, since they are ubiquitous on the beaches during major sargassum events. The other species live anchored to the seabed.

La composition chimique des sargasses

The chemical composition of Sargassum

Sargassum is photosynthetic, meaning that they capture nutrients dissolved in seawater and use photosynthesis to assimilate atmospheric carbon. Sargassum is chemically composed of 3 to 16% of proteins, up to 68% of polysaccharides (agglomerated sugar molecules), and 0.3 to 6% of lipids. But above all, the seaweed has an ability to capture and retain heavy metals such as cyanide or lead, thus presenting risks to health and the environment. Sargassum also has a high ash content and can potentially provide minerals and micro-nutrient beneficial to marine flora.

How does the reproduction process explain the growing quantities of Sargassum?

The Sargassum reproduction process is peculiar, and common to the different listed species, although it’s not possible to identify a “single” responsible for their proliferation. Sargassum natans and fluitans reproduce by vegetative fragmentation, meaning that a cut piece of seaweed grows back, explaining their capacity for proliferation. One of the studied theories is that chemical agents such as nitrates and phosphates present in the water, and particularly used in Brazil and Congo for intensive agriculture or mining activity, have a significant part to play in the growth of Sargassum. The nitrates and phosphates trickle through the soil and could be carried by the flow of the rivers Amazon and Orinoco in Latin America, and the Congo river in Africa (the three largest rivers on the planet) thus constituting significant nutrient inputs, captured by algae and promoting their development.


rapport ENSIP et ASTEE
rapport d’expertise collective de l’ANSES