Some are responsible for foul odors on the beaches or for the red, brown or even fluorescent green color of the waters. Others cause foam on the surface of the seas. Some are even toxic for shellfish, fish, but also for humans. For several years, they have been more and more talked about, because they cause closures of shellfish farms or coastal areas.
Microalgae – these micro-organisms living in the seas – are however not all harmful or harmful. They form the basis of the ocean’s food chain and have even produced a large part of the oxygen we breathe today. “5,000 species of microalgae have been identified worldwide. Among them, only 175 produce toxic substances for humans or marine animals”, explains Philipp Hess, expert in phycotoxins and head of the new research unit “Physiology and toxins toxic and harmful microalgae” of the French Research Institute for the Exploitation of the Sea (Ifremer), during of a press conference on Tuesday 7 June. Problem: Rising temperatures could lead to the proliferation of microalgae in the water. A question then arises: will global warming lead to an explosion of algae populations, or even make our seas and oceans toxic?
Migration from tropical seas to subtropical and temperate zones
The question is not so absurd, since Ifremer researchers carefully monitor the presence of toxic microalgae in our territorial waters and elsewhere in the world thanks to a monitoring network combining satellite observations and samples of water or shellfish in situ. It is thanks to their work that it was possible to identify the person responsible for respiratory disorders and flu-like states of 800 people who visited beaches on the Basque coast in August 2021: the microalgae Ostreopsis ovata. They were also the ones who analyzed the fluorescent green water near Nantes last year and who determined that this coloration visible to the naked eye was indeed caused by the proliferation of Lepidodinium chlorophorum, a microalgae that is harmless to human health – unlike green algae – but which can form a layer on beaches and give off an unpleasant smell. And for a few years, it is still these marine investigators who have observed the appearance in subtropical or temperate waters of toxic microalgae which usually prefer tropical seas.
These red-brown colored waters are due to an efflorescence of the microalga Lingulodinium polyedra on the coast of Morbihan up to the coast of Noirmoutier. Image taken on May 31 by the European Space Agency’s Sentinel-2 satellite. For the moment, the toxin concentrations measured in the shellfish production areas are well below the regulatory threshold.
Nantes ISOMer/Pierre Gernez University
This is the case of Ostreopsis ovata which is usually fond of tropical waters and which has been proliferating since the 2000s in the Mediterranean, a subtropical sea which is warming up and tending to become more tropical. Its presence in the Bay of Biscay last year suggests that it could follow the same dynamics in the future, although it is still too early to say with certainty that blooms [des augmentations rapides de la concentration de microalgues dans les eaux, NDLR] will perform there this summer.
Gambierdiscus, a tropical microalga present in the Pacific in the Caribbean basin and in the Indian Ocean progresses towards it also towards the subtropics. In recent years, it has been found near the Canary Islands, Madeira, the Azores, Tasmania and New Zealand. “We suspect that it extends towards even more temperate zones”, adds Philipp Hess. However, the blooms of this microalgae can poison the fish or seafood that ingests or filters it. And when these are in turn engulfed by humans, they cause digestive, neurological and cardiovascular disorders… Gambierdiscus is thus responsible for 50,000 to 100,000 cases of poisoning each year. His arrival in the Mediterranean would be very bad news.
From left to right: Dinophysis acuta, Ostreopsis cf. ovata and Gambierdiscus caribaeus, three toxic microalgae that have impacts on human health and activities.
Ifremer/Élizabeth Nézan and Nicolas Chomérat
Ifremer is also investigating the effects of climate change on Dinophysis, a microalga found in temperate waters that produces diarrheal toxins that can contaminate shellfish and the people who eat them. By testing several hypotheses according to the evolution of the temperature or the acidity of the water, the amounts of rain or the light intensity in the future, the researchers have reached a discouraging result. “Dinophysis will experience blooms until at least 2100 in European coastal waters, regardless of the IPCC climate scenario,” they write.
Warm springs promote blooms in summer
These black scenarios are not certain. “We do not have sufficient evidence to say that there will be more toxic microalgae on our coasts overall in the future. The situation will vary from year to year with regions more impacted and others spared. This remains to be demonstrated, but the he arrival on our coasts of tropical microalgae could be compensated by the disappearance of temperate species”, tempers Philipp Hess. Impossible, therefore, to know if the blooms will be more frequent, longer or shorter in the future. In recent years, the dynamics of microalgae on our coasts have nothing to predict. In 2021, the toxins produced by Dinophysis exceeded the regulatory threshold 20 times in the waters of mainland France, compared to 38 in 2020, 24 in 2019 and 36 in 2018, resulting in as many shellfish farming closures.
“What we do know, however, is that climate change will cause blooms that are increasingly difficult to predict, particularly in the face of increasingly frequent extreme events, such as Xynthia”, adds the researcher. This storm had, in 2010, brought large quantities of nutritive salts into the water, which had allowed the microalga Pseudo-nitzschia, which feeds on it, to develop. However, this microalga produces amnesic toxins which, after ingesting contaminated shellfish, can cause diarrhea, vomiting, and even neurological symptoms: headaches, confusion, disorientation, etc.
Another certainty of Ifremer: hot springs promote microalgae blooms in summer. The temperature records that have accumulated since April 2022 therefore do not bode well for the coming months. Where and when is another matter. “We can say that there will be blooms this summer, but obtaining a finer spatial and temporal scale is difficult, admits Philipp Hess. we are developing are still too expensive today”.