There would be nearly 100,000 species of microalgae, including 5,000 that we know very well.
And among these, 175 are considered toxic to humans or harmful to marine biodiversity.
Are these likely to proliferate more on our coasts in the future, under the effect of climate change?
This is the question posed on Tuesday by the French Research Institute for the Exploitation of the Sea (Ifremer), which is studying these microalgae more and more closely.
For several years, this institute has been observing the appearance of toxic microalgae in high concentrations, some of which come from tropical waters.
Like Ostreopsis.
Red-brown colored waters….
This is how we identify the presence in large quantities of Lingulodinium polyedra, a toxic microalgae which is increasingly present off the south-Breton coast.
“We know it is present almost every year in small quantities, invisible to the naked eye, begins Maud Lemoine, biologist at Ifremer and national coordinator of the Rephy-Refytox monitoring network, responsible for monitoring phytoplankton in our waters.
But we have also observed, in some years, greater concentrations, of the order of several hundred thousand cells per litre.
This was the case in 2003, then last August, and in recent weeks again.
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These pullulations are called blooms or efflorescences.
This is the first time that they have been observed two years in a row on this microalga off the coast of Brittany.
A microalgae under close surveillance
Worrying?
Lingulodinium polyedra releases yessotoxin, a toxin that contaminates the shellfish that feed on this microalgae, then which can have undesirable effects, for the moment unknown, on those who consume them.
"For the moment, the concentrations are well below the regulatory threshold beyond which the State can request the closure of a shellfish farming area", continues Maud Lemoine.
At Ifremer, Lingulodinium polyedra is still closely monitored.
If only because poisoning is not the only consequence.
"Like other microalgae, Lingulodinium polyedra rots quickly, consumed by marine bacteria which, in doing so, will use the oxygen in the water", continues Maud Lemoine.
The risk is then that of hypoxia, ie the reduction in the oxygen content of the water, which affects the marine organisms living nearby.
Starting with those who move too slowly to escape it.
Essential to life, but some with a side face
Lingulodinium polyedra is just one example of these microalgae that are growing more and more along our coasts.
On a global scale, there are nearly 100,000 species of microalgae, "including 5,000 that we know very well", specifies Philippe Hess, head of the Phytox* research unit, inaugurated in March at Ifremer to better understand the functioning of these microalgae.
On the one hand, they are essential to life at sea and on land.
"They produce half of the oxygen on the planet and are at the base of the ocean's food chain", resumes Philippe Hess.
On the other hand, they can be toxic to humans and/or harmful to marine biodiversity.
Of these 5,000 known species, 175 have this side face.
"They can cause fish or shellfish mortalities, respiratory or skin problems in swimmers or workers in regular contact with the sea, or even food poisoning through the consumption of shellfish", list Philippe Hess.
Even requiring the closure of bathing or shellfish production areas.
So that to the health and environmental impacts, we must add those that are potentially economic.
Ostreopsis, from tropical waters to the Basque Country
A growing threat from climate change?
"We do not have sufficient evidence to say that there will be more toxic microalgae on our coasts in the future overall," replies Philippe Hess.
The situation will vary from year to year, with regions more impacted and others spared.
On the other hand, in recent years, Ifremer scientists have observed the appearance of toxic microalgae, usually tropical, in temperate metropolitan waters.
A striking example is the ostreopsis ovata, well known to the Basques now.
The toxin it secretes, palytoxin, caused cases of breathing difficulties in swimmers and surfers last summer.
“Several beaches had to be closed and 800 people declared to the poison control center of the CHU of Bordeaux symptoms of intoxication”, recalls Philippe Hess.
There is nothing to say that the microalgae will still do its thing in the Bay of Biscay, continues the scientist.
But on the other side of the Pyrenees, in the Mediterranean, Ostreopsis ovata is now well established.
“It arrived at the beginning of the 2000s, thanks to the warming of the water, continues Philippe Hess.
This repeated presence can be seen as a witness to the rapid "tropicalization" of this semi-enclosed sea.
" Here again,
bathers pay the price.
“But we also observed mortalities of sea urchins and other animals living on the seabed,” continues the expert.
Other microalgae are in Ifremer's sights.
Some, again, are already in our waters.
Like Dinophysis, present from the English Channel to the Bay of Biscay, and whose toxin contaminates shellfish, causing diarrhea and other health risks in those who eat them.
Last summer, 20 toxic events involving it were recorded in mainland France, leading to the closure of shellfish farms.
It was 38 a year earlier, 24 in 2019, 36 in 2018… In other words, the years follow one another and are not always alike with these micro-algae.
On the other hand, "Dinophysis has the particularity of being able to feed itself through photosynthesis and by eating other microalgae, which makes it very adaptable to climate change", says Philippe Hess.
In other words, we are far from getting rid of it,
While waiting for the Gambierdiscus?
It is also to be feared that new toxic microalgae will reach our waters in the future, again due to global warming.
Philippe Hess cites the Gambierdiscus, which owes its name to the Gambier Islands, in the middle of the Pacific, where it appeared.
The geographic distribution of this microalgae continues to grow, thanks to the acidification of the oceans, another consequence of climate change.
“Cases have already been reported in the Canary Islands, Madeira, the Azores, list Philippe Hess.
And it is also found, in low concentrations, in the Mediterranean.
In its wake, Gambierdiscus brings ciguatera, a food poisoning caused by the consumption of fish and seafood contaminated with the toxin of the microalgae.
It causes digestive, neurological and cardiovascular disorders.
These proliferations of toxic microalgae are not easy, if not impossible, to counter.
The challenge, on the other hand, is to learn to adapt to it, slips Philippe Hess.
This implies, upstream, monitoring and studying them, to understand their functioning, their interactions with other living marine organisms, the chemical biodiversity of the toxins they produce and their consequences on humans and their activities.
This is the whole purpose of this new Phytox research unit.
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*
Phytox for “Physiology and toxins of toxic and
harmful microalgae”.
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