“They are super filters of the marine environment, with up to 25 liters of water filtered per day”, explains marine biologist Leïla Meistertzheim. “And what is more, they are a true bioaccumulation model of pollutants in general”, she adds.
Pesticides, bacteria, remnants of medicines … Actually, mussels pump and filter the water to feed on phytoplankton to store everything that comes along with it, so it is necessary to apply strict sanitary measures for those who are intended for human consumption.
Being the first affected by pollution, these organisms considered the “sentinels of the environment have long been used as” bioindicators “of the state of health of the seas, lakes or rivers in which they live. To the list of “monitored” substances, these mollusks could add contaminants that are still little known, such as microplastics and their additives (bisphenol A or phthalates).
Leïla Meistertzheim and her team installed, within the framework of the Tara Ocean Foundation campaign, mussel-filled pits on the shores of the estuaries of the Thames, Elba, and Sena rivers. After 1 month submerged, they were collected, dissected, and frozen or lyophilized, mainly to count the trapped particles and observe if there were chemical additives in the tissues.
The idea of covering the sea of mussel parks to absorb the ubiquitous microplastics, for now, is nothing more than an illusion. But for other pollutants, the process is much more advanced. “In some places, mussels and oysters are used as cleaners of the marine environment, for example for pesticides”, says Meistertzheim.
In that case, mussels collected in polluted waters “should not be eaten”, notes Richard Luthy of the Stanford University. Although not always a problem, says this environmental engineer, who also demonstrated the “suppression and deactivation” capabilities of bacteria such as E Coli that mussels have. “The mussels that eliminate fecal bacteria, also evacuate such bacteria in the form of feces or mucus. So it is a good thing”, he adds.
The consumption of mussels installed in eutrophication waters is not a problem, researchers clear up. This excess of certain nutrients, such as nitrogen and phosphorus from industry and agriculture, leads to a proliferation of the algae that feed on them, and an oxygen depletion that drowns the ecosystem.
Mussels allow a kind of “recycling” of these nutrients, by feeding on expanding algae “to gain weight and develop”, explains researcher Eve Galimany, who participated with the Milford laboratory of the United States NOAA agency in experiments on the Bronx River.
This principle is already applied among others by the Baltic Blue Growth project, which supports the pilot cultivation of mussels for animal feed in Sweden, Denmark, and the Baltic countries. “That could be part of the solution against eutrophication”, which is “one of the main problems of the Baltic Sea”, says Lena Tasse, the head of the project.
But if this process does not imply any risk for humans, why are these mussels dedicated to the feeding of birds or fish? Because, being smaller due to the low salinity of the Baltic Sea, they do not have much success among seafood lovers, Tasse replies.
Regarding the impact of microplastics and their additives on human health, little is known at the moment. According to a recent report from the World Wildlife Fund (WWF), a human being ingests up to 5 grams of microplastics every week, found in every corner of the ocean, including the shell of mussels.
A 2018 study published in “Environmental Pollution”, based on samples of mussels collected in the United Kingdom (UK), on the coast or in supermarkets, estimated that 70 pieces of plastic would be ingested per 100 grams of meat consumed. However, “I eat them”, says Leïla Meistertzheim. “A plate of mussels is not necessarily worse than the organic hamburger packaged in plastic”, she concludes.