Marine plastics harbor antibiotic-resistant bacteria. The contamination of turtles reveals this invisible health risk.
At Kélonia, the sea turtle care center on Réunion Island, some residents fall ill even while receiving treatment. Injured or weakened, they are rescued after swallowing fishing hooks, washing ashore, or ingesting plastic waste mistaken for jellyfish. But for several years now, a phenomenon has been worrying those in charge of the center: despite treatment, persistent infections appear in several turtles. Skin lesions, eye problems, diarrhea… And these infections are resistant to several antibiotics.
« Veterinarians did not understand where these multi-resistant bacteria were coming from that were making the turtles sick and complicating care, » says Philippe Jourand, an environmental microbiologist at UMR ENTROPIE.
One detail, however, eventually caught his attention: in the pumping basin that supplies the facilities with seawater, numerous plastic fragments were floating on the surface. Red, yellow, green, or white, they were visible to the naked eye. What if this waste had become a vector for pathogenic microbes?
The analyses would confirm this hunch.
Plastic fragments floating on the surface of the Indian Ocean. Colonized by microorganisms, this waste can transport pathogenic bacteria over long distances.© Adrian Fajeau GLOBICE
Microscopic life on waste
In the southwestern Indian Ocean, plastic pollution has now reached levels comparable to those observed in the Atlantic or Pacific Oceans. Research conducted by Margot Thibault, through 19 oceanographic campaigns, has revealed significant concentrations of debris in the southwestern Indian Ocean.
These floating fragments are primarily composed of polyethylene and polypropylene. They can drift for years on the ocean surface. But far from being inert, they quickly become a breeding ground for a whole community of microorganisms: bacteria, viruses, fungi, algae, and protists attach themselves to them, forming what researchers now call the « plastisphere. »
Scientists even refer to them as « hitchhikers of plastic pollution. » These floating debris do not only carry microbes; they also concentrate organic contaminants, pesticides, antibiotics, hormones, and heavy metals. Exposed to strong solar radiation, these biofilms become environments conducive to genetic exchange between microorganisms.
“ These plastics contain a chemical and microbial cocktail subjected to very strong UV radiation, which promotes the emergence and transmission of genes conferring resistance to antibiotics and metals ,” emphasizes Philippe Jourand.
Research conducted around Réunion Island shows that these biofilms are dominated by proteobacteria, a vast group encompassing numerous pathogens. Among them are bacteria of the genus Vibrio, some of which are responsible for cholera or diseases affecting coral reefs.
In Madagascar, a study conducted in a lagoon by researchers from the Institute of Fisheries and Marine Sciences (IH.SM) showed that nearly 90% of the macroplastics analyzed harbored bacteria potentially pathogenic to humans. This waste acts as a veritable microbial refuge, concentrating these pathogens at levels far exceeding those observed in the surrounding seawater.
These plastics constitute a potential reservoir of pathogenic and antibiotic-resistant species. If they contaminate the marine food chain or directly humans, it becomes a real health issue,” explains Rakotovao Raherimino, marine microbiologist at the Institute of Fisheries and Marine Sciences (IH.SM), in Madagascar.
The invisible underside of healthcare centers
For scientists, however, one crucial step remained: demonstrating that these microbes don’t just travel on plastics, but that they can actually contaminate marine life.
This is precisely what the study conducted at Kélonia has accomplished.
Scientists collected plastic fragments from the pumping basin at the care center and then identified the bacteria associated with them. They found several strains resistant to multiple antibiotics. Using genomic analyses, they then compared these to bacteria isolated from sick turtles.
The result is striking: the strains found on the plastics are identical—or very similar—to those infecting the treated animals.
This contamination is part of a trend observed for a long time at the Kélonia care center. After certain operations, the turtles excrete large quantities of plastic waste, which is systematically recorded by the teams. And the proportion of turtles excreting plastic increases year after year,” says Francis Schneider, a veterinarian who has worked with this facility since 2007.
Some analyses have even traced the origin of some of this waste back to Southeast Asia or China. For veterinarians, this widespread pollution now acts as a vast biological transport system across the oceans.
Seabirds, like this Barau’s petrel observed in the Indian Ocean, frequently ingest plastic waste and now serve as bioindicators of marine pollution.© Valentin Lauféron
Sentinels of marine contamination
Sea turtles thus appear as emblematic victims of a pollution far more complex than it seems. For behind the visible waste lies a largely invisible microbiological contamination.
At Kélonia, the study’s results have already led to recommendations for strengthening the treatment of the water supplying the pools, notably through UV sterilization systems capable of limiting the spread of microbial biofilms.
But for Philippe Jourand and Margot Thibault, the problem extends far beyond the care centers on Réunion Island. Seabirds, turtles, and other species are now becoming veritable bio-indicators of plastic pollution in the oceans. Many ingest these fragments, mistaking them for food, and in doing so, accumulate the microbes they carry.
The research conducted in the Indian Ocean is based on a broad scientific and community-based mobilization, bringing together researchers, students, veterinarians, care centers, and partners from the Global South. In Madagascar, Mauritius, and the Seychelles, several citizen science programs are already monitoring the evolution of this pollution on beaches and in marine life.
Because this floating waste no longer drifts alone. Invisible to the naked eye, billions of microorganisms now travel the oceans with it.
source : Le mag
