Fish are increasingly exposed to endocrine disruptors
Due to its chemical properties, microplastics can transport micropollutants into the digestive tract of fish, where they are then released by the action of the digestive fluid. Scientists at EPFL, in collaboration with other research institutes, have studied this process using the example of progesterone, which is often referred to as an endocrine disruptor.
When fish ingest microplastics, they often ingest progesterone as well. The chemical composition of the digestive fluids subsequently promotes its release in the fish's digestive system. This is the main finding of a study conducted jointly by scientists from EPFL, Eawag, Peking University and Oklahoma State University and published in the journal Environmental Science: Processes & Impacts.
"Our study shows that microplastics are an additional source of uptake of micropollutants such as progesterone, a steroid hormone found in the environment," says Florian Breider, head of EPFL's Central Environmental Laboratory and co-author of the study. "The microplastic in the water acts as a transport medium and acts like a sponge, as hydrophobic micropollutants preferentially attach to it," Breider explains. "Once in the fish, the contaminant molecules are released into the digestive tract due to the physical and chemical properties of the digestive fluids. Currently, no one knows if the micropollutants subsequently penetrate the intestinal wall and spread throughout the fish's body."
Three types of polymers
The study looks at microplastics from three different polymers (polyethylene, polypropylene and polystyrene) that are widely distributed in Swiss rivers and lakes. The scientists investigated the adsorption capacity of the polymers, i.e. how strongly they attract and bind micropollutants such as progesterone, and their desorption capacity, i.e. how quickly and how easily they can release micropollutants into the digestive tract of a fish. This allowed us to determine how progesterone molecules behave throughout the cycle.
Several studies have already been conducted on highly hydrophobic micropollutants such as polychlorinated biphenyls (PCBs). In the current study, the scientists found that progesterone, which is much less hydrophobic, behaves the same way in the presence of microplastics. However, progesterone is released into the digestive fluid at a higher rate. This suggests that ingestion of microplastics could contribute significantly to the exposure of aquatic organisms to progesterone, an endocrine disruptor. "If a one-kilogram organism consumes 4.5 liters of water per day, microplastics would account for 10 % of its daily progesterone intake from water as a source of progesterone exposure," Breider said.
Few studies in Switzerland
How the micropollutants transported by microplastics affect aquatic ecosystems and ultimately humans still needs to be studied in more detail. So far, only a few studies have been conducted on this, especially on freshwater organisms in Switzerland. This is likely due to the country's geographic location. "In countries bordering a sea or ocean, research on plastic pollution and associated micropollutants becomes an urgent task. That's not the case in Switzerland," Breider says. "But the awareness and the will to act are there, and we have the feeling that things are moving in the right direction," the researcher concludes.
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