German researchers found that mouse cells absorb microplastic particles about 10 times more often when they have been exposed to aquatic environments and developed a crust of biological matter, suggesting that they may enter animal tissue more frequently than previously known.

Microplastic particles, very small pieces of plastic that have become ubiquitous throughout nature, are commonly found in this weathered state but are often studied in pristine, unexposed conditions.

Their paper, published in Science Advances, came out of the University of Bayreuth’s new research center for microplastics that is funded by the German Research Foundation. Christian Bayreuth, the center’s speaker and a senior author of the new study, said he knew plastic developed biofilms from environmental exposure and wanted to explore how that affects their interactions with cells.

“Usually if you do ecotoxicological studies, most of the groups are using pristine particles which are grinded,” said Bayreuth, a professor of animal ecology at the University of Bayreuth who has studied microplastics for 11 years. “Therefore, we develop the idea that we test the response of the cells on pristines compared to the environmental-exposed particles.”

The scientists placed spherical microplastic particles 3 micrometers wide for weeks in an artificial freshwater pond, a saltwater aquarium or ultrapure water. The first two environments were teeming with plants, animals and microbes, and the plastic bits in them became coated with an “eco-corona” of biomolecules. No such crust developed in the third, sterile water environment.

All the microplastic spheres were added to mouse macrophage cells designed to glow if they internalize plastic. The cells, which cannot process plastic, nevertheless consumed the freshwater- and saltwater-exposed particles 10 times more often than the sterile particles.

“We were very surprised about the strength of the effect,” said Holger Kress, the paper’s other senior author and a biological physics professor at the University of Bayreuth. “We thought there's a difference, but initially we didn't know — maybe it is even reduced for the biomolecule coating, or it's increased — but we didn't expect a factor-of-10 effect.”

The researchers described the layer of carbohydrates, proteins, lipids and other organic matter as a “Trojan horse” that tricked the cells into absorbing the microplastic particles, which do not interact with the cells’ receptors on their own. 

Released into nature by humans in great quantities, microplastics are created when larger plastic pollution breaks down or originated in an array of consumer items. Humans ingest tens of thousands of microplastic particles each year from our air, water and food, including particles that rise through the food chain and into fish and other meat. Their health effects on people are not well understood, although they have been found to cause inflammation or premature cell death in some other animals.

Many studies on the toxic effects of microplastics use freshly ground plastic that never develops biofilms. Future studies should be more realistic by exposing their particles to marine environments, according to Kress.

“It would be helpful if more labs would use such particles which are incubated in fresh or saltwater or other bio environments,” he said, “because if only pristine plain microplastic patterns are used, then that might not be very representative of what's out there in nature.”

Laforsch’s team is currently investigating different shapes of microplastic particles beyond spheres because they may not interact in the same way. 

“It might be that different shapes and different polymer types may have different effects,” Laforsch said. “You always have to keep in mind that not every microplastic particle is similar to the other one.”

The article, “Environmental exposure enhances the internalization of microplastic particles into cells” was published Dec. 9 in Science Advances. The authors of the study were Anja Ramsperger, Vinay Narayana, Wolfgang Gross, John Mohanraj, Mukundan Thelakkat, Andreas Greiner, Holger Schmalz, Holger Kress and Christian Laforsch, University of Bayreuth. The lead author was Anja Ramsperger.