Not all microplastics are created equal – apparently identical polystyrene particles can damage cells in different ways –

Difficult to compare: microplastics from the same polymer can develop different biological effects depending on the manufacturer, as a study shows. Despite supposedly having the same chemical composition, some of these globules are more damaging to cells than others. These differences could also explain why toxicity studies have so far produced such conflicting results, the researchers report.

Microplastics have long been a global environmental problem. Such plastic particles, some of which are only a few micrometers in size, can be found almost everywhere. They exist in the oceans and in marine life, but even human infants are affected by microplastics. The particles can even get into the brain through food and damage cells there.

Partly contradicting research results

However, what harmful effects microplastics have in tissues and cells and what this depends on has so far only been partially clarified. In tests for this purpose, spherical polystyrene particles, which are specially manufactured for research purposes, are often used. Polystyrene is a widely used plastic, from which, for example, styrofoam is made.

Although the particles used in the microplastic investigations were mostly produced in the same way and also had an identical chemical composition, contradicting results were more often obtained in previous studies. A team led by Anja Ramsperger from the University of Bayreuth has now uncovered one of the possible sources of error in such research.

Polystyrene particles put to the test

The scientists examined polystyrene particles from two different manufacturers. One test candidate were beads from the US company Polysciences, the others came from the manufacturer Micromod from Rostock. Both companies stated that they manufacture the microplastics using so-called emulsion polymerization, and the size of the particles was almost identical. One difference: after production, Polysciences places its beads in 0.05 percent sodium azide to prevent bacterial growth. According to its own information, Micromod only uses water for this.

Ramsperger’s team examined the polystyrene microplastic on the one hand for its molecular composition and on the other hand for the electrical charge distribution on the surface of the particles, the so-called zeta potential. In the course of the study, they also exposed mouse model cells to the microplastic. The scientists measured the zeta potential of the beads at three different times: immediately after delivery, after they had washed the particles with ultrapure water, and after the interaction with the living cells.

Big differences in the distribution of charges

The result: Although the polystyrene particles were identical according to the manufacturers, they showed clear differences in their electrical charge and thus in their influence on living organisms. While the beads from Polysciences had an electrical potential of about minus 80 microvolt on delivery and after washing, the Micromod particles were almost uncharged.

This difference ultimately manifested itself in the model cells: the barely charged microplastic only attached to them externally. The Polysciences particles, on the other hand, were partially completely surrounded by cell structures and thus demonstrably disrupted the metabolism and cell growth.

“Our study impressively shows how problematic it is to try to make general statements about the health or ecological effects of microplastics,” says senior author Christian Laforsch from the University of Bayreuth. “The replicability of experiments must have the highest priority in microplastics research – especially when it comes to investigating health effects,” adds Ramsperger.

Trigger: groups of molecules on the surface

According to the researchers, the different properties of the polystyrene particles stem from the structure and arrangement of the long-chain plastic molecules, among other things. For example, they were able to detect negatively charged sulfate groups on the spherical surface of the particles from Polysciences, which means that the charge distribution there was significantly more homogeneous in contrast to the Micromod spheres.

The anionic sulfate groups in the microplastic produced by Polysciences ultimately ensure that the spheres develop intermolecular forces of attraction, so-called van der Waals forces. According to the researchers, this also explains why the globules can penetrate deeper into living tissue.

Better characterization needed

The scientists’ results demonstrate that even microplastics of the same polymer type are difficult to compare with one another. According to the researchers, the small differences between the apparently identical particles are essential for future investigations and must be carefully characterized before each study.

“There are numerous toxicological studies that aim to track down the effects of microplastics on living organisms. But only when we know the chemical composition and the surface properties of the particles used in detail can these studies be scientifically compared, ”Ramsperger sums up. “In the future, we will be taking a closer look at the microparticles used in our experiments in Bayreuth.” (Journal of Hazardous Materials, 2021; doi: 10.1016/j.jhazmat.2021.127961)

Source: University of Bayreuth

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