How microplastics spread through the food chain: Finnish researchers trace particle transfer from soil

20 Sep 2022 --- Researchers have found that sub-micron plastics (SMPs) are absorbed by plants, raising concern over the quantity of microplastics that are consumed through the food system. The study showed that the polymer type influences the biodistribution of the particles in lettuce (roots and leaves) and the number of particles transferred from the plants to insects feeding on the treated lettuce. 

The researchers concluded that plastic particles could potentially transfer from soil into food webs, and the chemical composition of plastics influences their biodistribution and trophic transfer in organisms. 

“Our finding suggests that different types of plastic SMPs have different fates and behavior in organisms and food webs. It is likely that other physicochemical properties of plastic
particles such as shape, size and density can influence the biological fate of these particles in food webs,” highlight the researchers. 

Study findings
The research team, led by biologist Fazel Monikh from the University of Eastern Finland, demonstrated this process in a laboratory by feeding tiny 250 nm particles of polystyrene and polyvinyl chloride to lettuce.

After 14 days, the researchers fed that lettuce to black soldier fly larvae, then gave those larvae to hungry roach fish after another five days. Once the fish fed on the insects for five days, the team dissected and imaged the tissues from each food chain (trophic) level.

The researchers found that plastic particles could potentially transfer from soil into food webs. Because these particles are hard to detect and can be altered during their physiological journeys, researchers encased the rare element gadolinium within the tiny plastics to more easily track them. The team used a scanning electron microscope (SEM) to ensure the plastic covered the metal completely to reduce its biological influence.

The finding that lettuce can take up SMPs from the soil and transfer them into the food chain indicates that the presence of sub-micron-sized plastic particles in soil could be associated with a potential health risk to herbivores and humans if these findings are found to be generalizable to other plants and crops and field settings.

Reason for concern?
WHO recently reviewed scientific information on microplastic particles (MP) in drinking water, drinking water sources and wastewater to evaluate the potential risks for human health. 

It concluded that the current knowledge base is insufficient to differentiate adverse effects associated with exposure to nano- and microplastic particles (NMP) from those of particles occurring naturally in the diet or inhaled. 

WHO stresses that although it is known that exposure to high concentrations of particulate matter (PM) is associated with respiratory effects, limited quantification of NMP in the air obviates a robust risk assessment. 

Thus, research to identify adverse effects intrinsic to NMP would provide guidance for an NMP-specific human health risk assessment. The available data do not allow firm conclusions on the risks to human health of inhalation or ingestion of NMP, but, as NMP are part of the PM mixture, the health impacts will not exceed those of PM.

Microplastics, including the smaller nanoplastics, are now ubiquitous in every environment, reaching from the oceans to remote Arctic islands. They are in our blood and the food we eat. 

Meanwhile, the European Commission recently released a draft proposal on restricting intentionally-added microplastics from various packaging, cosmetics, sports pitches and other general materials. While environmental officials are cautiously celebrating and calling for more stringent guidelines, some polymer experts say the move has no scientific basis. 

By Natalie Schwertheim