The big problem with microplastic pollution

Abstract

Plastic pollution is one of the most pervasive and sinister ecological threats worldwide. Most of us are aware of animals ingesting and dying from plastic waste. However, much less is known of the more insidious problem caused by much smaller microplastics, which are formed after larger pieces of plastic are degraded, by sunlight or wave action, into very tiny particles. Owing to their size, microplastics are extremely difficult to remove from the environment and are easily consumed by animals, yet their biological effects are unclear. For these reasons, Giacomo Limonta and scientists from the Universities of Siena and Ferrara, Italy, wanted to know the various ways in which microplastics impact animals, from their miniscule DNA molecules to their ability to move.The team tested the effect of two plastics – polyethylene (the principal source of microplastic globally) and toxic polystyrene – on a standard laboratory animal, the zebrafish. They exposed the fish to microplastics that were smaller than one-tenth of a millimetre, at concentrations similar to those found in highly polluted waters, for 20 days, before looking at how the plastics affected the fish's gills and intestines, their movement and the gene-expression patterns of their liver. The researchers focused on the liver because it detoxifies blood and contributes to immune function, while the intestine absorbs nutrients into the body and the gills are necessary for breathing. To identify how microplastics changed the structure of the gills and intestine, the researchers stained the tissues and scrutinised them under a microscope. Lastly, they used infrared sensors to observe how the pollution affected the fish's swimming behaviour.Overall, the microplastics severely compromised the zebrafish's immunity. For instance, microplastics elevated the activity of genes that help certain immune cells (lymphocytes) recognise liver tissue and trigger immune responses. This worrying finding suggests that the fish's own immune system could mount overzealous attacks on the liver, impairing its function. In addition, the team realised that germs might be able to infiltrate liver cells more easily, because genes linked to antimicrobial resistance and cell-to-cell attachment were less active. Meanwhile, genes for metabolic pathways that manage fat in the fish's bodies – including those for cholesterol and fatty-acid synthesis – were turned down. Given the importance of these molecules for immunity, the authors suggest that microplastics could seriously affect the fish's ability to fight off infections.The microscopic analysis also showed the layer of cells (mucosa) covering the intestine and gills was profoundly weakened by microplastics. Indeed, the intestine showed evidence of mucosal degradation (a sign of tissue damage) and it had fewer mucus-producing cells (goblet cells) essential for intestinal function. In the gills, the microplastics thickened the secondary lamellae (the flap structures where gases pass into and out of the body) and caused excessive mucus secretion, which together increases the distance the gasses travel to and from the bloodstream. Additionally, the microplastics elevated the number of neutrophils (immune cells) that respond to inflammation and kill bacteria in the gills and intestine, pointing to severe microbial infection. And when the team monitored the zebrafish's activity levels, the microplastics made them unusually active at night, indicating the damaging effect that these particles can have on an animal's internal clock.This study demonstrates that microplastics perversely impact all aspects of an animal's biology, from gene expression to first and secondary lines of defence against infection and their body clock. Considering the critical roles these play in everyday survival, microplastics have the potential to imperil whole populations of animals and wreak havoc on ecosystems. The alarming discoveries presented by the authors are yet another warning of the dangers of pollution and the need to reduce plastic use.