Microplastics Stunt the Growth of Worms and Make Them Lose Weight, Scientists Discover

Microplastics in the soil can stunt the growth of earthworms and make them lose weight, while also preventing certain plants from growing properly, according to research.

In recent years, increasing attention has been drawn to the impact of microplastics—tiny fragments of plastic—on the environment and the potential risks they pose to biodiversity and the functioning of ecosystems.

Scientists have recognized that soil ecosystems—particularly agricultural land—can absorb large quantities of microplastics. However, the effects of microplastics on these ecosystems—both above and below the ground—have remained relatively unknown.

"There is quite a lot of evidence of how microplastics are affecting animals and plants in aquatic ecosystems—like oceans, lakes and rivers—but only recently have scientists been focusing on what actually may happen in terrestrial ecosystems such as soils," Bas Boots, lead author of the research from Anglia Ruskin University in the U.K., told Newsweek.

"Some people consider soils as 'the poor man's rainforest,' because some soils are so rich in biodiversity, including animals, bacteria and fungi," he said. "Soils are the foundation of the food chain, as it supports plant growth, including the food we eat."

In an attempt to address some of the gaps in our understanding, Boots and his colleagues decided to investigate the impacts of microplastics in the soil on rosy-tipped earthworms (Aporrectodea rosea) and a type of plant known as perennial ryegrass (Lolium perenne).

For their experiments, the team added three different types of microplastics to soil that contained the worms and the ryegrass growing on top. Previous studies have demonstrated that these kinds of earthworms can move microplastics around in the soil.

"We decided to focus on Apporectodea rosea, which lives in the topsoil where it gains nutrition from eating soil organic matter," Boots said. "Its burrowing activity also moves soil around and produces soil aggregates, or small clumps, which in turn usually increases porosity, so plant roots can penetrate easier, water drains quicker and erosion can be avoided."

"As this earthworm eats soil it can also inadvertently ingest microplastics," he said. "It is a common earthworm in soils in temperate climates, so using this species as a model organism, we wanted to see if the addition of microplastics would have an effect on the animals' health and the soil environment they live in. If soil animals are negatively affected by this, we need to make sure that we do our best to avoid pollution of soils with microplastics."

The plastics studied were biodegradable polylactic acid (PLA), high-density polyethylene (HDPE) and microplastic clothing fibers. PLA is a plastic made from renewable sources such as corn starch or sugarcane which is often used in food-packaging, as well as in the healthcare, construction and textile industries. HDPE is a tough plastic commonly used to produce bottles. Ryegrass—which is an important agricultural crop—was used as a model plant.

The scientists conducted the study in the laboratory using so-called "mesocosms," which are controlled, experimental containers that can simulate conditions in the field. They observed these mini-ecosystems created for a 30-day period.

"We chose to focus on levels of microplastics which are relatively high, but not unrealistic for soils," Boots said. "At the rate we are going, more microplastics are predicted to end up in soils. We choose to focus on fibers as well, as they can be deposited from the air as fallout, and there are studies suggesting that they may end up on the soil when treated sewage solids are used as fertilizer."

Over the 30-day period, the scientists observed several different effects. Firstly, the scientists found that earthworms exposed to HDPE lost about 3.1 percent of their weight on average.

In comparison, worms living in a control environment without microplastics grew by 5.1 percent over the same period, according to a study published in the journal Environmental Science & Technology.

Meanwhile, soil exposed to PLA saw fewer ryegrass seeds germinating and a reduction in the height of ryegrass shoots. The latter observation was also seen in soil that was exposed to clothing fibers.

Finally, the team noticed that soil exposed to HDPE experienced a decrease in pH—meaning it became more acidic.

The researchers say that the study provides evidence that microplastics made from the substances studied can affect the development of ryegrass, the health of rosy-tipped earthworms and "basic, but crucial" soil properties. This indicates these materials could have significant impacts on the functioning of soil ecosystems, they say.

"Earthworms can be called 'ecosystem engineers' as they help maintain a healthy soil," Connor Russell, a co-author of the study from Anglia Ruskin, said in a statement. "They do this through ingesting dead organic matter, therefore contributing to the availability of nutrients.

"Their burrowing activity improves soil structure, helping with drainage and preventing erosion. It's therefore highly likely that any pollution that impacts the health of soil fauna, such as earthworms, may have cascading effects on other aspects of the soil ecosystem, such as plant growth."

Researchers stress however, that the reasons for the earthworms weight loss is unclear at present. "It may be that the response mechanisms to microplastics may be comparable in earthworms to that of the aquatic lugworms, which have been previously studied," Boots said in a statement. "These effects include the obstruction and irritation of the digestive tract, limiting the absorption of nutrients and reducing growth."

This article was updated to include additional comments from Bas Boots.