Scientists began reporting microplastic pollution in the ocean as far back as the 1970s, and in freshwater ecosystems such as the Great Lakes by the 2010s. But our understanding of these tiny particles’ impact on fish and other wildlife is only just catching up.

Now new research from the University of Toronto shows the harm to aquatic creatures is due to a wide range of factors that is not generally considered in toxicology testing – the plastics’ size, shape and chemical makeup.

In particular, the research shows larval fathead minnows exposed to microplastics collected from Lake Ontario developed almost six times more deformities compared to when they were exposed ‘pristine’ pre-consumer microplastics. This suggests microplastics in the lake soak up contaminants in the water and that it is these chemicals that are causing deformities.

The study’s authors argue researchers and policy-makers need to stop treating microplastics as a single contaminant, but rather understand they are multiple contaminants in one very small package.

Plastic particles are considered microplastics when they are five millimeters (0.2 inch) or smaller. These tiny bits of plastic come from many sources, such as plastic bags, water bottles and other things that started out larger.

In water, plastic breaks down when it’s exposed to light and wave action. Clothes made of nylon and other types of plastic also shed bits of lint as they are washed, and when wash water goes down the drain, it can carry that lint into rivers and the ocean, where fish and other aquatic creatures will eat it.

Scientists have found microplastics all over the world — even in mosquitoes’ bellies. Researchers in the United Kingdom recently found that mosquito larvae can eat tiny bits of plastic from the water in which they’re living and as the mosquitoes grow into adults, much of that plastic stays inside them. That means birds and bats that eat mosquitoes may be taking in a mouthful of plastic with every meal. And any other animal that eats those birds and bats is probably also getting a little microplastic.

Microplastics act like chemical sponges

Kennedy Bucci kneels, holding a jar and wearing one purple glove, and picks through microplastics on the shoreline.Kennedy Bucci, the University of Toronto PhD student who led the Lake Ontario study, says the most significant failing of most microplastic testing is that it doesn’t examine the impact of chemicals that hitch a ride on the particles after they end up in the water.

Microplastics act like a chemical sponge, soaking up contaminants such as persistent organic pollutants and heavy metals. The pesticide DDT and polychlorinated biphenyls (PCBs) are just two examples of pollution that have been found in plastics floating in the ocean.

“But most lab testing is done with pre-consumer microplastics,” says Bucci. “They’re easy to buy – they come in the mail.”

These pre-consumer microplastics are usually polyethylene or polypropylene and are used to make products such as shopping bags, food wrap, detergent bottles and even auto parts. While lab testing of pre-consumer microplastics has helped raise flags, results have offered conflicting evidence. Some show harmful effects to fish – liver stress, changes to gene expression and decreased metabolism – while others show no effect.

“The inconsistencies between studies in terms of whether or not an effect is detected are likely the consequence of ignoring the complexity and context of microplastics as an environmental contaminant,” Bucci and her fellow authors write.

Susanne Brander, an ecotoxicologist at Oregon State University who studies microplastics but was not involved in the Toronto study, agrees.

“A lot of experiments look at what’s happening over a few days or weeks,” she says. “But fish are exposed to microplastics at least intermittently throughout their lives.”

Zeroing in on an important prey species

The University of Toronto research focussed on larval fathead minnows, a significant prey species in North American rivers and lakes. They exposed the larvae to pre-consumer microplastics and microplastics that Bucci collected from Toronto’s Lake Ontario shoreline.

Before exposing the larvae, she put both types of plastic through a coffee grinder so the team could compare the effects of particles of the same shape and size.

Previous research has shown that the more jagged the shape of a microplastic, the more damage it does to the digestive tract of fish and other creatures. The same goes for size – the smaller the particle, the better it’s able to slip out of the digestive tract and into the body, where it can irritate internal organs and cause harmful inflammation.

After grinding the particles to a uniform size and shape, Bucci exposed the larvae to different treatments – pre-consumer polyethylene and polypropylene microplastics and microplastics made of the same mix of material, but sourced from Lake Ontario.

She also exposed some larvae to water in which each type of microplastic had soaked for 24 hours but from which the particles had been removed. This allowed Bucci to untangle the effect of the particles themselves from the chemical soup they leached out.

After 14 days, she examined the larvae for visible deformities. Environmental microplastics caused almost six times more deformities as the pre-consumer microplastics. The deformities included scoliosis of the spine, edema (fluid buildup around the eyes, heart or yolk sac), failure to hatch, and truncated tails.

“Although pre-consumer microplastics caused effects only when organisms were exposed to both the particles and the chemical leachates, the environmental microplastics caused effects when organisms were exposed to the chemical leachates alone, suggesting that the mechanism of effects are context‐dependent,” the authors write.

In other words, the microplastics collected from Lake Ontario likely soaked up contaminants from the water and it was these contaminants that caused the deformities. Bucci did not test the Lake Ontario microplastics to see what chemicals they contained, in part because it would have been difficult to untangle which chemical was responsible for which deformity.

Brander says the study was well-designed and its conclusion about treating microplastics as multiple stressors “is absolutely a perfect argument to be making.”

“We’re really in a new frontier here. We’re trying to assess not just the effect of a chemical, but that chemical and its interaction with this three-dimensional object. We’re trying to assess the risk of potentially 15 different things on one particle,” she says.