Tiny pieces of plastic are everywhere. They’re in the air we breathe, the water we drink, and the food we eat. By one estimate, some people ingest around a credit card’s worth of plastic every week. Microplastics have been found in human blood, placentas, and feces. But we don’t fully understand what all these minuscule bits of plastic are doing to us or other animals.
Now, new research in seabirds hints that it might affect gut microbiomes—the trillions of microbes that make a home in the intestines and play an important role in animals’ health, including our own. Seabirds ingest plastic from the ocean, which we know can accumulate in their stomachs. The research shows it leaves the birds with more potentially harmful microbes in the gut, including some that can break down plastics.
“It expands our view on what plastic pollution is doing to wildlife,” says Martin Wagner, a biologist researching the impact of plastics on ecosystems and human health at the Norwegian University of Science and Technology, who was not involved in the study. He finds the results “concerning.” We’ve long known that plastics can cause toxicity and physical injury to animals. The new evidence that animals’ microbiomes are affected too “really shows the wide spectrum of adverse effects that we get from plastic pollution, and microplastics in particular,” he says.
Plastic planet
Microplastics, miniature bits of plastic that measure less than five millimeters in diameter, are a type of pollution that’s been found in ecosystems all over the world. “We know that microplastics have reached very remote areas of the deep sea, the Arctic, the Tibetan Plateau,” says Gloria Fackelmann, a microbial biologist at Ulm University in Germany. “There are microplastics in rivers … and a lot of research is beginning to look at microplastics in soils as well.”
Researchers don’t really know how much plastic most animals are exposed to. But it is clear that seabirds are especially vulnerable. These birds spend a lot of their time on the high seas and eat fish at the water’s surface. They also ingest a lot of floating plastic.
Previous studies have found that these plastics can be incredibly harmful for seabirds. Animals with a stomach full of plastic can feel full, so they don’t eat enough and and end up starving to death. The chemicals that leach from plastic fragments can also be harmful, causing inflammation, for example. Because microbes can cling to the surfaces of plastics, Fackelmann and her colleagues wondered if microplastics might also affect the communities that make up the animals’ microbiomes.
Bad bacteria
Until now, only a few studies have looked at the potential impact of plastics on the microbiome. These have been experimental setups that involved feeding plastic to mice in a lab. Fackelmann and her colleagues wanted to find out what happens in a real-world setting instead.
Fackelmann’s colleagues examined seabirds from Canada and Portugal. Twenty-seven northern fulmars were collected by scientists working alongside Inuit hunters near Qikiqtarjuaq, Nunavut, and 58 Cory’s shearwaters that had died after colliding with buildings were collected on the Azores archipelago. Scientists then sampled the two ends of each bird’s intestinal tract—the proventriculus and the cloaca—to get an idea of what the microbiome was like at each.
The team also flushed out the birds’ gastrointestinal tracts to count the pieces of plastic and weigh the total amount in the gut of each animal.
The birds that had more pieces of microplastic in their guts had more diversity in their microbiomes. A wider variety of gut microbes has traditionally been considered a good thing. But that isn’t always the case, says Fackelmann. If the bacteria being introduced are harmful, then having more diversity would not be beneficial, she says.
To find out if the microbes being introduced might be “good” or “bad,” Fackelmann and her colleagues analyzed the microbiomes and looked up individual types of microbes in databases to learn what they do. They found that with more plastic, there were more microbes that are known to break down plastic. There were also more microbes that are known to be resistant to antibiotics and more with the potential to cause disease.
Fackelmann and her colleagues didn’t assess the health of the birds, so they don’t know if these microbes might have been making them unwell. “But if you accumulate pathogens and antibiotic-resistant microbes in your digestive system, that’s clearly not great,” says Wagner.
The study, which was published in the journal Nature Ecology and Evolution, shows that the levels of plastic already present in the environment are enough to affect animals’ microbiomes, says Fackelmann. The next step is to work out what this might mean for their health and the health of other animals, including humans, she says.
“When I read [the study], I thought about the whales we find beached with kilograms of plastic debris found in their bellies,” says Wagner. “It’s probably quite comparable to what birds have in their digestive systems, so it would be interesting to know if this happens in whales, dolphins, [and other marine animals] as well.”
Plastic people
We don’t yet know if the amount of plastic that humans ingest might be enough to shape our microbiomes. People ingest a lot less plastic than seabirds do, says Richard Thompson, a professor of marine biology at the University of Plymouth in the UK. The amount of plastic that gets into our bodies also depends on where we live and work. People who work in textile factories will have a higher exposure than those who work outdoors, for example.
And we don’t know the consequences of ingesting microbes that cling to the microplastics that get into our bodies. Humans are already exposed to plenty of disease-causing microbes that aren’t on plastics, Thompson points out. For example, we might worry that tiny bits of plastic might pick up nasty bugs in wastewater, and that these might somehow end up in our bodies. But overflows of wastewater regularly contaminate beaches and drinking water directly.
There’s a chance microbes that break down plastic will end up residing in our guts too. It’s difficult to know how—or whether—this will affect us. Microbes can evolve quickly, and they can swap genes with neighboring bugs. “Are we going to evolve to eat plastic? My answer would likely be no,” says Fackelmann. But the possibility that our guts will become home to more microbes that can break down plastic is “not beyond the realm of possibility,” she says.
There’s also the possibility that plastic pollution will affect us indirectly. Introducing more pathogenic microbes to birds and other animals could cause disease outbreaks, and one of the microbes that the team found to be correlated with plastic in the birds’ guts is thought to be able to jump from animals to humans. Wagner thinks it is unlikely that microbes seabirds pick up from floating plastic could eventually cause disease outbreaks in people. “But the more we disturb natural systems, the higher the likelihood of zoonosis [a disease jumping from animals to humans],” he adds.
Given the ubiquity of microplastics, studies like these are desperately needed to help us understand how plastic pollution affects living creatures, including humans, the researchers say.
“We’ve basically plasticized the globe,” says Wagner. “Everybody is exposed to microplastics and the chemicals in plastics—it’s just a matter of time until we figure out what it’s doing to our microbiome as well. And I cannot see any argument for why plastic ingestion would be beneficial.”