Northern fulmars and Cory’s shearwaters are masters of the sea and air, gliding above the waves and plunging into the water to snag fish, squid, and crustaceans. But because humans have so thoroughly corrupted the ocean with microplastics—at least 11 billion pounds of particles float at the surface, and that’s likely a huge underestimate—their diet now also includes substantial amounts of synthetic poison.
A study published today in the journal Nature Ecology & Evolution shows that those microplastics (defined as particles under 5 millimeters long) might be altering the seabirds’ gut microbiomes, with as-yet-unknown implications for their health. Another recent paper introduced the world to “plasticosis”: severe scarring in the digestive system of birds that had eaten plastic. With plastic pollution increasing exponentially along with plastic production, the new papers are a hint of the suffering to come.
The researchers behind today’s paper dissected 85 northern fulmars and Cory’s shearwaters caught in the wild. (Northern fulmars live around northern oceans and the Arctic; Cory’s shearwaters throughout the Atlantic.) Then the team flushed plastic particles out of the birds’ digestive tracts, looking for bits as small as 1 millimeter, and analyzed the species of microbes in the gut. When the researchers analyzed microplastics in the birds by mass, the greater the mass, the lower the gut microbiome diversity. But when they counted the number of plastic particles, “the more particles there were, the more diverse the microbiome was,” says Gloria Fackelmann, a microbiome biologist at Ulm University in Germany, and lead author of the study. In this case, diversity isn’t necessarily a good thing: The more particles, the more pathogenic and antibiotic-resistant microbes the researchers found in the gut.
In other words, a shift in the microbiome appears to favor potentially harmful, pathogenic microbes. Significantly, it happened among seabirds that had been eating “environmentally relevant” amounts of microplastics—meaning, what they found in their own habitat. (In previous laboratory studies, scientists have exposed various species to unrealistically high concentrations of microplastic.)
This paper didn’t track whether the birds became sickened by microbial diseases, “so we can’t say the seabirds that had more plastic were unhealthier,” says Fackelmann. But that will be one of the big questions as researchers try to parse what effects the particles might be having. As microplastics break down, they leach out their component chemicals—around 10,000 varieties are used in plastics, many of which are known to be toxic to life. They’re especially prone to leach in a hot, acidic place like a digestive tract. “This all paints a really scary picture,” says Britta Baechler, associate director of ocean plastics research at the Ocean Conservancy, who wasn’t involved in either of the new papers. The gut, she says, is “a very harsh environment—things can be released, and that includes pathogens, bacteria, but also chemical contaminants.”
As microplastics tumble through the ocean, they accumulate an extremely diverse community of viruses, algae, and even the tiny larvae of animals. (An especially common bacteria that scientists are finding on microplastics is Vibrio, which causes severe illness when people eat raw or undercooked seafood or are exposed to hurricane floodwaters.) This teeming world even has its own name: the plastisphere. When a fish or bird accidentally eats microplastic, it also eats that community of lifeforms. “If a seabird is ingesting more of these particles, and it does act as a vector, then you would have a higher diversity” of gut microbes, says Fackelmann.
This might be why her team got contrasting results in their analysis: The more individual microplastics in the gut, the greater the microbial diversity, but the higher mass of microplastics, the lower the diversity. The more particles a bird eats, the greater the chance that those hitchhiking microbes take hold in its gut. But if the bird has just eaten a higher mass of microplastics—fewer, but heavier pieces—it may have consumed fewer microbes from the outside world.
Meanwhile, particularly jagged microplastics might be scraping up the birds’ digestive systems, causing trauma that affects the microbiome. Indeed, the authors of the plasticosis paper found extensive trauma in the guts of wild flesh-footed shearwaters, birds that live along the coasts of Australia and New Zealand, that had eaten microplastics and macroplastics. (They also looked at plastic particles as small as 1 millimeter.) “When you ingest plastics, even small amounts of plastics, it alters the structure of the stomach, often very, very significantly,” says study coauthor Jennifer Lavers, a pollution ecologist at Adrift Lab, which researches the effects of plastic on sea life.
Specifically, they found catastrophic damage to the birds’ tubular glands, which produce mucus to provide a protective barrier for the inside of the stomach, as well as hydrochloric acid, which digests food. Without these key secretions, Lavers says, birds “also can’t digest and absorb proteins and other nutrients that keep you healthy and fit. So you’re really prone and susceptible to exposure to other bacteria, viruses, and pathogens.”
Scientists call this a “sublethal effect.” Even if the ingested pieces of plastic don’t immediately kill a bird, they can severely harm it. Lavers refers to it as the “one-two punch of plastics” because eating the material harms the birds outright, then potentially makes them more vulnerable to the pathogens they carry.
A major caveat to today’s paper—and the vast majority of microplastics research—is that most scientists haven’t been analyzing the smallest of plastic particles. But researchers using special equipment have recently been able to detect and quantify nanoplastics, on the scale of millionths of a meter. These are much, much more numerous in the environment. (This is also why the finding that there are 11 billion pounds of plastic floating on the ocean’s surface was probably a major underestimate, as that team was only considering particles down to a third of a millimeter.) But the process of observing nanoplastics remains difficult and expensive, so Fackelmann’s group can’t say how many might have been in the seabirds’ digestive systems, and how they too might influence the microbiome.
It’s not likely to be good news. Nanoplastics are so small that they can penetrate and harm individual cells. Experiments on fish show that if you feed them nanoplastics, the particles end up in their brains, causing damage. Other animal studies have also found that nanoplastics can pass through the gut barrier and migrate to other organs. Indeed, another paper Lavers published in January found even microplastics in the flesh-footed shearwaters’ kidneys and spleens, where they had caused significant damage. “The harm that we demonstrated in the plasticosis paper is likely conservative because we didn’t deal with particles in the nanoplastic spectrum,” says Lavers. “I personally think that’s quite terrifying because the harm in the plasticosis paper is quite overwhelming.”
Now scientists are racing to figure out whether ingested plastics can endanger not only individual animals, but whole populations. “Is this harm at the individual level—all of these different sublethal effects, exposure to chemicals, exposure to microbiome changes, plasticosis—is it sufficient to drive population decline?” asks Lavers.
The jury is still out on that, as scientists don’t have enough evidence to form a consensus. But Lavers believes in the precautionary principle. “A lot of the evidence that we have now is deeply concerning,” she says. “I think we need to let logic prevail and make a fairly safe, conservative assumption that plastics are currently driving population decline in some species.”