The plastics had been submerged in the ocean off Falmouth, England, for just a week, but in that time a thin layer of biofilm, a slimy mixture of slime and microbes, had already developed on their surface. Mikiel Voss, a microbiologist at the University of Exeter in England, had submerged five different types of plastic as a test. He and his colleagues wanted to know which of the myriad microbes living in the ocean would target these introduced materials.
The primary concern of Voss and his colleagues was pathogenic bacteria. To understand the extent to which plastic can be colonized by potentially deadly bacteria, the scientists injected wax moth larvae with the biofilm. After a week, four percent of the larvae die. But four weeks later, after Voss and his team let the plastics suffocate in the ocean a little longer, they repeated the test. This time, 65 percent of the wax moths died.
The scientists analyzed the biofilm: the plastics were covered in bacteria, including some known to make us sick. They found pathogenic bacteria responsible for causing urinary tract, skin and stomach infections, pneumonia and other diseases. To make matters worse, these bacteria also carry a wide range of antimicrobial resistance genes. “Plastics that you find in water are quickly colonized by bacteria, including pathogens,” says Voss. “And it doesn’t really matter what plastic it is.”
It’s not just bacteria that gets on plastics. Biofilms on marine plastics can also harbor parasites, viruses and toxic algae. Because plastic pollution in the sea is so ubiquitous—found everywhere from the bottom of the Mariana Trench to Arctic beaches—scientists are concerned that plastic is carrying these human pathogens across the oceans.
But whether plastics harbor pathogen populations dense enough to be truly dangerous, and whether they carry them to new areas, are difficult questions to answer.
There are good reasons to believe that plastics accumulate and spread pathogens around the world. Linda Amaral-Zettler, a microbiologist at the Royal Netherlands Institute for Marine Research, coined the term plastisphere of the new ecosystem that plastics are creating, says that plastic is different from other solid surfaces often found in the ocean – such as logs, shells and rocks – because plastic is durable, long-lasting and much of it floats. “It gives him mobility,” she says.
Plastics can travel long distances. After the 2011 earthquake and tsunami in Japan, for example, many identifiable Japanese objects were washed up on the west coast of North America. This stretcher, says Amaral Zettler, has “the potential to transport whatever is attached to it.”
Recent laboratory work also shows that some typically terrestrial disease-causing parasites can survive in seawater and infect marine mammals. Karen Shapiro, an infectious disease expert at the University of California, Davis, has shown that these protozoan parasites—specifically, Toxoplasma gondii, Cryptosporidium minorand Giardia enterica— can attach to microplastics in seawater. This could change where, when and how these parasites accumulate in the ocean.
“If they’re hitchhiking with plastics that happen to be in the same sewer outlet, river or overland storm drain, then they’re going to end up where the plastic ends up,” explains Shapiro. This could be in clams on the sea floor or drifting on currents in the middle of the ocean.
The next step, Shapiro explains, is to look for a similar link between parasites and plastics outside the lab.
That microplastic pollution appears to be a breeding ground for pathogens also raises a long-term concern for Vos — that plastics may be promoting the spread of antibiotic resistance. Bacteria can exchange genes, and because the bacteria are in close contact on the surface of small microplastics, the level of horizontal gene transfer between them is high, he says. Plastics can also put bacteria in close contact with pesticides and other pollutants, which also adhere to biofilms. This promotes the development of antimicrobial resistance.
“We don’t know that much about it,” says Voss, “but there are potentially interesting ways that bacteria can experience stronger selection [for antimicrobial resistance] on plastics, but also have more opportunities to exchange genes that could provide resilience.
In addition to posing potential risks to human health, plastic-borne pathogens can threaten marine ecosystems and food supply chains, Amaral-Zetler says. Millions of people rely on seafood as a source of protein, and there are many pathogens that infect the fish and shellfish we eat. It may be possible, Amaral-Zettler says, for microplastics to spread disease between different aquacultures and fisheries.
Although we don’t fully understand the risks, these studies are another good argument for limiting plastic pollution, says Voss. “There can be nothing positive about plastics with pathogens floating around.”
This article is from the Hakai Journal, an online publication for science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
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