Harmful bacteria found ordinary sea plastic

Most waters are naturally teeming with microbial life and the Mediterranean is no exception. Now the microscopic marine organisms of the Mediterranean Sea have a new mode of locomotion. You’re hitchhiking on a growing fleet of plastic ships: microfibers.

In a recent study published Nov. 30 in the journal Plus one, a team of biologists from the Sorbonne Université in France have discovered 195 species of bacteria living on microfibers floating in the Mediterranean Sea. According to their analysis, a single microfiber could be home to more than 2,600 bacterial cells. While not all marine microbes on the plastic particles were dangerous, the researchers were particularly concerned about the amount of bacterial species that could be potentially harmful to wildlife and humans.

“Plastics are a relatively new substrate in the ocean,” says Ana Luzia de Figueiredo Lacerda, study author and plastic marine pollution researcher at Sorbonne Université. “We discover what lives [on plastics] to see the diversity of bacterial groups and what may be potentially pathogenic or invasive among those groups.”

A 2020 United Nations Environment Program report estimates that 730 tonnes of plastic waste enters the Mediterranean Sea every day, leaving more than 64 million tiny floating particles per square kilometer in certain areas, including plastic microfibers. In fact, the Mediterranean Sea has the highest concentration of microfibers of any major ocean basin in the world. These small synthetic threads are released from sources such as frayed fishing nets, textile manufacturers, or laundry masses, Lacerda explains. “It’s one of the most abundant types of microplastics in the oceans,” she says. The high salinity and density of Mediterranean waters may also result in larger concentrations of the fibers floating near the surface, the new study finds.

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Across the world’s oceans, plastic pollution has created a new artificial community for marine microbes – what researchers call the “plastisphere.” Free-floating bacteria and other microbiota can secrete sticky molecules that help them attach to substrates such as wood, microalgae, or sediment. Once attached, the bacteria produce more of these sticky molecules for more microbes to colonize, causing a biofilm to grow, explained Karen Shapiro, an infectious disease expert at the University of California, Davis, in an email to PopSci. But the problem with plastics is that they last much longer than natural substrates in marine environments, increasing the risk and spread of microbial contamination, Lacerda says. Some types of plastic are less dense than seawater and float to the surface where they can be carried long distances by ocean currents.

“The plastic acts like a boat for these organisms,” explains Lacerda. “They transport species across regions, which could lead to changes in the functioning of the natural system.”

When colonized by bacteria, the plastics can smell like food to sea creatures that accidentally consume them. Not only does this mean that the microplastics are making their way up the food chain, previous studies have shown that toxic chemicals in plastics can cause hormonal imbalances that affect the growth and reproduction of some wildlife groups, including orcas and oysters.

Photomicrograph of floating fibers collected in the coastal zone of the north-west Mediterranean (A) and scanning electron microscopy (SEM) images of their bacterial communities (B) showing elongated and rounded cells and the sticky molecular compounds that form biofilms (CF ). Pedrotti et al., 2022, PLOS ONE

To find out what types of bacteria might be harboring microfibers in the Mediterranean Sea, Lacerda and her colleagues collected samples from the north-west end near the coasts of Monaco and Nice, France. After isolating the microfibers, the team used microscopy and DNA sequencing to identify the types of bacteria on the fibers and compared them to the free-floating bacteria in the water. Among the 195 species living on the microfibers, Lacerda and the authors highlighted a “large amount” of pathogens Vibrio parahaemolyticus, a bacterium that can cause seafood poisoning in humans.

Previous studies have found pathogenic marine microorganisms on plastics in the sea, such as Aeromonas salmonicida, which can infect and kill salmon, and Arcobacter Species that can cause disease in humans. “In one particular sample, the authors found almost a third of the bacterial species V.parahaemolyticus, that’s a remarkable fraction and potentially worrying given its pathogenic potential,” Shapiro, who was not involved in the recent research but has also studied the plastisphere, wrote in her email. “These anthropogenically derived fibers that end up in our oceans could mediate disease transmission for marine life, but also for people who consume shellfish, which can concentrate these pollutants.” V. parahaemolyticus thrives in warm brackish water where filter feeders like oysters are typically cultivated.

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Locating where microfibers — and the harmful species they contain — are plentiful can help people know if certain bodies of water are safe for bathing, farming, or fishing, Lacerda says. Climate change could promote the spread and pathogenicity of temperature-affected microorganisms in plastic, e.g V. parahaemolyticus. “As the temperature of the ocean increases, virulence and [plastic] With increasing temperature, the adhesion of the organism also increases,” says Lacerda. This is particularly important in a sandy sea like the Mediterranean, which is warming faster than other regions of the world. As a result, “we might expect the Mediterranean plastisphere to respond more quickly to climate change,” Lacerda notes.

According to Shapiro, the results of the Mediterranean study add to the growing evidence that marine bacteria that thrive on plastic waste is “a global phenomenon that deserves more attention.” She and Lacerda both agree that more research into the interactions between pathogens and various pollutants, such as plastics, is needed to better understand how humans are altering — and damaging — marine ecosystems.

“I think the general public should be aware of this issue and understand that plastic pollution in the ocean not only affects marine life, it can also affect us,” Lacerda said. As the plastic problem continues to grow, she adds, “We need to look for a different way of life.”

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