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Plastics were once a symbol of modern life. Today, they’re one of the most pressing environmental problems of the 21st century. Among the many threats they pose, microplastics—tiny plastic fragments found in rivers, seas, and oceans—have raised serious concerns about their impact on human and environmental health.

Microplastics in aquatic environments don’t exist in isolation. They readily accumulate biofilms, which are complex communities of bacteria and microorganisms. This transformation of microplastics into microbe-laden rafts raises concerns about the potential for the widespread dispersal of pathogens. These contaminated plastic particles could travel from freshwater sources to the oceans and potentially infiltrate the food chain.

But recent European research offers a surprising twist: saltwater seems to act as a natural filter, stopping many freshwater pathogens from surviving the journey to the sea.

A Scientific Expedition Through Europe's Rivers

To investigate how microplastic-related bacteria change as they travel from freshwater to saltwater, a team of French scientists launched an ambitious seven-month research expedition. They sailed along nine of Europe’s most important rivers—including the Rhine and the Seine—collecting samples from river mouths up to the areas just above major cities, where human impact on water quality becomes more intense.

At each location, the team collected water samples and microplastic fragments using fine mesh nets. They were especially interested in how salinity—the level of salt in the water—affected microbial life on the plastic. 

To dig even deeper, the researchers also placed clean pieces of plastic (made of polyethylene, nylon, and polyoxymethylene) in the water. After a month of floating, these plastic samples were retrieved and analysed for bacterial DNA.

Every precaution was taken to avoid contamination. The microplastics were sterilised with alcohol, tools were disinfected, and samples were flash-frozen in liquid nitrogen. Cutting-edge DNA sequencing and infrared spectrometry were then used to identify both the types of plastic and the microbes living on them.

Saltwater as a Microbial Barrier

The results surprised even the researchers. The bacterial communities on microplastics differed significantly between freshwater, estuarine (where rivers meet the sea), and marine environments. Freshwater plastics hosted far more diverse and abundant bacteria compared to those found in the sea.

Even more striking: pathogenic bacteria like Aeromonas, Acidovorax, Arcobacter, and Prevotella were found only in rivers, not in saltwater. In the sea, only one type of pathogen—Vibrio—was dominant. Crucially, there was no evidence that freshwater pathogens had crossed over into marine environments via plastic.

This suggests that salinity works as a powerful natural filter, making it nearly impossible for river-based microbes to survive in the ocean. The researchers called this effect a “strong selective pressure”—a kind of environmental sieve that limits the spread of potentially harmful organisms.

One Troubling Exception

While most of the findings were reassuring, there was one notable exception. For the first time, scientists identified the bacteria Shewanella putrefaciens living on a microplastic sample from river water. Though not commonly dangerous, this bacterium can cause intestinal, skin, and soft tissue infections in humans.

Luckily, Shewanella was not found in saltwater samples—again confirming the protective role of salinity. Another interesting takeaway? The type of plastic didn’t seem to matter much for which microbes colonised it. The majority of collected microplastics were made of polyethylene (45%) and polypropylene (12%), but the bacteria didn’t seem to have strong preferences.

Earlier studies had suggested that different plastic types might influence the formation of biofilm, but this research—based on real-life environmental sampling—didn’t confirm that link.

What This Means for EU Policy and the Planet

Beyond scientific curiosity, these findings have real-world implications. European Union policies are already targeting plastic pollution, especially microplastics. Regulations like REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the Water Framework Directive (WFD), and the Marine Strategy Framework Directive (MSFD) are part of the EU’s legal toolkit to protect rivers, lakes, and seas.

New research offers strong scientific evidence indicating that biofilms on microplastics can harbour harmful microbes, although their transfer between freshwater and saltwater seems restricted. These findings are crucial for developing efficient environmental strategies focused on both decreasing plastic pollution and safeguarding public health.

The researchers emphasise that more studies are needed—especially on viruses and single-cell organisms that could also use plastic as transport. Understanding how tides, temperature, or other environmental factors affect these microbial communities is another key research frontier.

A Reminder from Science

In a world where plastic has reached even the most remote corners of the planet, studies like this one highlight the importance of scientific research in protecting our future. They don’t just help us understand what’s happening—they also equip us to make smarter choices for our health, ecosystems, and policies.

So, while microplastics are still a major environmental concern, it’s reassuring to know that nature—specifically, a dash of salt—might be doing more to protect us than we once thought.

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