Biofilms could change the way we track plastic pollution

Biofilms – thin, sticky layers made by microorganisms – may hold the key to understanding how microplastics move and settle in the environment and even inside our bodies.

Predicting where these tiny particles accumulate has proven tricky, influenced by many factors like water flow and sediment texture.

Scientists from the Massachusetts Institute of Technology (MIT) have found that biofilms play a bigger role than previously thought. These microbial layers often coat sandy riverbeds and seashores, changing how microplastics interact with the ground beneath them.

Biofilms and plastic pollution

Traditionally, experiments on microplastic movement have used bare sand. But as the researchers point out, real environments are full of life.

Bacteria, fungi, and algae create what are known as extracellular polymeric substances (EPS). These sticky materials coat surfaces and change how riverbeds behave.

The team set out to study how EPS affects microplastic buildup. They used a flow tank filled with fine sand. Some experiments had just sand, while others included a biological material simulating natural biofilms. In certain tests, vertical plastic rods stood in for mangrove roots.

For three hours, water mixed with tiny plastic particles flowed through the tank. Afterward, the researchers photographed the sand bed under ultraviolet light. The plastic particles glowed, allowing for precise measurements of their concentrations.

Less biofilm, more plastic

Two major findings emerged from the study. Around the rods – mimicking plant roots – turbulence reduced plastic buildup. But more surprising was what happened with the biofilm-coated sand.

The experiments showed a clear pattern: microplastics were less likely to settle in sand beds covered with biofilm. This difference pointed to a subtle but important role biofilms play in shaping where plastic pollution builds up.

“These biological films fill the pore spaces between the sediment grains,” noted MIT postdoctoral researcher Hyoungchul Park. “That makes the deposited particles – the particles that land on the bed – more exposed to the forces generated by the flow, which makes it easier for them to be resuspended.”

“What we found was that in a channel with the same flow conditions and the same vegetation and the same sand bed, if one is without EPS and one is with EPS, then the one without EPS has a much higher deposition rate than the one with EPS.”

Why it matters for pollution control

Biofilms could make a big difference in how we monitor pollution. Study co-author Heidi Nepf noted that the biofilm blocks the plastics from accumulating in the bed because they can’t go deep into the bed.

“They just stay right on the surface, and then they get picked up and moved elsewhere. So, if I spilled a large amount of microplastic in two rivers, and one had a sandy or gravel bottom, and one was muddier with more biofilm, I would expect more of the microplastics to be retained in the sandy or gravelly river,” explained Nepf.

This discovery could change how we plan cleanup and monitoring efforts. While many other factors – like water turbulence and the roughness of the riverbed – still matter, biofilms offer a new “lens” for scientists.

“They’re trying to determine what kinds of habitats these plastics are in, and this gives a framework for how you might categorize those habitats,” said Nepf. “It gives guidance to where you should go to find more plastics versus less.”

Identifying vulnerable hotspots

Mangrove ecosystems offer a good example. Park suggests that the sandy edges of these areas might become microplastic hotspots. These outer zones have less biofilm and more open sand, making it easier for plastics to settle.

“The sandy outer regions may be potential hotspots for microplastic accumulation,” said Park, making them key places to prioritize for monitoring and protection.

Identifying these vulnerable zones is crucial for focusing cleanup efforts and reducing environmental risks.

By knowing where plastics are likely to build up, scientists and policymakers can better target conservation strategies and deploy limited resources more efficiently. Over time, this targeted approach could help slow the spread of microplastics in sensitive ecosystems.

The full study was published in the journal Geophysical Research Letters.

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