The ubiquitous threat of microplastics, tiny particles that permeate every corner of our environment and even our bodies, poses one of the key environmental questions of our time. Scientists and environmental experts are constantly faced with the challenge of predicting where these particles, smaller than five millimeters, will accumulate the most. The accurate identification of so-called "hotspots" is crucial for directing remediation efforts and mitigating their harmful impact. Recent scientific research brings a revolutionary insight into this problem, revealing that one previously overlooked biological factor plays a decisive role in the fate of these particles in aquatic ecosystems.

The secret life of riverbeds: The role of biofilm

At the bottom of rivers, streams, and along sea coasts, the surfaces of sand and sediments are not lifeless. They are covered with a thin, sticky layer known as biofilm. This biological coating is created by communities of microorganisms such as bacteria, algae, and fungi. They secrete substances known as extracellular polymeric substances (EPS), which create a sticky matrix that binds sediment particles together. Although it was long thought that such a sticky surface could promote the trapping of microplastics, new research conducted at the Massachusetts Institute of Technology (MIT) shows the exact opposite. It seems that biofilm acts as a kind of "protective layer" that reduces the deposition of microplastics.

The study found that, under otherwise equal conditions, sediments enriched with biofilm retain significantly fewer microplastic particles. The reason lies in the physical interaction between the particles, the water, and the substrate itself. The biofilm fills the microscopic spaces between the sand grains, making the surface smoother and less "adherent" to incoming particles. When a microplastic particle lands on such a surface, it cannot penetrate deeper into the sediment but remains more exposed at the very top. Because of this, the water flow much more easily lifts it and carries it further downstream. In contrast, bare sandy beds act as a trap, allowing particles to settle deeper among the sand grains, where they are protected from water currents and are more difficult to re-mobilize.

An innovative experiment that illuminates the processes

To reach these conclusions, the researchers designed a precise laboratory experiment. They used a special flow tank, a kind of miniature river simulator, the bottom of which was lined with fine sand. In some experiments, vertical plastic pipes were also placed in the sand to simulate the presence of roots, for example, in mangrove ecosystems. The key part of the research was the comparison of two types of substrates: one consisting of pure sand and the other where the sand was mixed with a biological material that faithfully mimics natural biofilm and its extracellular polymeric substances.

During the experiment, water mixed with tiny plastic particles was pumped through the tank for three hours. Afterward, the surface of the riverbed was photographed under ultraviolet (UV) light. This technique allowed the plastic particles to fluoresce, or glow, which gave the scientists the ability to precisely and quantitatively measure their concentration on different surfaces. The results were unequivocal and revealed two key phenomena that affect the deposition of microplastics.

Unexpected allies in the fight against pollution

The first observed phenomenon was related to turbulence. Immediately around the simulated roots, increased water swirling prevented the deposition of particles, creating a kind of "protected zone." However, the second, even more significant finding, related to the influence of the biofilm itself. As the proportion of simulated biofilm in the sediment increased, the amount of accumulated plastic decreased.

The scientific team concluded that the sticky polymers from the biofilm fill the pores and irregularities between the sand grains, leaving less space for "catching" microparticles. Because the particles were more exposed on the surface, rather than hidden in the sediment, the forces generated by the water flow could more easily lift them again and carry them away. This means that a river with a sandy or gravelly bottom without significant biological activity will likely retain a much larger share of microplastics than a river whose bottom is muddy and covered with a rich layer of biofilm. Bare sandy areas thus become potential hotspots for microplastic accumulation, while biologically active areas have a greater capacity for self-cleaning.

Practical applications: Where to look for microplastics?

This research provides an invaluable "tool" for ecologists and environmental protection agencies. It provides clear guidelines on where monitoring and remediation efforts should be focused. Instead of random sampling, it is now possible to identify with greater certainty the habitats that are more susceptible to the accumulation of these dangerous particles. For example, in complex ecosystems like mangrove forests, the outer, sandy edges exposed to stronger currents are likely to be places with high concentrations of microplastics. On the other hand, the inner zones, where the bottom is muddy and rich in biofilm, could contain significantly less plastic waste.

This approach allows for a more efficient use of resources and the direction of protective measures to where they are most needed. The identification of sandy outer regions as potential hotspots makes them priority zones for monitoring and protection. Moreover, these findings open the door to new pollution mitigation strategies. It is suggested that environmental restoration measures, such as reforesting coastal areas with plants that promote biofilm growth, could actively help reduce the accumulation of microplastics in aquatic systems. It highlights the powerful role that biological and physical factors play in shaping particle transport processes, offering nature-based solutions to a human-made problem.

Source: Massachusetts Institute of Technology