Biofilms Found To Reduce Microplastic Accumulation

The pervasive problem of microplastic pollution in the environment—and increasingly, within our own bodies—is gaining global attention. Yet, despite growing concern, predicting where these microscopic pollutants are likely to accumulate remains a scientific challenge. The key reason lies in the complex interplay of environmental variables that govern how these particles move and settle. Now, new research from the Massachusetts Institute of Technology (MIT) sheds light on a critical factor influencing the distribution of microplastics: biofilms.

The study, published in the journal Geophysical Research Letters, highlights how the presence of biofilms—sticky biopolymer layers produced by microorganisms like bacteria and algae—affects the way microplastics behave in aquatic environments. According to the researchers, these biofilms can significantly reduce the accumulation of microplastics in sediments by increasing the likelihood that the particles will be resuspended and carried away by water flow.

This discovery offers valuable insights into where microplastics are most likely to build up, and how environmental restoration strategies could mitigate this accumulation. The paper was authored by MIT postdoctoral researcher Hyoungchul Park and civil and environmental engineering professor Heidi Nepf.

“Microplastics are definitely in the news a lot, and we don’t fully understand where the hotspots of accumulation are likely to be,” says Nepf. “This work gives a little bit of guidance” on the environmental factors that contribute to the transport and retention of microplastics and other fine particles.

Historically, most research on microplastic deposition has focused on flow over bare sand. But as Park points out, natural aquatic systems are far more complex. “In nature, there are a lot of microorganisms such as bacteria, fungi, and algae,” he explains. “When they adhere to the streambed, they generate some sticky things.” These substances—technically known as extracellular polymeric substances (EPS)—can drastically alter the sediment’s physical properties.

To simulate natural conditions and isolate the impact of biofilms, Park and Nepf designed a series of experiments in a flow tank with a bottom lined with fine sand. Some experiments included plastic rods to mimic mangrove roots, while others used a mix of sand and biological material to replicate the presence of biofilms. Water infused with tiny fluorescent plastic particles was pumped through the tank for three hours, and ultraviolet lighting allowed the researchers to measure where and how many particles settled.

The results were striking. In areas without biofilms, plastic particles were more likely to become embedded in the sand. In contrast, when biofilms were present, the accumulation of plastic particles decreased significantly. The researchers discovered that the biofilms filled the spaces between sand grains, reducing the depth to which microplastics could penetrate. As a result, the particles remained closer to the surface and were more susceptible to being swept away by water currents.

“These biological films fill the pore spaces between the sediment grains,” Park explains. “That makes the deposited particles more exposed to the forces generated by the flow, which makes it easier for them to be resuspended.”

Nepf elaborates: “The biofilm is blocking 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.” For example, if a large volume of microplastics were introduced into two rivers—one with a sandy or gravel bottom and another with a muddier, biofilm-rich bed—the sandy river would likely retain more of the microplastics.

This understanding not only clarifies how microplastics behave in varied sediment types but also serves as a tool for prioritizing monitoring efforts. It can help scientists and environmental agencies focus their efforts on specific areas that are more prone to accumulation.

Park adds that this framework may be especially useful in complex ecosystems like mangroves. The outer, sandier edges of such systems may become microplastic hotspots, while the interior areas with richer biofilm content may see less accumulation. Therefore, monitoring and conservation efforts might be most effective if concentrated on these sandy peripheries.

The implications of the findings are far-reaching. According to Isabella Schalko, a research scientist at ETH Zurich who was not involved in the study, the work suggests that restoration methods such as promoting vegetation or encouraging biofilm development could help mitigate microplastic buildup in rivers and coastal zones. “It highlights the powerful role of biological and physical features in shaping particle transport processes,” she says.

Funded by Shell International Exploration and Production through the MIT Energy Initiative, the study adds a crucial dimension to our understanding of microplastic pollution. In a world increasingly impacted by human-induced environmental change, such findings underscore the importance of incorporating biological dynamics into environmental policy and pollution control strategies.