The issue of microplastics accumulating in our environment—and even within our bodies—is becoming increasingly urgent. However, pinpointing where these pervasive particles tend to concentrate has proven challenging, given the multitude of factors influencing their distribution and deposition.
Recent research from MIT highlights one crucial factor in determining where microplastics are likely to accumulate: the presence of biofilms. These thin, sticky layers created by microorganisms can form on various surfaces, including sandy riverbeds and shorelines. The findings reveal that microparticles are less likely to settle in areas enriched with biofilms, as they become more susceptible to being resuspended by flowing water and carried away.
Published in the journal Geophysical Research Letters, the research was conducted by Hyoungchul Park, a postdoctoral researcher at MIT, and civil and environmental engineering professor Heidi Nepf. “Microplastics are a hot topic right now,” Nepf notes, “and we still don’t fully understand where the hotspots for their accumulation are. This research provides valuable guidelines on environmental factors influencing microparticle distribution.”
Most studies on microplastic transport have focused on bare sand surfaces. “In natural environments, microorganisms such as bacteria and algae create biofilms that can influence the sediment dynamics,” Park explains. The research aimed to clarify how these biological substances affect the movement of microplastics.
The study used a flow tank with a base of fine sand, sometimes incorporating vertical plastic tubes to mimic mangrove roots. In select experiments, the sand was mixed with biological material to replicate the biofilms typically found in various aquatic environments.
For three hours, water mixed with tiny plastic particles circulated through the tank, after which researchers photographed the sediment surface under ultraviolet light to measure the concentration of fluorescent microplastics.
The results revealed two phenomena influencing plastic accumulation on different surfaces. Near the simulated roots, turbulence limited particle deposition. Furthermore, as biofilm presence increased in the sediment, plastic accumulation decreased.
Nepf and Park concluded that biofilms occupy the spaces between sand grains, leaving fewer opportunities for microplastics to settle in. As particles were less embedded in the sediment, they were more exposed and easily resuspended by water movement.
“These biological films occupy pore spaces in the sediments,” Park elaborates. “Consequently, particles that make contact with the surface remain exposed to the flow forces, making it easier for them to be swept away. Our findings indicate that in environments with identical flow conditions, those without biofilms will exhibit significantly higher microplastic deposition than those with EPS.”
Nepf adds, “Biofilms essentially prevent plastics from sinking deeper into the sediment. They stay on the surface and are easily displaced. If we dumped microplastics into two rivers—one with sandy or gravelly bottoms and the other muddier with more biofilm—we’d expect greater retention in the sandy substrate.”
Other factors, such as water turbulence and sediment roughness, complicate this scenario. However, this research offers a valuable framework for understanding microplastic habitats and where to focus remediation efforts. “It aids researchers in identifying the environments where plastics may accumulate more, providing clear guidance on where to investigate further,” she explains.
For instance, in mangrove ecosystems, microplastics might accumulate in sandy outer edges rather than in biofilm-rich inner areas. Consequently, these outer zones could be targeted as priority areas for monitoring and protective measures.
Isabella Schalko, a research scientist at ETH Zurich not involved in this study, calls the findings relevant: “They imply that restoration strategies, such as re-vegetation or enhancing biofilm growth, could mitigate microplastic buildup in water systems. This research underscores the significant role biological and physical elements play in influencing particle transport.”
This research received funding from Shell International Exploration and Production through the MIT Energy Initiative.
Photo credit & article inspired by: Massachusetts Institute of Technology
