From Slime to Supper: How freshwater biofilms respond to microplastic pollution and shape food-web transfer

Researchers collecting environmental biofilm samples

Microplastics (particles <5 mm in size) are environmentally ubiquitous contaminants that have garnered significant attention from both the media and researchers in recent years. Broadly speaking, there are two main types of microplastic pollution. Primary microplastics are intentionally produced and originate from commercial or industrial sources, such as cosmetic exfoliant beads and plastic pellets used for extrusion and injection molding. In contrast, secondary microplastics are generated through the weathering and breakdown of larger plastic products, such as fragments from a water bottle you forgot to recycle or a plastic bag that blew past you on a windy walk.

Among all forms of microplastics, microfibres are by far the most abundant in studies of aquatic environments and biota.

Fast Fact: Researchers have found that a single load of laundry (6 kg) from a household of four can release up to 18 million microfibres per wash.

No matter the source, microplastic particles are easily transported through surface runoff or wind into a wide range of environments, with the majority eventually making their way into surface waters. My research focuses on the interactions between microplastic contaminants and freshwater biofilms, helping to inform the fate, transport, and ecological effects of microfibres in Canada’s rivers.

Red polyester microfibres isolated from laboratory-cultured biofilm samples

What is a biofilm?

Biofilms are the slimy films that grow on the surfaces of submerged objects in aquatic environments. They are composed of complex communities of microorganisms and play a key role in mediating nutrient cycling within these ecosystems. Biofilms form the foundation of aquatic food webs and support a variety of higher trophic organisms, including benthic invertebrates and some fish species.

In nutrient-poor aquatic systems and under flow conditions where plants struggle to grow, biofilms can serve as the primary driver of ecosystem productivity. Studying these slimy wonders helps fill a critical gap in our understanding of the bottom-up effects that microplastics may have on whole freshwater ecosystems.

Once microplastic pollution enters the water column, it quickly sinks and interacts with benthic biofilms. Preliminary results from my research indicate that polyethylene terephthalate (PET) microfibres rapidly partition into the biofilm matrix in as little as 24 hours, with partitioning increasing proportionately to microfibre concentration. While the magnitude of partitioning plateaus at high concentrations, it can be further enhanced by the addition of phosphorus, a nutrient that is present in nearly all of Ontario’s freshwater systems as a result of agricultural activity.

We observed both structural and functional biofilm responses to microplastic exposure, including shifts in bacterial community composition and a tendency for certain bacterial taxa to aggregate around microfibres. These changes result in biofilms with a more clumped, rather than evenly dispersed, structure.

Confocal laser scanning microscopy (CLSM) imaging of low (left) and high (right) concentration microplastic treatments in laboratory cultured biofilms. Note the green and yellow (denoting bacteria) aggregating around the red microfibres.

What’s next?

The next stage of my research aims to characterize the potential for trophic transfer of microfibres embedded within laboratory-cultured biofilms. To do this, biofilms containing microfibres will be fed to the aquatic grazing file ramshorn snail (Planorbella pilsbryi). This work will provide key insights into whether biofilms act as a source or a sink for microplastic pollution, and how these contaminants may move upward through freshwater food webs.

Written by Daniel S.