Aquatic Biofilms—Sink or Source of Microplastics?

In this post, Gabriela Kalčikova talks about her recently published paper on microplastics being a source or sink for aquatic biofilms. Microplastics interact with aquatic biofilms, which crucially impacts the fate and behavior of microplastics in aquatic ecosystems.

Microplastics in aquatic environments

This recent Critical perspective paper discusses the importance of aquatic biofilms for the behavior, fate, and ecotoxicity of microplastics. The main question is, “do aquatic biofilms act as source or sinks for biofilms?”. Once microplastics enter aquatic ecosystems, they attract a diverse microbial community of bacteria, fungi and protozoa. Together with algae and diatoms, they form a biofilm on their surface (see Fig. 1). The growth of the biofilm on the microplastics can increase their particle size and density, stimulating microplastics to (slowly) sediment. During this process, microplastics may become ingested by various organisms because they cannot detect the plastic particles within biofilms. The presence of algae in biofilm can also “flavor” microplastics, thus attracting zooplankton. In addition, biofilms on microplastics have a much higher sorption capacity for various pollutants compared to pristine microplastics. This is important for assessing the toxicity of microplastics, their fate and effects of co-occurring pollutants.

Figure 1: A microplastic particle overgrown by a biofilm after four weeks of incubation in stream water (Photo by G. Kalčikova)

Biofilms: Source and Sinks?

In contrast to the implications of biofilm growth on microplastics and related consequences on their fate and effect, microplastics can also interact with substrate-associated biofilms (e.g., periphyton). These biofilms are complex microbial communities. They adhere to submerged surfaces such as rocks, sediments, and coarse particulate organic matter. They are also literally ubiquitous in aquatic ecosystems. Very small microplastics ≤1 μm (and of course nanoplastics) can penetrate the biofilm matrix. In the other hand, larger particles can adhere to the outer layer of biofilms. Over time, they can be overgrown by microorganisms and remain in biofilm. They can also undergo further fragmentation.

Microplastics trapped in the biofilm matrix can be remobilized with the detached part of the biofilm due to shear stress and seasonal changes, but also due to the action of pollutants (e.g. antibiotics, pesticides), which affect the structure and functions of the biofilm.

The immobilization, degradation/fragmentation, and release dynamics of microplastics in biofilms remain unclear. These knowledge gaps represent a significant blind spot. The reason being: processes involved may be critical to fully understand the fate of microplastics in the aquatic environment. This aspect is also reflected in microplastics under the influence of multiple interacting factors.

Here is also a link to a short video, if you want to see microplastics with a attached rotifer:

The paper “Aquatic Biofilms—Sink or Source of Microplastics? A Critical Reflection on Current Knowledge” was authored by Gabriela Kalčikova and Mirco Bundschuh and published as open access article in Environmental Toxicology and Chemistry.