A Plastic Problem with a Microbial Twist
Our planet faces a mounting challenge: it is awash in plastic pollution. Microplastics (MPs), those tiny plastic fragments less than 5 millimeters in size, are everywhere – in our water, soil, air, and even ice. Plastics vary greatly in chemical composition and their physical properties and can be altered by environmental exposure. These MPs are particularly abundant in freshwater, posing a growing threat to both ecological health and potentially human well-being. But the story gets more complex: microplastics aren’t just inert debris; they are miniature habitats teeming with microbial life that colonize plastic surfaces, forming complex communities known as the “plastisphere”. A biofilm is a key component of the plastisphere, inhabited by diverse microorganisms like bacteria, fungi, algae, and protozoa, and varies depending on the environment, season, and location. This variation, in turn, may influence how these microplastics interact with aquatic organisms.
The «Gustatory Trap» Hypothesis:
One of the most concerning aspects of plastic pollution is that fish and other aquatic creatures consume microplastics. It’s well-documented that plastic particles are showing up in the digestive tracts of various animals, but why? Do they mistake it for food?
Our research suggests that biofilms growing on microplastic surfaces may create a “gustatory trap”. Imagine a fish encountering a piece of plastic covered in a “tasty” layer of microbes. The biofilm might make the plastic seem like food, through taste, visual cues, or perceived nutritional value, drawing the fish in. However, if the fish is unable to distinguish the plastic from their natural diet, it leads to potential ingestion. This can disrupt feeding behavior, increase vulnerability to predators, expend unnecessary energy, and potentially disrupt food webs and energy transfer.
Our Experiment: Creating a Real-World Plastisphere
We designed a mesocosm experiment mimicking the natural environment of the climbing perch. We created a unique in situ experimental setup within the species' habitat, enabling us to observe natural biofilm formation on EPS pellets without risking further pollution. We exposed floating microplastics to the Am Chua irrigation canal for two, six, and fourteen days, allowing biofilms to develop under authentic environmental conditions. The pellets visibly changed color, from white to yellowish-green, as a diverse community of microorganisms (protozoa, cyanobacteria, algae, amoebae, and fungi) colonized the surface. These “biofouled” pellets closely resembled real-world microplastic particles found in polluted waters. We then observed how the fish responded to these biofouled pellets compared to clean EPS and natural food.
What We Discovered: The Biofilm Effect
Our findings revealed that climbing perch did frequently grasp the biofouled EPS pellets but often rejected them after oral testing.
· Climbing perch more frequently grasped the floating EPS pellets with a mature biofilm, likely due to visual and/or chemical cues mimicking food
· Fish ultimately rejected all EPS pellets after intraoral testing, while all feed pellets were ingested
· Despite the initial attraction, the fish rejected the plastic, suggesting the biofilm imparted a taste that anabas found unpalatable
· Fish grasped the well-developed biofilm plastic more frequently and spent less time “testing” it with their mouths.
We believe the climbing perch employs a multi-step decision-making process regarding the potential food-object: first, visual assessment as initial attractiveness to the fish (Does it look like food?); then, olfactory evaluation upon approaching to the pellets (Does it smell like food?); and finally, grasping and oral testing (How does it feel and taste in the mouth?). This integrated sensory information helps the fish decide whether to swallow or reject the object. While the biofouling process seems to act as a deterrent = preventing ingestion, ultimately, the presence of a biofilm diminished the climbing perch”s ability to avoid contact with the plastic debris, causing the fish to express their natural foraging behavior that likely results in energy expenditure and increased predation risk. Thus, this indicates that MP debris can induce behavioral changes in fish without requiring them to ingest the plastic.
Ecological Consequences: More Than Just Ingestion
This study highlights the complex interactions between microplastics, microbial communities, and aquatic life. Even without ingesting the plastic, the presence of biofouled microplastics can have ecological consequences. Floating plastics stimulate foraging behavior in climbing perch, prompting them to move towards the surface, potentially increasing energy expenditure and predation risk with no benefit. Our study shows that the mere presence of microplastic debris can induce behavioral changes in aquatic organisms.
Looking Ahead: A Call for More Research
While our study provides valuable insights into the feeding ecology of climbing perch, it’s important to remember that sensory-mediated feeding behaviors can vary greatly among fish species. The type of plastic, its size and buoyancy, and the duration of exposure can all influence how different fish respond.
Future research should focus on comparative studies across diverse fish species, detailed characterization of biofilm composition, and assessments of toxicity in real-world ecosystems. This will give us a clearer picture of the true risks that plastic pollution poses to aquatic food webs. By understanding how biofilms influence these interactions, we can develop more effective strategies to mitigate the impact of plastic pollution on our planet.
The study is published: Floating Microplastics with Biofilm Changes Feeding Behavior of Climbing Perch Anabas testudineus, Microplastics 2025, 4(3), 62; https://doi.org/10.3390/microplastics4030062