Combined effects of application-specific plastic composition and environmental conditions on algal pigments and microbial activity of freshwater biofilms

Plastic pollution is extremely complex due to the variety of materials resulting from the mixing of synthetic polymers and chemical additives to match their intended applications. Such application-dependent chemical compositions complicate the assessment of plastics' ecotoxicological effects and impacts on biological communities and ecosystem functioning. Our study investigates how plastics affect freshwater microbial community composition and metabolic activity. To that end, we selected eight plastic materials from four polymer types and grouped them by their regulatory classification application ("medical or food grade" vs. "non-medical or food grade") to distinguish material effects from application-dependent chemical composition. After six weeks of biofilm development in three freshwater basins, we analyzed microbial biomass, metabolic functions (photosynthesis, respiration, esterase activity), and algal pigment composition on plastic surfaces and on a chemically inert control. We found that microbial biomass and metabolic activity were more controlled by environmental factors (e.g. suspended solids and light exposure) than by the polymer material and its application, except for esterase activity which was significantly higher in biofilms found on plastics than those found on the control. Algal pigment composition was controlled by both application-dependent plastic composition and environmental conditions. Our study emphasizes the need to consider material properties (e.g. polymer type), intended application (e.g. specific chemical composition) and environmental conditions (e.g. light, suspended solids) to better understand plastic-microbe interactions, which would allow for improved guidance and consequently effective future legislation on polymers and chemical additives.