Microbial colonization and function of biofilms developing on plastics and bioplastics in a pristine mountain stream ecosystem

Abstract Streams naturally receive allochthonous particulate organic materials, but human activity may contribute additional plastic litter inputs, which can affect ecosystem functioning. Our objective was to assess the effect of plastic and bioplastic inputs on microbial biomass and function in a pristine mountain stream. To do that, fragments of plastics (polyethylene—PE, polypropylene—PP), bioplastics (polyhydroxyalkanoate—PHA, polylactic acid—PLA), and wood (as a natural material) were immersed in the stream and collected after 120, 202, 316, and 383 d. Fungal, prokaryotic, and algal biomass, and autotrophic and heterotrophic functions (net primary production, extracellular enzyme activities, and nutrient uptake) were analyzed together with organic matter weight changes over time. Wood showed high fungal and prokaryotic biomass, phosphatase and β‐glucosidase activities, and a significant weight loss, as related to the degradation process. In contrast, plastics and bioplastics did not lose weight and were mainly colonized by algae, suggesting that they serve as an inert surface and over‐enhance primary production. However, phenol oxidase (ligninolytic enzyme) activity in plastics and bioplastics was similar to that in wood and increased with time together with heterotrophic biomass. This indicates that plastispheres can contribute to the degradation of plastics and bioplastics, which may become a carbon source in the long term. Overall, the observed dynamics of microbial biomass and metabolism in the plastispheres point to their potential effect on stream nutrient and carbon cycles. The study highlights the need for careful human activities in pristine mountain watersheds to avoid altering their ecosystem functioning.