The entrainment of polyester microfibers modifies the structure and function of periphytic biofilms

Abstract As plastics undergo degradation, they give rise to microplastics (MPs), such as polyester microfibers (PMFs), which are increasingly recognized for their potential impact on microbial communities. Despite a growing body of the literature on MP effects, there is a gap in understanding prolonged PMF exposure (≥ 1 month) on stream periphyton across an extensive concentration gradient. This study addresses this gap by investigating the response of periphyton exposed to increasing PMF concentrations (0–22,000 PMF L −1 ) in stream mesocosms. As PMF concentrations increased, total periphyton biomass remained unaffected, while algal and bacterial biomass decreased and increased, respectively. Higher PMF concentrations also modified coarse algal community structure (measured as changes in chlorophyll b : c ) and decreased light harvesting efficiency. Increased bacterial abundance was accompanied by elevated respiration, shifting the system from net autotrophy to net heterotrophy at 10,000–12,000 PMF L −1 . Additionally, bacterial community composition was altered along with reductions in β -1,4-glucosidase activities. Despite reduced algal biomass, higher PMF concentrations appeared to support bacterial growth. Many periphyton attributes, including nutrient composition, phosphorus removal, gross primary production, and maximum electron transport rate of photosystem II, were unaffected. This study underscores the multifaceted implications of PMF contamination on the structure and function of periphyton in stream ecosystems.