Accumulation of polypropylene microplastics within freshwater substrate biofilms alters microbial assembly processes and functionality

Abstract Freshwater biofilms are vital to biogeochemical cycling in aquatic ecosystems. However, there has been limited in-depth investigation into the impact of polypropylene microplastics (PP-MPs)—newly emerging pollutants—on their functional performance. This study investigated the effects of PP-MPs on the physiology, community structure, and functional performance of freshwater biofilms. Experimental analyses were conducted to assess the accumulation of 200 μm PP-MPs by biofilms, along with physiological and ecological responses, and alterations in microbial community structure and function. The results demonstrated significant accumulation of PP-MPs by biofilms, with the MP concentration within biofilms increasing with water-column MP concentration, yet reaching a saturation threshold when the water-column concentration reached 150 mg/L. Physiological assessments revealed that low-concentration PP-MPs (≤100 mg/L) promoted extracellular polymeric substance (EPS) secretion, whereas high concentrations (≥150 mg/L) suppressed EPS production and decreased chlorophyll-a content, suggesting a potential negative effect on photosynthetic activity and carbon fixation. Elevated lactate dehydrogenase activity indicated cell membrane damage under high PP-MPs exposure conditions. Microbial community analysis revealed that high concentrations of PP-MPs significantly reduced biofilm α-diversity and modified the relative abundance of dominant phyla (e.g., Cyanobacteria and Proteobacteria), suggesting potential effects on carbon, nitrogen, iron, and phosphorus cycling. Functional profiling further revealed disruptions in key metabolic pathways, including pentose metabolism and unsaturated fatty acid biosynthesis. These findings provide critical insights into the ecological risks of PP-MPs in freshwater ecosystems.