Fate and accumulation of legacy and emerging PFAS in periphytic biofilms in riverine mesocosms

Per- and polyfluoroalkyl substances (PFAS) have been widely used over the past 70 years, leading to contamination of ecosystems and raising concerns about their effects on the environment and health. Next-generation PFAS, designed as less bioaccumulative alternatives to legacy compounds, remain poorly characterized regarding their environmental fate and health effects. Understanding the environmental fate of these substances, particularly in aquatic ecosystems where they are ubiquitous and contaminate all trophic levels, is essential. Periphytic biofilms, composed of bacteria, microalgae, protists, and fungi embedded in extracellular polymeric substance (EPS), play a key role in PFAS bioconcentration and could serve as integrative samples for water monitoring. However, mechanisms of PFAS sorption by periphytic biofilms and the environmental fate of next-generation PFAS within these communities remain poorly understood due to limited controlled studies. To address this knowledge gap, biofilm in a controlled river mesocosm was exposed to environmentally relevant concentrations (100 ng. L⁻¹) of a legacy (PFOS) and a next-generation (6:2 FTS) PFAS for two months. This simplified ecosystem enabled investigation of bioconcentration dynamics and whether effects observed in natural biofilms are reproduced under controlled conditions, while monitoring EPS composition (proteins, polysaccharides, humic acids) and microbial descriptors (biomass, surface coverage, chlorophyll-a). PFAS bioconcentrated in the biofilm throughout the experiment (PFOS: 1228.4 ng.g⁻¹, logBCF 4.19; 6:2 FTS: 57.5 ng.g⁻¹, logBCF 2.93). Polysaccharide concentration decreased after two weeks, increasing the Protein/Polysaccharide ratio, particularly for PFOS until one month. These results provide insights into the bioconcentration potential of PFAS, the influence of biofilm composition, and the potential contribution of biotransformation processes.