Résumé
Per- and polyfluoroalkyl substances (PFAS), often referred to as "forever chemicals," encompass over 14,000 compounds used across industries such as food packaging, textiles, firefighting foams, cosmetics, and aeronautics[1]. Since the 1950s, their extreme persistence and widespread use have led to global contamination of ecosystems, causing significant threats to human and environmental health. Recently introduced short-chain and next-generation PFAS, developed as supposedly less bioaccumulative replacements for phased-out legacy compounds, are increasingly linked to similar environmental and health risks[2]. In this context, understanding the environmental behaviour of these substances, particularly in aquatic ecosystems where they are now ubiquitous and contaminate all levels of the trophic network, is essential. Periphytic microbial communities, structured as biofilms, are key players in the bioaccumulation of organic pollutants in aquatic ecosystems. Those biofilms, composed of bacteria, microalgae, protists, and fungi embedded in a matrix of extracellular polymeric substances (EPS), have been shown to accumulate PFAS and therefore, have been proposed as potential integrative samples for water monitoring[3], [4]. However, the capacities of bioaccumulation and the mechanisms by which PFAS are sorbed by biofilms remain poorly understood, due to the lack of studies conducted under controlled conditions. Moreover, little is known about the behaviour in environmental matrices of next-generation PFAS, although their bioaccumulative properties have already been highlighted in the literature [3], [4]. To tackle this challenge, a specifically designed and custom-built river mesocosm pilot system was used under controlled conditions (light/dark 12:12, temperature ≥19°C, nutrients, and flow ≈ 300 L/h). Within this dedicated setup, biofilm was exposed to environmentally relevant concentrations (100 ng/L) of legacy (PFOS) and next-generation PFAS (6:2 FTS) for two months. Following the exposure, the removal of PFAS within the mesocosm, as well as their bioaccumulation in the biofilm, was quantified using LC-MS/MS (Waters Xevo TQ-XS). Additionally, the EPS was characterized in terms of quantity and its components (proteins, polysaccharides, and humic acids), along with biomass and microbial community descriptors (surface coverage assessed via microscopy and chlorophyll-a content). Marked differences were observed between PFOS and 6:2 FTS in terms of removal efficiency, bioaccumulation factors, and associated descriptors, reflecting distinct bioaccumulation and biotransformation behaviours. The identified descriptors were mainly related to EPS composition, highlighting its key role in PFAS sorption, retention, and environmental fate. Overall, these results improve our understanding of PFAS–biofilm interactions and the mechanisms governing PFAS transfer, accumulation, and persistence in natural aquatic ecosystems.