Reconsidering anaerobic biotransformation of per- and polyfluoroalkyl substances (PFAS) through coupled N/Fe/S/C biogeochemical cycling.

Per- and polyfluoroalkyl substances (PFAS) are persistent and bioaccumulative compounds that pose long-term risks to ecosystems and human health. While PFAS were long considered biologically inert under anaerobic conditions, emerging evidence shows that selected PFAS can undergo microbial biotransformation and, in some cases, biodefluorination in biogeochemically active environments. However, the current understanding remains constrained, as PFAS biotransformation has been interpreted through molecular structure, individual microbial strains, or isolated enzymes, while the concurrent nitrogen, iron, sulfur, and carbon (N/Fe/S/C) biogeochemical cycles that define redox regimes, electron flow, and microbial community assembly are rarely treated as key factors. This review reframes anaerobic PFAS biotransformation through the perspective of coupled biogeochemical processes. We first systematically summarize the current evidence on anaerobic PFAS biotransformation, then examine how PFAS molecular structure and redox environment regulate transformation kinetics, efficiency, and pathways, with particular attention to the distinct defluorination behavior of polyfluorinated PFAS. We further discuss how denitrification, iron reduction, sulfur cycling, and methanogenesis shape the PFAS-biotransforming environment by regulating electron availability, microbial assembly, and metabolic interactions. In addition, molecular mechanisms involving localized oxygenase or hydroxylative activation, reductive C-F bond cleavage, electron bifurcation, and fluoride detoxification are evaluated. By incorporating PFAS biotransformation into the context of natural elemental cycling, this review provides a conceptual framework for predicting PFAS fate, assessing biodefluorination, and improving treatment strategies by considering redox conditions and biogeochemical processes.