Twin Threats in the Deepest Ocean: Novel PFAS Drive Exposure While Legacy PFAS Drive Risk

Although per- and polyfluoroalkyl substances (PFAS) are widespread in global ecosystems, their presence in the hadal zone, particularly that of novel compounds, remains unexplored. In this study, 15 PFAS were detected in amphipods from the Mariana, Mussau, and New Britain Trenches (ranging from 0.4-37.5 ng g^-1 dry weight), whereas all seawater and sediment samples fell below the detection limit. We quantified organism-water partitioning, analyzed structural and concentration similarity via COSMO-RS, applied neural networks to predict bioaccumulation, and prioritized PFAS risks using a persistence-bioaccumulation-toxicity framework. Short-chain novel PFAS (e.g., PFBA and PFPeA) formed the largest share of total PFAS loads (up to 4.2 ng g^-1 dw) but contributed minimally to risk. In contrast, long-chain PFAS (PFTrDA and PFUnDA), though less abundant, exhibited substantially higher risk potential. The novel compound F-53B was detected exclusively in Mariana amphipods. Overall, the accumulation patterns across PFAS classes reflect the combined influence of external exposure and internal partitioning constraints. These findings demonstrate that structural modification does not inherently reduce PFAS hazards and highlight the necessity of including hadal organisms in global chemical risk evaluation.