Nominal Concentration-Based Assessment of Structure-Dependent PFAS Responses in the Freshwater Diatom Cyclotella meneghiniana Using Dual Physiological Endpoints

Per- and polyfluoroalkyl substances (PFAS) are persistent, structurally diverse contaminants whose ecological hazard assessment remains challenging, particularly for primary producers. Conventional growth-based algal bioassays often overlook sublethal responses associated with chronic exposure. Here, we developed a 7-day multigenerational bioassay using the freshwater diatom Cyclotella meneghiniana that integrates chlorophyll fluorescence (photosynthetic biomass) and lipid accumulation (oxidative stress and carbon reallocation) as complementary endpoints. Ten PFAS spanning carbon chain lengths from C4 to C11 and representing both perfluoroalkyl carboxylates and sulfonates were evaluated under identical nominal exposure conditions. Distinct structure-dependent toxicity patterns emerged. Long-chain perfluoroalkyl carboxylic acids elicited the strongest responses, whereas short-chain compounds produced no measurable effects. Among the sulfonates, PFHxS reduced chlorophyll fluorescence with limited effects on lipid accumulation, whereas PFOS produced no measurable response. Exploratory species sensitivity distribution analyses indicated compound-specific differences, although predicted no-effect concentrations should be interpreted cautiously because they combine literature-derived growth endpoints with endpoint-specific values generated in this study. This dual-endpoint diatom bioassay provides a proof-of-concept platform for structure-informed PFAS hazard ranking rather than regulatory toxicity assessment.