Short-chain perfluorobutyric acid (PFBA) disrupts anaerobic ammonium oxidation in riverbed sediments and reshapes the functional microbial community.

Perfluorobutanoic acid (PFBA), a representative short-chain perfluoroalkyl carboxylic acid, is increasingly detected in aquatic environments, yet its impacts on anaerobic nitrogen cycling remain poorly understood. Here, a laboratory-scale simulated river sediment system was established to investigate the concentration-dependent effects of PFBA (1 ng/L-50 mg/L) on Anammox performance, microbial communities, functional genes, and PFBA transformation. Low PFBA concentrations (≤1 μg/L) stimulated nitrogen removal, with NH4+-N conversion and NO2--N removal increasing by 30.8% and 38.9%, respectively. In contrast, inhibition emerged at 1 mg/L and became pronounced at ≥ 10 mg/L. At 50 mg/L, NO2--N and TN removal rates decreased to 7.58 and 5.22 μg/(g·h), respectively, and remained suppressed during recovery. High PFBA exposure reduced extracellular polymeric substances (EPS) -associated protein content and key Anammox enzyme activities, accompanied by downregulation of hao, hdh, and hzs and increased relative abundance of nrf, indicating a shift from autotrophic Anammox toward heterotrophic dissimilatory nitrate reduction to ammonium (DNRA). Notably, 91.7% of PFBA was apparently transformed at 50 mg/L, with shorter-chain products such as perfluoropropanoic acid and trifluoroacetic acid being qualitatively identified, suggesting sequential dealkylation and decarboxylation. However, a complete mass balance was not established because the transformation products were not quantified. These findings demonstrate that PFBA exerts concentration-dependent effects on sedimentary nitrogen cycling by disrupting Anammox metabolism and reshaping microbial functional pathways, providing new insights into per- and polyfluoroalkyl substances (PFAS) ecological risks and sediment bioremediation.