Masked ecological risk: Stable anammox performance conceals resistance genes propagation under short-term...

Dioctadecyldimethylammonium chloride (DODMAC) and potassium sorbate (PS) are two prevalent non-antibiotic antimicrobials that frequently co-exist in sewage. However, their impacts on anaerobic ammonium oxidation (anammox) systems remains unclear. This study investigated the short-term effects of single and combined DODMAC (0.5 and 8 mg/L) and PS (5 and 20 mg/L) stress over 40 operational cycles. Remarkably, anammox activity was not inhibited, and a metabolic shift led to pronounced NO₃^--N accumulation under 8 mg/L DODMAC stress. Total nitrogen removal efficiency was maintained or slightly enhanced under PS stress due to stimulated denitrification. However, this seemingly stable performance under short-term stress masked a critical ecological threat: significant enrichment of resistance genes (RGs). The abundance of intracellular RGs increased substantially, with intI1 (an integron gene) reaching up to 8.5 times that of CK under the stress of DODMAC and/or PS. Network analysis identified Bdellovibrio, Dokdonella and Acinetobacter as key potential RGs hosts enriched (p < 0.05). Horizontal gene transfer mediated by mobile genetic elements (especially intI1) was the primary driver of RGs dissemination. These findings demonstrated that functional stability did not equate to ecological safety. This highlighted the urgent need to look beyond conventional performance metrics when assessing the environmental impacts of emerging contaminants.