Lomefloxacin-induced stress on groundwater denitrification and its recovery potential.

The co-contamination of antibiotics and nitrate has raised global concerns, as antibiotics exert an inhibitory effect on denitrification in the environment. However, the recovery potential of groundwater systems exposed to low antibiotic concentrations, especially at ng/L levels, remains underexplored. This study focused on lomefloxacin (LOM)-a contaminant frequently detected in groundwater-and investigated denitrification recovery dynamics within 58 days following acute exposure to LOM via batch experiments.The results indicated that, in comparison to the carbon source, the impact of LOM at groundwater-relevant concentrations (ng/L - μg/L) on denitrification is a secondary factor. Denitrification inhibition by LOM is reversible. Denitrification can recover up to ∼80% after 12 days at concentrations ranging from 10 ng/L to 100 μg/L, while the 1 mg/L LOM group demonstrated 45.8% recovery after 41 days. The recovery time and extent of denitrification are determined by the concentration effect of antibiotics. At low concentrations, LOM induced quorum sensing, sequentially regulating carbon metabolism and multidrug resistance mechanisms, thereby facilitating the rapid recovery of denitrification after a short-term inhibition. Conversely, at high concentrations, LOM exerted bactericidal or bacteriostatic effects, suppressing denitrification. Despite the subsequent growth of fluoroquinolone (FQ) resistant bacteria and carbon source supplementation, the sustained inhibition of the cell cycle by LOM resulted in a slow and limited recovery of denitrification. This emphasized the challenges in recovering from occasional high antibiotic pollution events and highlighted the secondary risks linked to prolonged antibiotic exposure, such as the dissemination of FQ- and multidrug-resistant genes.