Research progress on the impact of antibiotics in groundwater systems on denitrification

The microbial denitrification process is the most important form of nitrate-nitrogen removal in groundwater. The reclaimed water reuse and livestock breeding caused antibiotic pollution usually co-occurs with nitrate. Therefore, it is necessary to conduct in-depth research on the effects of denitrification and the generation, accumulation, and dissemination of antibiotic resistance genes (ARGs) by antibiotics and their forms to comprehensively analyze the reasons for the increased concentration of nitrate in groundwater. Many studies in recent years have identified the dissociation/complexation, adsorption forms (interlayer adsorption/surface adsorption), and the products of hydrolysis and microbial degradation of antibiotics in groundwater systems; and have elucidated the inhibitory mechanism of antibiotics on the denitrification process from the perspectives of denitrifying microbial communities, the types and activities of functional enzymes, abundance of functional genes, as well as the production and transmission pathways of ARGs. The main conclusions are as follows: (1) In groundwater systems, antibiotics exist in various forms, and different forms of antibiotics exhibit significant differences in toxicity to microorganisms. (2) In the presence of antibiotics at levels ranging from nanograms to micrograms per liter, the denitrification process in groundwater is inhibited. Antibiotics alter the microbial community structure, suppress enzymatic activity, and increase the abundance of ARGs. Under the synergistic effects of these actions, the kinetics of nitrate degradation shift from zero-order to first-order. (3) During the antibiotic-induced inhibition of denitrification, there is also an increase in the emission of the greenhouse gas N₂O. Antibiotics primarily affect the expression of the functional gene nosZ, and the concentration of N₂O shows a negative exponential relationship with nosZ abundance. Based on the review of relevant literature, the prospects for future research are put forward: (1) quantitatively identifying the existence forms of typical antibiotics after entering groundwater systems; (2) elucidating the impact of antibiotics in different existence forms on denitrifying microbial communities, the types and activities of functional enzymes, and the abundance and diversity of functional genes; (3) exploring the dynamic process of denitrification functional genes under different existence forms and input modes of antibiotics, and establishing models for the production, enrichment, and transmission of ARGs; (4) combining field observations and laboratory experiments to study the denitrification process in groundwater systems under complex pollution from molecular biology, environmental chemistry, and hydrogeology perspectives. This research can provide a theoretical basis for addressing the increasingly complex groundwater pollution prevention and drinking water safety assurance.