Microbial responses underlying the denitrification kinetic shifting exposed to ng/L- and μg/L-level lomefloxacin in groundwater.

The emerging co-contaminant of antibiotics and nitrate has acquired great concerns worldwide, which poses a potential impact on denitrification in the ecological environment, but little is known about the groundwater system at lower antibiotic concentration, especially ng/L-level. Herein the frequently detected Lomefloxacin (LOM) in groundwater was selected to explore its influences on denitrification kinetics and microbial dynamic responses. The NO3--N removals in ng/L-μg/L LOM-amended reactors (8.7-44.9%) performed far lower than that in control (76.1%). LOM can inhibit denitrification even at ng/L-level. The kinetic characteristic shifted from zero- to first-order once inhibition occurred. This observation is the synergistic effects of microbial community, enzyme activity, and antibiotic resistance genes (ARGs). The enzyme activities were inhibited immediately, whereas microbial community and ARGs exhibited hysteresis responses at ng/L-level. The enrichment of non-corresponding ARG types suggested LOM's co-selection effects. Brevundimonas were potential antibiotic resistant bacteria. Exposed to μg/L-level LOM, denitrification underwent a 6-d lag phase. The more sensitive enzyme activities and microbial community and the enrichment of ARGs with less abundance were investigated. These findings clarify the microbial response mechanism underlying the denitrification kinetic shifting exposed to low-concentrations of LOM, which is the potential process for heightening nitrate accumulation in groundwater.