Unraveling the mechanism of norfloxacin removal and fate of antibiotics resistance genes (ARGs) in the sulfur-mediated autotrophic denitrification via metagenomic and metatranscriptomic analyses

The co-contamination of antibiotics and nitrogen has attracted widespread concerns due to its potential harm to ecological safety and human health. Sulfur-driven autotrophic denitrification (SAD) with low sludge production rate was adopted to treat antibiotics laden-organic deficient wastewater. Herein, a lab-scale sequencing batch reactor (SBR) was established to explore the simultaneous removal of nitrate and antibiotics, i.e. Norfloxacin (NOR), as well as microbial response mechanism of SAD sludge system towards NOR exposure. About 80.78 % of NOR was removed by SAD sludge when the influent NOR level was 0.5 mg/L, in which biodegradation was dominant removal route. The nitrate removal efficiency decreased slightly from 98.37 ± 0.58 % to 96.58 ± 1.03 % in the presence of NOR. Thiobacillus and Sulfurimonas were the most abundant sulfur-oxidizing bacteria (SOB) in SAD system, but Thiobacillus was more sensitive to NOR. The up-regulated genes related to Xenobiotics biodegradation and metabolism and CYP450 indicated the occurrence of NOR biotransformation in SAD system. The resistance of SAD sludge to the exposure of NOR was mainly ascribed to antibiotic efflux. And the effect of antibiotic inactivation was enhanced after long-term fed with NOR. The NOR exposure resulted in the increased level of antibiotics resistance genes (ARGs) and mobile genetic elements (MGEs). Besides, the enhanced ARG-MGE co-existence patterns further reveals the higher horizontal mobility potential of ARGs under NOR exposure pressures. The most enriched sulfur oxidizing bacterium Thiobacillus was a potential host for most of ARGs. This study provides a new insight for the treatment of NOR-laden wastewater with low C/N ratio based on the sulfur-mediated biological process.