Effects of sulfamethoxazole on nitrogen transformation and antibiotic resistance genes in short-cut nitrification and denitrification process treating mariculture wastewater

• High NH 4 + -N and TIN removal was realized during 0.0-1.2 mg/L SMX. • 2.0 mg/L SMX deteriorated NH 4 + -N removal by inhibiting AOB and nitrification genes. • SMX significantly affected the electron donors and transfer related genes. • SMX dose induced the diversity of ARB and ARGs. The influential mechanism of sulfamethoxazole (SMX) on short-cut nitrification and denitrification (SCND) and antibiotic resistance genes (ARGs) was evaluated in a sequencing batch reactor (SBR). Excellent removal for NH 4 + -N and total inorganic nitrogen (TIN) was realized with above 95.7 and 93.0% under 0.0-1.2 mg/L SMX, while 2.0 mg/L SMX induced the deterioration of their removal to 53.5 and 55.2%, respectively. SMX altered the microbial composition by showing positive correlation to microbes, resulting in the dominant microorganisms changing from Denitromonas to Vibrio and occurrence of more antibiotic resistance bacteria (ARB). SMX dosage suppressed significantly ammonia oxidizing bacteria (AOB) Nitrosomonas , but enriched and diversified the denitrifying bacteria (DNB). 2.0 mg/L SMX led to the emergence of heterotrophic nitrification-aerobic denitrification (HNAD) as a nitrogen removal pathway and strengthened the sulfide-oxidizing autotrophic denitrification (SOAD) process by enrichment of SOAD bacteria ( Candidatus_Thiobios , Sedimenticola ). Metagenomic analysis revealed that 2.0 mg/L SMX decreased the NH 4 + -N removal by reducing nitrification related genes ( amoABC , hao ) at genetic level. Compared with 0.0 mg/L SMX, 2.0 mg/L SMX significantly inhibited the electron supply for heterotrophic denitrifying process by restraining the carbon source metabolism related genes (such as ACSS , sucC , mdh ), but stimulated the electron generation for autotrophic denitrification by enhancing sulfide oxidation related genes aprA and aprB . Moreover, 2.0 mg/L SMX induced higher diversity and relative percentage of antibiotic resistance genes (ARGs) but barely affected the potential ability of dissemination of ARGs, revealing SCND was a promising system for controlling transmission of ARGs under SMX treating mariculture wastewater.