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The impact of carbon/nitrogen (C/N) ratio on sequencing batch biofilm reactor (SBBR) initiated with different seed sludges for treating actual mariculture effluent was explored. Increasing the C/N ratio significantly enhanced the nitrogen removal efficiency, achieving average removal efficiency of 95% for ammonia nitrogen and 73% for total nitrogen at ratio of 30, while the impact of seed sludge was minimal. High C/N ratio promoted the secretion of tightly bound extracellular polymeric substances (TB-EPS), which showed significant correlation with nitrogen removal. Interactions between bacteria and archaea were enhanced and conditionally rare or abundant taxa were the keystone taxa. High C/N ratio inhibited the relative abundance of ammonia-oxidizing archaea (Candidatus_Nitrosopumilus) and bacteria (Nitrosomonas), but promoted the heterotrophic nitrification-aerobic denitrification bacteria (Halomonas). The expression of nitrogen removal functional genes significantly correlated with functional genera. This study emphasized the crucial role of high C/N ratios in biological nitrogen removal from actual mariculture effluent., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

The construction of aerobic denitrification (AD) systems in an antibiotic-stressed environment is a serious challenge. This study investigated strategy of cyclic stress with concentration gradient (5-30 mg/L) of sulfamethoxazole (SMX) in a sequencing batch reactor (SBR), to achieve operation of AD. Total nitrogen removal efficiency of system increased from about 10 % to 95 %. Original response of abundant-rare genera to antibiotics was changed by SMX stress, particularly conditionally rare or abundant taxa (CRAT). AD process depends on synergistic effect of heterotrophic nitrifying aerobic denitrification bacteria (Paracoccus, Thauera, Hypomicrobium, etc). AmoABC, napA, and nirK were functionally co-expressed with multiple antibiotic resistance genes (ARGs) (acrR, ereAB, and mdtO), facilitating AD process. ARGs and TCA cycling synergistically enhance the antioxidant and electron transport capacities of AD process. Antibiotic efflux pump mechanism played an important role in operation of AD. The study provides strong support for regulating activated sludge to achieve in situ AD function., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)