Wastewater from marine recirculating aquaculture systems (RAS) poses significant treatment challenges due to its high nitrate concentrations and low carbon-to-nitrogen ratio. The additional presence of antibiotic residues such as florfenicol creates mixed pollution that further impairs denitrification and destabilizes microbial communities. This study systematically compared the multi-scale responses of heterotrophic (HDN), sulfur-autotrophic (SADN), and mixotrophic (MDN) denitrification under 4.0 mg L⁻¹ florfenicol stress in synthetic recirculating mariculture wastewater (salinity = 3.5 %). The MDN system achieved the highest denitrification rate (46.83 mg NOₓ⁻-N h⁻¹ g⁻¹ MLVSS) and maintained nitrate removal efficiency above 98 % with minimal fluctuations under florfenicol stress. Although the HDN system exhibited high NAR/NIR activities and electron transport system activity (ETSA), nitrite accumulation occurred due to functional imbalance. The SADN system, relying on sulfur oxidation, demonstrated limited metabolic flexibility and electron supply capacity under florfenicol stress, resulting in slower nitrate reduction. Metagenomic and KEGG pathway revealed significant differences in microbial community composition and functional potential among HDN, SADN and MDN denitrification systems: HDN was dominated by biofilm-forming Proteobacteria, with broad antibiotic resistance gene (ARG) enrichment and stronger mobile genetic element co-localization; SADN showed enrichment of Campylobacterota, consistent with sulfur oxidation-based pathways, and Mantel tests confirmed tighter ARG-environment coupling; MDN showed the highest microbial diversity with elevated Planctomycetota, functional enrichment in carbohydrate and energy metabolism, and stable quorum sensing-electron transfer coupling with functional redundancy. These findings provide new insights for designing robust treatment strategies for recirculating mariculture wastewater subject to compound pollution from nitrate and antibiotics.