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.　©　2024　Elsevier　Ltd