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　nitrificationaerobic　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.