Marine　recirculating　aquaculture　systems　(marine　RAS)　are　increasingly　utilized　for　their　high　productivity　and　minimal　environmental　impact.　However,　effective　nutrient　management　and　biosecurity　measures　remain　crucial　for　sustainable　operation　of　marine　RAS.　This　study　successfully　established　a　simultaneous　nitrification　and　denitrification　(SND)　process　using　a　biofloc　technology　(BFT)–ultrafiltration　(UF)　combined　approach.　A　stable　autotrophic　marine　nitrification　system　was　rapidly　initiated　within　23　days　using　a　ceramic　ring　(CR)–granular　activated　carbon　(GAC)–UF　process　(CGUF),　followed　by　the　substitution　of　polyhydroxyalkanoates　(PHA)　for　GAC　filter　to　achieve　rapid　start-up　of　SND　in　24　h　(CG/PUF).　Soluble　reactive　phosphate　(SRP)　was　removed　in-situ　via　biofloc　adsorption.　The　co-existence　of　ammonia-oxidizing　archaea　(Candidatus_Nitrosopumilus)　and　denitrifying　bacteria　(Stenotrophomonas,　Ruegeria　and　Ilumatobacter)　synergistic　promoted　the　SND　start-up　in　CG/PUF　process.　Stenotrophomonas　emerged　as　the　keystone　species　which　closely　linked　to　amoA/B/C,　hao　and　nosZ　genes　(p　−2∙　h−1　(LMH)　for　180　days,　with　a　2-day　intermittent　hydraulic　flushing　keeping　transmembrane　pressure　below　50　kPa.　These　findings　provide　insights　for　broader　application　of　BFT–UF　combined　process　in　marine　RAS.　©　2024　Elsevier　B.V.