Conventional　biological　nutrient　removal　processes　rely　on　external　aeration　and　produce　significant　carbon　dioxide　(CO2)　emissions.　This　study　constructed　a　phototrophic　simultaneous　nitrification-denitrification　phosphorus　removal　(P-SNDPR)　system　to　treat　low　carbon　to　nitrogen　(C/N)　ratios　wastewater　and　investigated　the　impact　of　sludge　retention　time　(SRT)　on　nutrient　removal　performance,　nitrogen　conversion　pathway,　and　microbial　structure.　Results　showed　that　the　P-SNDPR　system　at　SRT　of　15　days　had　the　highest　nutrient　removal　capacity,　achieving　over　85%　and　98%　removal　of　nitrogen　and　phosphorus,　respectively,　meanwhile　maintaining　minimal　CO2　emissions.　Nitrogen　removal　was　mainly　through　assimilation　at　SRTs　of　5　and　10　days,　and　nitrification-denitrification　at　SRTs　of　15　and　20　days.　Stable　partial　nitrification　was　facilitated　by　photoinhibition　and　low　DO　levels.　Flow　cytometry　sorting　technique　results　revealed　SRT　drove　community　structural　changes　in　translational　activity　(BONCAT+)　microbes,　where　BONCAT+　microbes　were　mainly　simultaneous　nitrogen　and　phosphorus　removal　bacteria　(Candidatus　Accumulibacter),　denitrifying　bacteria　(Candidatus　Competibacter　and　Plasticicumulans),　ammonia-oxidizing　bacteria　(Nitrosomonas),　and　microalgae　(Chlorella　and　Dictyosphaerium).　The　P-SNDPR　system　represents　a　novel,　carbon-neutral　process　for　efficient　nutrient　removal　from　low　C/N　ratio　wastewater　without　aeration　and　external　carbon　source　additions.