Photo-enhanced　Biological　Phosphorus　Removal　(PEBPR)　systems,　promising　wastewater　treatment　technology,　offer　efficient　phosphorus　removal　without　external　oxygen.　However,　comprehending　the　impact　of　sludge　retention　time　(SRT)　on　the　system　is　crucial　for　successful　implementation.　This　study　investigated　the　SRT　effect　on　nutrient　fate,　microbial　community,　and　bacterial　phototolerance　in　PEBPR　systems.　PEBPR　systems　exhibited　good　bacterial　phototolerance　at　SRT　of　10,　15,　and　20　d,　with　optimal　phosphorus-accumulation　metabolism　observed　at　SRT　of　10　and　15d.　However,　at　SRT　of　5d,　increased　light　sensitivity　and　glycogen-accumulating　organisms　(GAOs)　growth　resulted　in　poor　P　removal　(71.9%).　Accumulibacter-IIC　were　the　dominant　P　accumulating　organisms　(PAOs)　at　SRT　of　10,　15,　and　20　d.　Accumulibacter-I,　IIC　and　IIF　were　the　major　PAOs　at　SRT　of　5　d.　The　decrease　in　SRT　promoted　the　microalgal　population　diversity,　and　Dictyosphaerium　and　Chlorella　were　the　major　microalgal　species　in　this　study.　Flow　cytometry　results　revealed　high　light　intensity　triggered　intracellular　Fe2+　efflux,　limiting　translation　activity　and　metabolism.　Moreover,　PAOs　had　lower　phototolerance　than　GAOs　due　to　Poly-P　bound　intracellular　Mg2+　affecting　enzyme　activity.　This　study　provides　an　in-depth　understanding　of　PEBPR　systems　operation　strategy　toward　environmentally　sustainable　wastewater　treatment.　©　2023　Elsevier　Ltd