Low　temperatures　limit　the　denitrification　wastewater　in　activated　sludge　systems,　but　this　can　be　mitigated　by　addition　of　redox　mediators　(RMs).　Here,　the　effects　of　chlorophyll　(Chl),　1,2-naphthoquinone-4-sulfonic　acid　(NQS),　humic　acid　(HA),　and　riboflavin　(RF),　each　tested　at　three　concentrations,　were　compared　for　denitrification　performance　at　low　temperature,　by　monitoring　the　produced　extracellular　polymeric　substances　(EPS),　and　characterizing　microbial　communities　and　their　metabolic　potential.　Chl　increased　the　denitrification　rate　most,　namely　4.12　-fold　compared　to　the　control,　followed　by　NQS　(2.62　-fold　increase)　and　HA　(1.35　-fold　increase),　but　RF　had　an　inhibitory　effect.　Chl　promoted　the　secretion　of　tryptophan-like　and　tyrosine　-like　proteins　in　the　EPS　and　aided　the　conversion　of　protein　from　tightly　bound　EPS　into　loosely　bound　EPS,　which　improved　the　material　transfer　efficiency.　NQS,　HA,　and　RF　also　altered　the　EPS　components.　The　four　RMs　affected　the　microbial　community　structure,　whereby　both　conditionally　abundant　taxa　(CAT)　and　conditionally　rare　or　abundant　taxa　(CRAT)　were　key　taxa.　Among　them,　CRAT　members　interacted　most　with　the　other　taxa.　Chl　promoted　Flavobacterium　enrichment　in　low　-temperature　activated　sludge　systems　.　In　addition,　Chl　promoted　the　abundance　of　nitrate　reduction　genes　narGHI　and　napAB　and　of　nitrite　reduction　genes　nirKS　,　norBC　,　and　nosZ　.　Moreover,　Chl　increased　abundance　of　genes　involved　in　acetate　metabolism　and　in　the　TCA　cycle,　thereby　improving　carbon　source　utilization.　This　study　increases　our　understanding　of　the　enhancement　of　lowtemperature　activated　sludge　by　RMs,　and　demonstrates　positive　effects,　in　particular　by　Chl.