The competition of two Rhodocyclaceae-affiliated bacterial populations was assessed during the formation and maturation of aerobic granular sludge in two different types of anaerobic-aerobic sequencing batch reactors. Zoogloea spp. (57%) were predominant in the bacterial community of aerobic granules cultivated in a 2.5-L bubble-column SBR under wash-out conditions selecting for a fast-settling biomass. Zoogloea spp. were favored over “Candidatus Accumulibacter phosphatis” relatives by high transient biomass specific organic loads leaking into the aeration phases. Accumulibacter proliferation over Zoogloea spp. and enhanced dephosphatation were observed after 80 days, as soon as the granular sludge concentration (>7 gVSS•L-1) and the sludge blanket (>30 cm) were sufficiently high to ensure full anaerobic carbon uptake. Formation of up to 2-mm granular aggregates was also observed in a 2.0-L conventional stirred-tank SBR operated at biomass steady-state for Accumulibacter enrichment from activated sludge. Granules formed in this reactor showed a low abundance of Zoogloea spp. (<5% during the first 20 days). The bacterial community was predominated by Accumulibacter (50-60%) and exhibited enhanced orthophosphate cycling profiles. Ensuring full anaerobic carbon uptake by progressively adapting the food-to-microorganism ratio and the anaerobic contact time is proposed to be the key for cultivating dephosphatating aerobic granules
