The "Aerobic Granular Sludge" (AGS) technology for wastewater treatment is based on dense microbial biofilm in the form of granular aggregates that have an excellent settling property allowing high biomass concentrations in bioreactors. This promising biological treatment of wastewater is a cost-effective and land-saving alternative to the conventional flocculent activated-sludge technology. A complex microbial community is intertwined in the dense biofilm structure of AGS, and little is known about the roles and the interactions between microorganisms within this matrix. Moreover, there is a need to understand the fundamental biological mechanisms underlying the AGS process to control its stability and performance. In this context, the physiology and the structural organization of the AGS microbial populations involved in biological phosphorus removal from the influent are investigated in lab-scale conditions. With the experimental set-up and conditions used, for a microbial community dominated by the genus Ca. Accumulibacter phosphatis, acetate, propionate, and probably some amino acids (like aspartate or glutamate) could support the biological phosphorus removal from the influent. Glycine was shown to interfere with this metabolism with an effect proportional to its concentration. Concerning the use of acetate and glucose, microorganisms belonging to Ca. Accumulibacter phosphatis and to an unknown genus of Propionibacteriaceae were likely the primary consumers, respectively. Investigating the organisms involved in the biological phosphorus removal revealed Ca. Accumulibacter phosphatis as the primary organism responsible for phosphorus treatment performance. Finally, the spatial organization of the microorganisms in the AGS showed clusters of microcolonies rather than an organization in layers.