Photogranules are spherical microbial aggregates that were discovered in 2015, and are of potential use in environmental engineering for wastewater treatment. The use of photogranule deserves attention because a photogranule-based process may represent a sustainable alternative for wastewater treatment, and could replace costly aeration of conventional activated sludge process by light. A photogranule is composed of an outer interwoven layer of filamentous cyanobacteria embedding heterotrophic bacteria. This syntrophic association between autotrophic and heterotrophic microorganisms can remove organic carbon pollutants from wastewater with light as driving force. One the one hand, cyanobacteria take up available carbon dioxide and provide oxygen through photosynthesis. On the other hand, heterotrophic bacteria use available oxygen as final electron acceptor to oxidize organic matter to produce carbon dioxide used by cyanobacteria. This virtuous circle is modulated by light and carbon availabilities. In my talk, I will present a spatially explicit description of microbial activities of individual photogranules, using a combination of microelectrode experiments and modelling. Local oxygen concentrations were monitored using a Clark-type microelectrode at different depths inside photogranules exposed to gradients of light and organic carbon. The concomitant presence of oxic and anoxic zones inside photogranule will be discussed in the context of wastewater treatment.