Oxygenic photogranules (OPGs) are dense spherical microbial aggregates that are used for self-aerating wastewater treatment. Filamentous cyanobacteria are key granulating species, providing structural backbone and integrity in OPGs. Currently, the mechanisms for selecting filamentous cyanobacteria and their ability to form spherical OPGs are not well understood. Literature shows that iron (Fe) availability plays a significant role in the cyanobacterial growth and colony formation. Moreover, Fe availability is known to be dependent upon light, which is an essential growth substrate for the phototrophs. This research has investigated the role of Fe in the photogranulation phenomenon and the impact of light intensity on Fe availability. The central hypothesis of this research is that the limitation of available Fe selects filamentous cyanobacteria which drives the photogranulation process. It is also hypothesized that the fate of Fe availability is significantly influenced by light intensity, thereby affecting the rate of photogranulation. The characteristics and fate of Fe was investigated in hydrostatic and hydrodynamic batches where activated sludge inoculum transforms into OPGs under illumination. Early sharp release of Fe into the bulk liquid occurred along with the development of anaerobic conditions and the presence of light (i.e., photochemical Fe reduction). This bulk-liquid Fe, however, declined quickly and remained low as batches continued and photosynthetic oxygenation prevailed. Fe associated with biomass-bound extracellular polymeric substances (bEPS-Fe) declined over the course of batches and reached steady values around the time when mature OPGs appeared. Pellet Fe fraction, consisting of intracellular Fe, Fe precipitates/minerals, and unextracted bEPS-Fe, increased during the cultivation period. Spectrophotometric techniques revealed the presence of amorphous ferric oxides in biomass, which did not undergo significant changes by photogranulation. The results demonstrated that the limitation of available Fe, via decreases in bEPS and bulk liquid fractions, induces cyanobacterial community to form a granule to utilize Fe precipitates/minerals over time. Results also showed the limitation of Fe availability in bulk liquid and bEPS fractions increased with light intensity which, in turn, accelerated the rate of photogranulation. Light intensity also influenced the cyanobacterial physiology in terms of motility and pigment in the OPG biomass. Overall, these research findings are expected to enhance our fundamental knowledge on photogranulation phenomenon.