Photogranule Growth and Cyanobacterial Dominance Correlate with Significant Reductions in Extracellular Polymeric Substances (EPS) and EPS-Bound Iron

This study investigates the relationships between granule size, cyanobacterial enrichment, and the dynamics of iron (Fe) and extracellular polymeric substances (EPS) in oxygenic photogranules cultivated in a sequencing batch reactor treating real municipal wastewater. As photogranules increased in size, reaching up to 2.5 mm, filamentous cyanobacteria became increasingly dominant among phototrophs, coinciding with a significant rise in Fe concentration within the biomass pellet (i.e., total biomass excluding EPS). A strong correlation (r = 0.97) between cyanobacterial abundance and Fe in the biomass pellet suggests that Fe contributes to cyanobacterial enrichment. This cyanobacteria enrichment and Fe accumulation within the biomass pellet occurred along with notable declines in both EPS and EPS-bound Fe, suggesting the utilization of EPS and Fe from the extracellular matrix for cellular growth. When EPS and EPS-bound Fe substantially declined, cyanobacterial abundance also began to decrease, suggesting growth limitation. Concurrently, photogranules became more compact, spherical, and less hairy, suggesting enhanced cyanobacterial aggregation under EPS- and Fe-limited conditions. Other metals─manganese, copper, and zinc─showed weak correlations with cyanobacterial trends and different distribution patterns within granules than Fe. This study highlights the critical role of Fe in regulating cyanobacterial dominance, aggregation, and granule morphology in photogranule-based treatment systems.