Transient cyanobacteria drive hydrostatic photogranulation but disappear during microbial succession

Photogranulation transforms activated sludge into spatially organized photogranules, yet the role and fate of cyanobacterial strains that trigger this process remain poorly understood. Here, we tested the ability of seven strains of filamentous cyanobacteria to induce photogranule or mat morphotypes in replicated hydrostatic incubations. Despite successful granulation in all replicates when granule-forming strains were added, sequencing revealed that these keystone cyanobacteria responsible for photogranulation were outcompeted during community succession and absent from mature photogranules. A time-series experiment demonstrated that the added granule-forming strain <i>Geitlerinema</i> sp. A4 dominated the cyanobacterial community early on but was gradually replaced by mat-forming taxa, such as <i>Tychonema</i> and <i>Planktothrix</i>. Nevertheless, the granule morphotype persisted even after the founder strain vanished. These findings highlight that transiently dominant organisms can act as microbial engineers, steering ecosystem trajectories without remaining in the final community. This study refines our understanding of microbial succession and has practical implications for selecting the inoculum in biotechnological processes using photogranules.