Cyanobacterial abundance drives photogranulation

Oxygenic photogranules couple oxygen production through photosynthesis to heterotrophic respiration. They may be used in bioprocess engineering applications like wastewater treatment. We demonstrate that photogranulation does not require hydrodynamic shear and washout but also happens under static, batch conditions. Instead, it appears that the formation of these granules strongly depends on the initial presence and abundance of filamentous, motile cyanobacteria. In replicated augmentation experiments, we tested additions of cyanobacteria over a wide concentration gradient. The addition of small numbers of cyanobacteria statistically improved photogranulation, while too many added cyanobacteria led to the formation of microbial mats. Photogranulation is thus driven by entirely different factors than those typically postulated for anaerobic and aerobic granules