The Coupled Effect of Light and Iron on the Photogranulation Phenomenon.

Cyanobacteria occasionally self-immobilize and form spherical aggregates. This photogranulation phenomenon is central for oxygenic photogranules, which present potential for aeration-free and net-autotrophic wastewater treatment. Light and iron are tightly coupled via photochemical cycling of Fe, suggesting that phototrophic systems continually respond to their combined effects. Thus far, photogranulation has not been investigated from this important aspect. Here, we studied the effects of light intensity on the fate of Fe and their combined effects on the photogranulation process. Photogranules were batch-cultivated with the activated sludge inoculum under three photosynthetic photon flux densities: 27, 180, and 450 μmol/m2·s. Photogranules were formed within a week under 450 μmol/m2·s compared to 2-3 and 4-5 weeks under 180 and 27 μmol/m2·s, respectively. Batches under 450 μmol/m2·s showed faster but lower quantity of Fe(II) release into bulk liquids compared to the other two sets. However, when ferrozine was added, this set showed substantially more Fe(II), indicating that Fe(II) released by photoreduction undergoes fast turnover. Fe linked with extracellular polymeric substances (EPS), FeEPS, diminished significantly faster under 450 μmol/m2·s, while the granular shape in all three batches appeared along with the depletion of this FeEPS pool. We conclude that light intensity has a major influence on the availability of Fe, and light and Fe together impact the speed and characteristics of photogranulation.