Filamentous cyanobacteria are an essential element of oxygenic photogranules for granule-based wastewater treatment with photosynthetic aeration. Currently, mechanisms for the selection of this microbial group and their development in the granular structure are not well understood. Here, we studied the characteristics and fate of iron in photogranulation that proceeds in a hydrostatic environment with an activated sludge (AS) inoculum. We found that the level of Fe in bulk liquids (Fe BL ) sharply increased due to the decay of the inoculum but quickly diminished along with the bloom of microalgae and the advent of the oxic environment. Iron linked with extracellular polymeric substances (Fe EPS ) continued to decline but reached steady low values, which occurred along with the appearance of granular structure. Strong negative correlations were found between Fe EPS and the pigments specific for cyanobacteria. Spectroscopies revealed the presence of amorphous ferric oxides in pellet biomass, which seemed to remain unaltered during the photogranulation process. These results suggest that the availability of Fe EPS in AS inoculumsafter algal bloomselects cyanobacteria, and the limitation of this Fe pool becomes an important driver for cyanobacteria to granulate in a hydrostatic environment. We therefore propose that the availability of iron has a strong influence on the photogranulation process.
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.
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