Unveiling the role of ferrous ion in driving microalgae granulation from salt-tolerant strains for mariculture wastewater treatment

Clarifying the elusive mechanisms governing saline-adapted microalgae granulation holds significant promise, particularly in the practical implementation of microalgal-bacterial granular sludge (MBGS) for mariculture wastewater treatment. This study successfully developed MBGS with exceptional settleability and nutrient removal efficiency through a mixed culture of pure Spirulina platensis and Chlorella sp. GY-H4. The addition of Fe2+ was identified as a facilitator for granulation initiation and the maintenance of a stable granular structure. This effect was attributed to the stimulation of extracellular protein secretion, fostering the accumulation of exogenous heterotrophic and exopolysaccharide-producing cyanobacteria. However, an excess of Fe2+ resulted in irregular granular morphology and reduced nutrient removal, which was linked to the expanded anaerobic zone within the granules and the inhibition of functional gene expression.