Efficient control of cyanobacterial blooms with calcium peroxide: Threshold and mechanism

This study explored the feasibility and mechanism of cyanobacterial blooms control by calcium peroxide (CaO₂). The obtained results demonstrated a strong inhibitory effect of CaO₂ on cyanobacterial growth. The removal chlorophyll-a rate reached 31.4 %, while optimal/maximal quantum yield of PSII (Fv/Fm) decreased to 50 % after CaO₂ treatment at a concentration of 100 mg L^-1 for 96 h. Two main mechanisms were involved in the treatment of cyanobacterial bloom with CaO₂, namely oxidative damage and cyanobacterial colony formation. It was found that CaO₂ released reactive oxygen species (ROS), namely hydroxyl radicals (·OH), singlet oxygen (¹O₂), and superoxide radicals (·O₂^-), inhibiting the activity of antioxidant enzymes in cyanobacterial cells and resulting in intracellular oxidation imbalance. Cyanobacteria can resist oxidative damage by releasing extracellular polymeric substances (EPS). These EPS can combine with CaO₂-derived Ca, forming large cyanobacterial aggregates and, consequently, accelerating cell sedimentation. In addition, CaO₂ caused programmed cell death (PCD) of cyanobacteria and irreversible damage to the ultrastructure characteristic of the cyanobacterial cells. The apoptotic rate was greatly improved at 100 mg L^-1 CaO₂. On the other hand, the results obtained using qRT-PCR analysis confirmed the contribution of CaO₂ to the down-regulation of photosynthesis-related genes (rbcL and psaB), the up-regulation of microcystins (mcyA and mcyD), the up-regulation of the oxidation system: peroxiredoxin (prx) through oxidative mechanisms. The present study proposes a novel treatment method for water-containing cyanobacterial blooms using CaO₂.