Unveiling the susceptibility mechanism of Microcystis to consecutive sub-lethal oxidative stress-Enhancing oxidation...

The use of H₂O₂ to mitigate cyanobacterial blooms has gained popularity due to its selectivity. Previous research has shown that consecutive low-dose H₂O₂ are far more effective in suppressing cyanobacteria than a single higher dose, minimizing damage to co-existing organisms in the aquatic ecosystem. However, the underlying mechanism remains unclear. This study aimed to investigate the mechanism underlying this sensitivity by monitoring the progression from oxidative stress to cell death in Microcystis induced by consecutive low doses of H₂O₂ (3 + 5 mg/L, with an interval time of 4 h). The initial application of H₂O₂ (3 mg/L) resulted in a rapid increase in the transcription of antioxidant genes (gpx, 2-cys prx, trxA and sod) within 1 h, and returned to baseline levels within 8 h. The addition of a second H₂O₂ led to a significant increase in glutathione peroxidase (gene and product) and glutathione within 24 h. The cell death following consecutive H₂O₂ stress was classified as regulated cell death (RCD), characterized by the upregulated metacaspase genes, increased caspase-like activity, modulation of the mazEF system, DNA fragmentation, cell vacuolization, and membrane disruption. Interestingly, the RCD process coincided with the fluctuation of glutathione cycle. Validation experiments demonstrated that exogenous glutathione can promote the gene expression and activity of metacaspase, while inhibition of glutathione biosynthesis led to decreased intracellular glutathione and suppressed metacaspase activity and gene expression. Therefore, glutathione may play a vital role in the connection between oxidative stress and RCD during consecutive H₂O₂ treatment. These results reveal the inherent vulnerability of Microcystis to consecutive oxidative stress, providing a biological mechanism for a sustainable strategy to mitigate cyanobacterial bloom.