Effects of UVC/H₂O₂ on dissolved organic matter derived from a cyanobacterial bloom

Cyanobacterial blooms release complex metabolite mixtures, yet their degradation during water treatment remains poorly understood. This study investigates the degradation of cyanometabolite-rich natural bloom extracts using UVC/H₂O₂ to generate hydroxyl radicals (HO^•). By applying LC-HR-MS/MS combined with computational metabolomics, we identified nine microcystins (MCs), four anabaenopeptins (APs), and sixteen microginins (MGs), proposing structures for five new congeners and two novel transformation products (TPs). UVC/H₂O₂ accelerated cyanopeptide degradation compared to individual control treatments. Kinetics were described by a second-order time-dependent polynomial model, reflecting matrix heterogeneity, dynamic HO^• scavenging by oxidation intermediates, and H₂O₂ self-scavenging. Molecular networking and network annotation propagation (NAP) facilitated the detection of known MC TPs alongside new oxidative cleavage products for MGs and APs. Compositional profiling revealed distinct molecular restructuring post-treatment, characterized by decreasing nitrogen-rich peptides and a concurrent relative increase in lipids and fatty acids. Overall, this workflow provides crucial insights into HO^•-driven transformation pathways within complex cyano-HAB mixtures during advanced oxidation.