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

Cyanobacterial harmful algae blooms (cyano-HABs) release complex mixtures of metabolites into aquatic ecosystems, including potent toxins. While the degradation of several individual cyanotoxins by reactive species such as the hydroxyl radical (HO •) is well documented, the fate of cyanometabolites in bloom-derived mixtures during water treatment remains poorly understood. This study investigates the degradation of cyanometabolite-rich aqueous extracts from a natural cyanobacterial bloom using UVC/H2O2 to generate HO •. To assess the chemical composition of the samples before and after treatment, we applied an analytical pipeline combining LC-HR-MS/MS with computational metabolomics and statistical analysis. This workflow enabled the identification of nine microcystins (MC), MC-LR, MC-RR, MC-YR, MC-HilR, [D-Asp3]MC-LR, [D-Asp3]MC-RR, MC-WR, MC-Prhcys(O) and MC-(H4)YR, four anabaenopeptins (AP), AP A, AP B, AP F and Oscillamide Y, and seventeen microginins (MG), for which structures of five new congeners and two transformation products (TP) were proposed. UVC/H2O2 accelerated the degradation of cyanopeptides as compared to UVC or H2O2 control treatments. The second-order kinetic models observed were consistent with studies showing the dual role of background dissolved organic matter (DOM) as both a competitive radical scavenger and a photosensitizer. Molecular networking and network annotation propagation (NAP) facilitated the detection of several known TP of MC, and new oxidative cleavage products of MGs and APs. Compositional profiling of treated samples demonstrated molecular restructuring, with nitrogen-rich peptides decreasing, while lipids and fatty acids concurrently increasing. Overall, this study provides valuable insights into • OH-driven transformation pathways in complex cyano-HAB derived mixtures, necessary for developing water treatment strategies.