Cyanobacterial blooms occur at increased frequency and intensity in freshwater sources, leading to the release of potentially harmful cyanobacteria metabolites (cyano-metabolites) in natural waters that can serve as resources for drinking water. Recognizing this issue, the World Health Organization has established guidelines for the concentrations of four cyano-metabolites. Water suppliers need to account for a probable intensification of blooming events due to climate change and eutrophication by better understanding the efficiency of treatment strategies, such as ozonation, to abate cyano-metabolites and their associated toxicity. However, of the over 2500 cyano-metabolites currently known (CyanoMetDB), the ozonation of less than a dozen has been extensively studied. This is due, in part, to the absence of commercially available purified reagents for the vast majority of cyano-metabolites. Instead, these compounds can currently only be obtained as complex mixtures extracted from laboratory-grown cyanobacteria, which makes it impossible to perform studies with individual compounds. To overcome this obstacle, a multi-compound O₃ competition kinetic method was developed to determine second-order rate constants for the reactions of O₃ with 31 cyano-metabolites extracted from two cyanobacterial strains (from <10² to 7.3 × 10⁷ M^-1 s^-1) and representing a wide range of cyano-metabolite structures. This method uses sixteen competitors that have been validated to simultaneously cover apparent second-order O₃ rate constants between 10²-10⁷ M^-1 s^-1. In addition, ozonation products of selected cyano-metabolites were investigated by LC-HRMS/MS, combining mechanistic information of O₃ reactions and suspect screening with MS² fragmentation fingerprint match between cyano-metabolites and their potential ozonation products. This study is a step forward towards better understanding the transformation of the large variety of cyano-metabolites during ozonation.

Anita Lopes Souto

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