Photochemical Degradation of Saxitoxins in Surface Waters

Photochemical degradation has been shown to be a significant pathway in the environmental fate of many cyanotoxins, compounds produced by harmful cyanobacteria. However, there has been a lack of research on the photochemical fate of saxitoxins. This project evaluated the direct and sensitized photodegradation of three saxitoxin analogues including saxitoxin (STX), gonyautoxin-2 and -3 (GTX 2/3), and n-sulfocarbamoyl-gonyautoxin-1 and -2 (C 1/2). Irradiation of STX and analogues in the presence or absence of dissolved organic matter (DOM) revealed that none of the toxins undergo direct photolysis at pH 6 or 8, and only STX and GTX 2/3 undergo sensitized photolysis at pH 8. Sensitized photolysis half-lives ranged from 1 to 10 h for STX and GTX 2/3. The contribution of reactive intermediates including singlet oxygen (O-1(2)), triplet-state dissolved organic matter ((DOM)-D-3*), and hydroxyl radicals ((OH)-O-center dot) was assessed, with (DOM)-D-3* accounting for the majority of STX and GTX 2/3 degradation. Additionally, three transformation product candidates were identified for the photolysis of STX, and an electron-transfer mechanism was proposed. The kinetics, mechanism, and products of saxitoxin phototransformation are essential to understanding the persistence of these toxins in surface waters and assessing the impacts of harmful algal blooms.