Deciphering the photodegradation fate of Brevetoxins: Kinetics, degradation pathways, and toxicity evaluation

Karenia brevis blooms have emerged as an increasing global concern due to their capacity to produce brevetoxins (BTXs), which are potent marine neurotoxins. The ecological impact of BTXs is significantly modulated by the natural attenuation process, with photodegradation playing a pivotal role. Nevertheless, the fate of BTXs following photodegradation remains inadequately understood. This study elucidates the photodegradation kinetics of three representative BTXs and identifies their transformation products (TPs). The observed first-order photodegradation rate coefficient, based on the logarithmic transformation of dissolved BTXs concentrations against irradiation time, ranged from 0.84 to 2.49 h^-1. Utilizing high-resolution liquid chromatography‒mass spectrometry, three, four, and five TPs were identified for BTX-1, BTX-2, and BTX-3, respectively. All TPs underwent substantial structural modifications with significant mass reductions. Proposed primary photodegradation pathways encompass Ring-opening, alkyl chain cleavage, hydrolysis, dehydroxylation, epoxidation, and β-elimination. Additionally, in silico prediction indicated that the biodegradability of TPs was significantly higher than that of the parent toxins. However, certain TPs exhibited greater toxicities compared to their parent toxins. The coexistence of multiple BTXs and TPs may lead to synergistic toxicity, thereby posing secondary risks to aquatic organisms. Overall, this research will enhance a comprehensive understanding of the environmental behaviors and risks associated with BTXs.