Persistence and degradation of cyanobacterial paralytic shellfish poisons (PSPs) in freshwaters

Abstract

Paralytic shellfish poisons (PSPs) extracted from the cyanobacterium Anabaena circinalis persisted for over 90 days when incubated in freshwater. The primary transformation reaction was desulfocarbamoylation of the predominant, low toxicity C-toxins, to the more potent decarbamoylgonyautoxins (dc-GTXs). This transformation caused an initial increase in sample toxicity, in spite of an overall decrease in total toxin concentration (on a molar basis) with time. Incubation of PSPs in irrigation drainage water resulted in a six-fold increase in toxicity after 10 days, with the solution remaining more toxic than the original solution for over 60 days. The dc-GTXs thus formed, and the gonyautoxins (GTXs) present in the original sample, degraded slowly for the duration of the 90 day incubations. Similar results were recorded when PSPs were incubated with sterile deionised water, and with sterile deionised, protein precipitated water, but the build-up of dc-GTXs proceeded at a slower rate than with the drainage water. The other reaction noted to be significant was the slow epimerisation of the less stable β-epimers (C2, GTX3) to α-epimers (C1, GTX2). The degradation of C-toxins, gonyautoxins (GTXs) and dc-GTXs could be modelled by simple first-order loss kinetics. When PSPs were incubated at 90°C, the same loss processes and reaction order was observed. The failure of protein precipitation and high temperature to alter the reaction processes, indicates that desulfocarbamoylation of C-toxins, and degradation off GTXs and dc-GTXs is chemically, and not enzymatically, mediated. Upon the collapse of PSP producing A. circinalis blooms, and the subsequent release of toxins into the water column, contaminated water may become more toxic in the short term before the eventual degradation of PSPs to non-toxic products. Unless significant dilution occurs, PSP contaminated waters may contain toxins for more than 3 months.