Sunlight concurrently reduces Prymnesium parvum elicited acute toxicity to fish and prymnesins

Prymnesium parvum continues to spread globally, producing harmful algal blooms that release toxins known to cause fish kills. While previous work has identified possible P. parvum toxin(s) (e.g., prymnesins, fatty acids, fatty acid amides) and investigated treatment strategies targeted at minimizing cell abundance, studies examining efficacy of treatment approaches to remove toxins are lacking. To understand influences of sunlight on toxins stability and toxicity to fish, acutely toxic P. parvum cultures were exposed to three light scenarios (lab dark control, field dark, and field light) and then evaluated for acute toxicity to fish and prymnesins abundance. Previous work showed acute toxicity to fathead minnow larvae was ameliorated after 2 h of sunlight exposure, and results observed herein found an identical trend. Acute toxicity disappeared in light exposed filtrate, but filtrate exposed to 35 ^°C without sunlight remained acutely toxic to fish. Additionally, six prymnesins were identified through high-resolution mass spectrometry and abundance corresponded to acute toxicity levels. Prymnesins were present at the highest level in filtrate that was acutely toxic but diminished in filtrate that was exposed to light and correspondingly ameliorated acute toxicity to fish. These findings suggest prymnesins are responsible for measured acute toxicity and are photo-labile, which represents an important implication for treatment strategies.