Dissecting the toxicity and mitigating the impact of harmful Prymnesium blooms in the UK waters of the Norfolk Broads

Prymnesium parvum is a toxin-producing microalga that causes harmful algal blooms (HABs) globally, frequently leading to massive fish kills that have adverse ecological and economic implications for natural waterways and aquaculture alike. The dramatic effects observed on fish are likely due to algae-produced polyether toxins, known as the prymnesins, but these compounds had not been detected in environmental samples, which has resulted in ambiguity about the true ichthyotoxic entities. Using qPCR, we found elevated levels of both P. parvum and its lytic virus, PpDNAV-BW1, in a fish-killing bloom on the Norfolk Broads, United Kingdom, in March 2015, a site historically plagued by P. parvum blooms. 16S rRNA gene sequencing confirmed that P. parvum dominated the bloom microbial community and that the microbial species diversity changed after recovery of the ecosystem. We also detected, for the first time, the recently discovered B-type prymnesin toxins in Broads waterway samples and gill tissue isolated from a dead fish taken from the study site. Furthermore, Norfolk Broads P. parvum isolates unambiguously produced B-type toxins in laboratory-grown cultures. In addition, a 2-year longitudinal study of the Broads study site showed P. parvum blooms positively correlated with increased temperature and that PpDNAV plays a significant role in P. parvum bloom demise in this natural setting. Finally, we used a field trial to show that treatment with low doses of hydrogen peroxide represents an effective strategy to mitigate blooms of P. parvum in enclosed water bodies.