Effects of cyanobacterium Phormidium nigroviride K3 on zebrafish embryos and genomic insights into its toxic potential

Cyanobacterial blooms in freshwater systems pose significant environmental and public health risks, largely due to their production of toxic secondary metabolites. This study investigated the effects of Phormidium nigroviride K3 extracts on early zebrafish development, focusing on organismal toxicity and gene expression changes. Acute toxicity was assessed by monitoring developmental deformities and evaluating relative changes in the expression of six genes associated with xenobiotic metabolism, apoptosis, endoplasmic reticulum stress, and endocrine processes. Whole-genome sequencing, in silico genome annotation and mining were conducted to identify biosynthetic gene clusters involved in toxin production. Zebrafish embryos exposed to P. nigroviride K3 extracts developed spinal deformities, pericardial edema, yolk sac edema and reduced eye size, with spinal deformities being the most prevalent malformation (EC50 of 215.6 µg d.w. mL⁻¹). Significant alterations in the expression of genes involved in xenobiotic metabolism were detected, including a dose-dependent 4.6-fold induction of cyp1a1 and a 4.47-fold increase in abcb4 expression at the highest extract concentration. Genome analysis of P. nigroviride K3 revealed 15 biosynthetic gene clusters for secondary metabolites, including a match with the Cylindrospermopsis raciborskii AWT205 cluster, responsible for the cyanotoxin cylindrospermopsin biosynthesis. The LC-MS/MS analysis confirmed the production of cylindrospermopsin in P. nigroviride K3, providing new insights into cyanotoxin biosynthesis in Phormidium species, a genus that has been underexplored in the context of toxin production. These findings expand cyanobacterial genomic databases, enhancing our understanding of cyanobacterial toxic potential. Such knowledge is crucial for predicting and mitigating the risks associated with cyanotoxins in aquatic ecosystems.