International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 167 sequencing and transcriptome analysis have played an important role in the discovery of toxin genes. American scientists analyzed marine diatom toxicity-producing gene clusters (such as the dodolalic acid synthesis gene cluster dabA-dabD of pseudo-rhombus algae) through comparative transcriptome analysis; similarly, genome sequencing revealed the composition of multiple toxin synthesis gene clusters in dinoflagellate and cyanobacteria (Lorenzi et al., 2019). In addition, metabolomics and isotope labeling techniques are also used to correlate specific metabolites with candidate genes, providing auxiliary evidence for the analysis of toxin synthesis pathways. Figure 1 Morphology of Chlamydomonas asymmetrica (A), Dunaliella salina (B), and Scenedesmus obtusiusculus (C), after exposure to 1 µg/mL MC-LR and CYL for 15, 24, 48, and 72 h. Control: non-treated cell cultures. Bar = 10 µm (Adopted from Teneva et al., 2023) 3.2 Structural characteristics of key toxin synthesis gene clusters Toxin synthesis genes in toxin-producing algae usually exist in clusters, encoding enzymes required for the complete biosynthetic pathway. Taking the cyanobacteria microcystis toxin as an example, the mcy gene cluster of M. aeruginosa PCC7806 spans about 55 kb, including a total of 10 open reading frames of mcyA-mcyJ, encoding biosynthetic enzymes such as multimodal polyketone synthetase (PKS) and non-ribosomal peptide synthetase (NRPS) (Rhee et al., 2012). The cluster is closely arranged in two directions of operon structures transcribed, providing a complete enzyme system for the synthesis of a complex peptide loop. Similarly, the gene cluster (ndaA–ndaF) of the cyclogenic toxin Nodularin is highly homologous to the mcy cluster and shares some of the core enzymes. The synthesized gene cluster of dinoflagellate paralytic cadoxin (PSTs) contains more than 20 genes, encoding enzymes such as PKS and aminotransferases required to synthesize STX; the oxalic acid
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