| Summary: | The frequency of harmful algal blooms (HABs) has increased in China in recent years. Information about harmful dinoflagellates and paralytic shellfish toxins (PSTs) is still limited in China, especially in the Beibu Gulf, where PSTs in shellfish have exceeded food safety guidelines on multiple occasions. To explore the nature of the threat from PSTs in the region, eight <i>Alexandrium</i> strains were isolated from waters of the Beibu Gulf and examined using phylogenetic analyses of large subunit (LSU) rDNA, small subunit (SSU) rDNA, and internal transcribed spacer (ITS) sequences. Their toxin composition profiles were also determined using liquid chromatography-tandem mass spectrometry (LC-MS/MS). All eight strains clustered in the phylogenetic tree with <i>A. pseudogonyaulax</i>, <i>A. affine</i>, and <i>A. tamiyavanichii</i> from other locations, forming three well-resolved groups. The intraspecific genetic distances of the three <i>Alexandrium</i> species were significantly smaller than interspecific genetic distances for <i>Alexandrium</i> species. Beibu Gulf isolates were therefore classified as <i>A. pseudogonyaulax</i>, <i>A. affine</i>, and <i>A. tamiyavanichii</i>. No PSTs were identified in <i>A. pseudogonyaulax</i>, but low levels of gonyautoxins (GTXs) 1 to 5, and saxitoxin (STX) were detected in <i>A. tamiyavanichii</i> (a total of 4.60 fmol/cell). The extremely low level of toxicity is inconsistent with PST detection above regulatory levels on multiple occasions within the Beibu Gulf, suggesting that higher toxicity strains may occur in those waters, but were unsampled. Other explanations including biotransformation of PSTs in shellfish and the presence of other PST-producing algae are also suggested. Understanding the toxicity and phylogeny of <i>Alexandrium</i> species provides foundational data for the protection of public health in the Beibu Gulf region and the mitigation of HAB events.
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