Discovery of Fukuyoa yasumotoi (Dinophyceae) from the Xisha Islands, South China Sea: A comprehensive study on morphology, molecular phylogeny and toxicity.

The epiphytic dinoflagellate genus Fukuyoa, known for producing ciguatoxins, significantly contributes to ciguatera poisoning (CP) in humans and impacts marine food webs. This study reports the first observation and isolation of two Fukuyoa strains, SIO-DF176 and SIO-DF181, from the Xisha Islands in the South China Sea, both identified as Fukuyoa yasumotoi through morphological and molecular techniques. Molecular analysis, including assessments of small subunit (SSU) rDNA, internal transcribed spacer (ITS) rDNA, and large subunit (LSU) rDNA (D1-D3 and D8-D10 regions), revealed close relationships to strains from Japan and Singapore. The analysis of LSU rDNA regions D8-D10 did not differentiate between F. yasumotoi and F. koreensis as distinct morphospecies, underscoring the need for additional physicochemical data from both species for accurate classification. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was employed to examine the profiles of intracellular and extracellular toxins. Strain SIO-DF176 produced 44-methylgambierone at an intracellular concentration of 10.24 pg/cell, while strain SIO-DF181 produced it at 0.59 pg/cell. Additionally, the extracellular toxin of strain SIO-DF181 comprised 44-methylgambierone at 1.34 pg/cell, whereas SIO-DF176 displayed only trace amounts of 44-methylgambierone, which did not reach the limit of quantitation. Strain SIO-DF181 produced gambierone both intracellularly and extracellularly, with concentrations of 56.22 fg/cell and 66.04 fg/cell, respectively, while strain SIO-DF176 did not produce gambierone. Notably, neither strain produced Pacific ciguatoxin (P-CTX-2 or P-CTX-3). This study highlights the presence of F. yasumotoi in the northwestern South China Sea and its potential CP risk.