The dinoflagellate Alexandrium minutum is known to form massive discoloration and has the ability to produce the neurotoxins saxitoxins, which were often associated with paralytic shellfish poisoning in humans. The species’ unique physiological adaptations to environmental changes make it highly adaptable to varying conditions, allowing it to thrive in diverse habitats. It is, however, remains unclear of the processes facilitating these successful adaptations. In this study, the genetic diversity of the natural bloom populations of A. minutum was investigated through genotyping using high-throughput amplicon sequencing of the 18S rDNA metabarcode. To infer the ploidy levels of cells during the bloom events, the relative DNA contents of the bloom populations were obtained flow cytometrically. Our results indicated a degree of polymorphism within the bloom subpopulations. The sequence variant diversity was positively associated with the planozygote/planomeiocyte-dominated subpopulations, but negatively correlated with the vegetative haploidic subpopulation. The results further suggested that frequent sexual reproduction and the incessant excystment mode of the species, as shown by the linkage disequilibrium, contributed to the source of bloom development and enabled the low-density background population to rapidly reach higher densities. These processes were likely to restrict population differentiation, as indicated by the Fst value, thus sustaining the high clonal diversity within the bloom populations. Understanding the genetic diversity within natural populations of this bloom-forming toxigenic species can provide valuable insights into the species' ability to persist and evolve in response to environmental challenges.
 

Alexandrium minutum, cyst, diploid, haploid, HAB, population genetic