Sexual processes and life cycle transitions of the tropical Pacific Alexandrium minutum (Dinophyceae)

Resting cysts of harmful marine dinoflagellates, including Alexandrium species, play an important role in their bloom dynamics, where massive cyst germination is regarded as the natural phenomenon that initiates blooms. The life cycle of Alexandrium minutum from the temperate region has been intensively studied, however, related studies on its tropical counterparts are scarce. In this study, sexual reproduction of a toxigenic A. minutum (tropical Pacific ribotype) was investigated in a laboratory setting. Gamete expression was observed in both compatible cross‐mating and self‐crossed cultures, but sexual induction was observed only in the compatible cross‐mating cultures, confirming the heterothallic nature of the species. Resting cysts were successfully produced in 41 out of 91 pairwise combinations of cross‐mating strains. The crossing matrix results showed that the mating system of the tropical Pacific A. minutum was complex, with at least four distinct mating groups observed. The resting cysts had a relatively shorter dormancy period (5–8 days). The rapid encystment‐excystment processes and a short cyst dormancy period in this tropical Pacific A. minutum are believed to play crucial roles in governing the bloom and its dynamics in the tropical coastal region.