The frequency and intensity of harmful algal bloom (HAB) events have been increasing in many places around the world. Heterosigma akashiwo is a marine raphidophyte species known to cause HABs in many places across tropical and temperate climates. Studies of temperate strains have identified that H. akashiwo blooms are driven by mass activation of cysts which, for this species, can only form at <15°C temperatures. Although these temperatures do not occur in the tropics, there have been no comparative studies for tropical H. akashiwo. This study aimed to investigate whether tropical H. akashiwo can form cysts under warm temperatures, therefore having different responses from temperate strains. Results showed that tropical H. akashiwo were similar with temperate strains and could only form round cyst-like structures at 5°C but not 25°C. We also observed novel response of a decline and recovery in cell densities at 25°C. The decline was interrupted when the cultures were diluted, implying a tendency for H. akashiwo to rapidly spread and accumulate within surrounding waters, thereby facilitating blooms. This behaviour presents unique bloom concerns. Close monitoring of H. akashiwo distribution patterns is needed for better assessment of the bloom threat posed within tropical waters.

中文翻译：

---

Heterosigma akashiwo（Raphidophyceae）细胞群密度的下降和恢复作为该物种的新开花驱动因素

有害藻华 (HAB) 事件的频率和强度在世界上许多地方都在增加。赤壁异形是一种已知会在热带和温带气候的许多地方引起 HAB 的海洋针叶植物。对温带菌株的研究发现，H. akashiwo水华是由孢囊的大量激活驱动的，对于这个物种，只能在 <15°C 的温度下形成。虽然这些温度不会出现在热带地区，但还没有热带地区的比较研究H. akashiwo. 本研究旨在调查热带是否H. akashiwo可以在温暖的温度下形成包囊，因此对温带菌株有不同的反应。结果表明，热带H. akashiwo与温带菌株相似，只能在 5°C 而不是 25°C 下形成圆形囊状结构。我们还观察到 25°C 时细胞密度下降和恢复的新反应。当培养物被稀释时，下降被中断，这意味着倾向于H. akashiwo在周围水域迅速蔓延和积累，从而促进开花。这种行为提出了独特的绽放问题。密切监测H. akashiwo需要分布模式来更好地评估热带水域内造成的水华威胁。