Encystment of raphidophycean red tide flagellates

属于异形异形藻属和查顿藻属的短尾鞭毛藻在夏季会引起日本沿海的赤潮和黄尾等养殖鱼类的死亡。这些鞭毛虫在其生活史中有越冬的包囊阶段。这些包囊在引发赤潮并将其分布扩大到新的地区方面发挥了重要作用。因此，弄清赤潮鞭毛虫水华发生的过程和条件，对了解赤潮鞭毛虫的水华动态具有重要意义。然而，关于包裹体的过程和条件的信息很少。本文研究了赤潮鱼在广岛湾北部内陆海的包裹化过程。并对卵形隐孢子虫、疣状隐孢子虫和赤潮隐翅虫的包囊条件进行了研究。结果表明：(1)采集赤潮末期赤潮自然种群，在黑暗中与高压灭菌的海洋沉积物孵育，种群中的部分细胞转变为胞体前小细胞，最后形成环孢子虫，与野外发现的赤潮螺旋藻孢子一致。根据在广岛湾北部进行的赤潮每周测量，新形成的海藻在水华高峰期后基本不能萌发，但在2周后才能萌发。(3)在实验室培养条件下，在卵形海藻和赤潮海藻中，氮素限制能有效地诱导小细胞的形态变化，如预囊化小细胞和滨海海藻。然而，这些诱导的卵圆线虫和赤潮鱼的小细胞并没有转化为包囊。经限氮处理后，卵圆盖藻细胞可在黑暗中存活4周以上。

Raphidophyceanflagellates belonging to the genera Heterosigma and Chattonella cause red tides and kill cultured fish such as yellowtail in the coastal sea of Japan in summer. These flagellates have overwintering cyst stages during their life cycles. The cysts play an important role to initiate red tides and to expand their distribution to new areas. Therefore, to clarify the process and conditions for encystments is essential to understand bloom dynamics of these red tide flagellates. However, there is a paucity of information on the process and conditions for encystments. In the present study encystment processes of H.akashiwo were investigated in northern Hiroshima Bay, the Seto Inland Sea. And the conditions for encystment were examined on C.ovata, C.verruculosa, and H.akashiwo in culture. The following results were obtained.(1)When a natural population of H.akashiwo in the final stage of a red tide was collected and incubated in the dark with autoclaved marine sediment, a portion of the cells in the population changed into preencystment small cells and finally into cycts, which were identical to H.akashiwo cysts found in the field. The newly formed cycts required at least2 weeks of mandatory dormancy period before germination.(2)According to the weekly measurements of a red tide of H.akashiwo carried out in northern Hiroshima Bay, newly formed cycts were substantially supplied to the bottom after the peak of bloom, and they could not germinate for at least a week, but could do so after2 weeks.(3)In C.ovata and H.akashiwo, nitrogen limitation was effective in inducing morphological changes into small cells like preencystment small cells of C.antiqua and C.marina under the laboratory culture conditions. These induced small cells of C.ovata and H.akashiwo, however, failed to change into cysts. After the treatment of N-limitation, cells of C.ovata could survive in the dark for more than 4 weeks.