Encystment of raphidophycean red tide flagellates

在夏季，日本沿海海域的刺藻鞭毛虫属和查顿菌属引起赤潮，并杀死黄尾鱼等养殖鱼类。这些鞭毛虫在其生命周期中有越冬囊肿阶段。这些囊肿在引发赤潮和扩大其分布到新的地区方面起着重要作用。因此，弄清这些赤潮鞭毛虫的成囊过程和条件，对了解赤潮鞭毛虫的华流动态至关重要。然而，关于这种方法的过程和条件的信息还很缺乏。本研究在濑户内海广岛湾北部调查了赤藻的成窝过程。并对体外培养的卵形菌、疣状菌和赤石菌的成囊条件进行了研究。得到了以下结果：(1)采集赤潮末期赤石夜蛾自然种群，用高压海水沉淀物在黑暗中孵育，种群中部分细胞转变为囊前小细胞，最终形成周期，与野外发现的赤石夜蛾囊相同。新形成的周期在萌发前至少需要2周的强制休眠期。(2)根据在广岛湾北部进行的赤谷赤潮的每周测量，新形成的周期在高潮后大量供应到底部，至少在一周内不能发芽，但在2周后可以发芽。(3)在实验室培养条件下，氮限制能有效诱导c.o ovata和h.a akashhiwo的小细胞形态变化，如c.a antiqua和c.a marina的囊前小细胞。然而，这些诱导的小细胞没有变成囊肿。经限氮处理后，卵形弧菌细胞可在暗处存活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.