有害有毒赤潮の生物学(32)有害赤潮ラフィド藻ヘテロシグマー5

摘要

The life cycle of the noxious raphidophyte Heterosigma akashiwo was summarized with special reference to overwintering cells. There are three types of reports on resisting cells, i.e. benthic stage cells, cysts, and resting cells. Motile cells of H. akashiwo aggregated to form encapsulated masses of non-μmotile cells in a benthic stage. Benthic stage cells kept in darkness exhibited survival for more than 15 weeks when stored at 5°C and 10°C. The cysts of H. akashiwo were identified from surface sediments collected in northern Hiroshima Bay of the Seto Inland Sea, Japan. The cysts were spherical with the diameter of 8-12 μm and were covered with mucilaginous materials to which various particles such as sand grain and pieces of diatom frustule adhered. The cysts were solitary in general, but formed a cluster of several cysts occasionally. Live cysts had auto-fluorescing chloroplasts (4-11 per cyst) under blue-light excitation. The nuclei of cysts were smaller than those of vegetative cells. The germination successfully occurred within one or two days when incubated in filtered-autoclaved seawater at 22°C with light, but they can germinate in the dark. Resting cells were induced to form when vegetative cells were subjected to low temperature (10°C) and darkness for 14-16 days. These cells recovered to vegetative cells under the conditions of light and suitable temperature. The size and number of chloroplasts were different between cysts and benthic stage cells, but they were thought to be identical thing due to their tolerance ability to darkness and low temperature (winter) and morphological features.