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Alexandrium tamarense is a toxic dinoflagellate known to cause paralytic shellfish poisoning to human and marine animals. Previous studies observed the depletion of macronutrients during the A.tamarense blooming periods in Osaka Bay, Japan, and suggest the different nutrient availabilities between surface and bottom layers allow A.tamarense to grow. In this study, the field observation and Algal growth potential(AGP) bioassay with A.tamarence and non-toxic, dominant diatom Skeletonema sp. in Osaka Bay are conducted to compare the nutrient availabilities during the period of A.tamarense occurrences in the Bay. The field observations confirmed the depletion of dissolved inorganic nitrogen, PO4-P, and SiO2-Si concentrations with the A.tamarense occurences in spring. The phosphorous limitation to both speicies at surface layer were frequently determined by the AGP bioassay in spring. The bioassay also revealed the higher AGP of both species at deeper water than those at surface water and different limiting nutrients between surface and bottom waters or between A.tamarense and Skeletonema sp. These different nutrient availabilities probably allow A.tamarense and dominant diatoms to coexist and grow Osaka Bay during spring.

有毒渦鞭毛藻 Alexandrium tamarense 広島湾株による溶有態有機リン(DOP)の取り込みと排出に関して、広島湾汲み置き海水を用いたパッチ培養と半連続培養による実験を行った。バッチ培養では、培養液中のリン酸塩濃度約2μMを境に、それ以下で有意な正味のDOP取り込み、それ以上で正味の排出が見られた。半連続培養では、A. tamarense は希釈率が大きく、培地中の十分なリン酸塩を利用して活発に増殖している場合にDOPの正味の排出を示した。以上のことから、予備的実験結果ではあるが、A. tamarense 広島湾株はリン酸塩濃度が低い場合にはDOPを利用し、リン酸塩濃度が高くて活発に増殖している場合にはDOPを排出することが示唆された。, Capability of utilization and excretion of dissolved organic phosphorus (DOP) by the toxic dinoflagellate Alexandrium tamarense (Hiroshima Bay strain) was investigated with both batch and semi-continuous cultures using natural aged-seawater from Hiroshima Bay. In the batch cultures, significant net DOP uptake was detected in phosphate concentrations below 2μM, while net DOP excretion was observed above this concentration. The semi-continuous culture showed that A. tamarense excretes DOP at high dilution rates. At these rates, this organism grows fast by utilizing sufficient phosphate in the medium. These experiments, although preliminary, suggest that A. tamareme (Hiroshima Bay strain) can utilize DOP of natural seawaters under a condition of low phosphate concentration and excretes DOP under a condition of high phosphate concentration.