1United Graduate School of Agricultural Science, Ehime University, Matsuyama, Ehime 790-0905, 2Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-8502, Japan
In eutrophic coastal environments in Japan, red tides have caused mass mortality of cultured and wild fish. It is known that the concentration of organic phosphorus is usually higher than that of inorganic phosphorus in such seawaters, and that many phytoplankton produce alkaline phosphatase (AP). To elucidate the contribution of organic phosphorus to the growth of red tide phytoplankton, organic phosphorus utilization and AP production of Karenia mikimotoi, Skeletonema costatum and Heterosigma akashiwo were examined. K. mikimotoi and S. costatum used organic phosphorus compounds (phosphomonoesters, ADP, and ATP) as a sole phosphorus source. These phytoplankton produced AP under orthophosphate concentration of less than 0.2 and 0.25μM, respectively, and their AP activities were 115 and 0.273 fmol/cell/min, respectively. It was shown that these phytoplankton have sufficient potential AP activity necessary for the uptake of phosphorus to maintain saturated growth. On the other hand, H. akashiwo used ADP and ATP, but not phosphomonoesters, as phosphorus sources; AP production was not found. It is suggested that organic phosphorus contributes to the outbreaks of red tide in eutrophic coastal seawaters.