中筋川下流域に発生する淡水赤潮の原因種Ｐｌａｇｉｏｓｅｌｍｉｓ ｓｐ．（８７株）の増殖生理特性

Abstract

Freshwater red tides by Plagioselmis sp., a Cryptophyceae, frequently occur in the lower part of the Nakasuji River during the cold seasons. The present paper describes the growth characteristics of Plagioselmis sp. (strain 87) isolated from the river water in 1987. The optimum tempreature and light intensity for the growth of Plagioselmis sp. (strain 87) were found to be 10-25°C and 1, 000-14, 000lx. This alga preferred the neutral and lower alkaline pH (7-9).5 and lower salinity (0-3.4‰). The amounts of nitrate, phosphate, and vitamin B12 required to maximize both growth rate and cell yields were 5mg N/l, 0.15mg P/l, and 50ng/ml, respectively. The falf-saturation constants of nitrate, phosphate, and vitamin B12 based on the growth rate were estimated to be 0.15mg N/l, 0.0078mg P/l, and 2.3ng/l, respectively. The minimum cell quotas of nitrogen, phosphorus, and vitamin B12 were 1.5×10-2ng N/cell, 5.5×10-4ng P/cell, and 2.5×10-4pg/cell, respectively. Phosphate concentration, but not nitrogen and vitamin B12 concentration, in the Nakasuji River water was found to reach almost the level which can maximize the growth rate of this alga. Growth potential in the water was estimated to be more than 6×103cells/ml from the quotients of those nutrient concentrations divided by each minimum cell quota of the alga. These results suggest that the water of the Nakasuji River has the potential of growing enough algae for the abundance of red tide during the cold seasons.