1Fisheries Technology Institute, Hyogo Prefectural Technology Center for Agriculture, Forestry and Fisheries, Akashi, Hyogo 674-0093, Japan
In order to elucidate the mechanism of Porphyra bleaching by Coscinodiscus wailesii blooming, effects of nitrogen (N), phosphorus (P) and silicon (Si) on C. wailesii growth under the culture were examined. The maximum growth rate (μm ) was 1.0-1.3 divisions/d at 20°C and 0.46-0.49 divisions/d at 9°C nutrient saturated conditions respectively. The half-saturation constants for growth (Ks) of nitrate, phosphate, and silicate were estimated as 1.4 μM-N, 0.39 μM-P, 5.7-6.2 μM-Si, respectively. The minimum cell quotas of N, P and Si were calculated as 0.44 nM-N/cell, 0.030-0.040 nM-P/cell and 1.4 nM-Si/cell, respectively. C. wailesii did not grow on a wide variety of amino acid and organic phosphorus compounds of different molecular structure as a sole N or P source. The present study demonstrated that comsumptions of the nutrients per a cell of C. wailesii are significantly larger than the other species causing Porphyra bleaching. Decrease of orthophosphate or silicate concentration in Harima-Nada during the period from February to March is presumed to be the limiting factors of C. wailesii bloom in spring.