Nitrate and phosphate uptake kinetics of the harmful diatom Eucampia zodiacus Ehrenberg, a causative organism in the bleaching of aquacultured Porphyra thalli

The diatom Eucampia zodiacus Ehrenberg is a harmful diatom which indirectly causes bleaching of aquacultured Nori (Porphyra thalli) through competitive utilization of nutrients during bloom events. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of E. zodiacus, Harima-Nada strain. Maximum uptake rates (ρmax), which were obtained by short-term experiments, were 0.777 and 0.916 pmol cell−1 h−1 for nitrate and 0.244 and 0.550 pmol cell−1 h−1 for phosphate at 9 and 20 °C, respectively. The half-saturation constants for uptake (Ks) were 2.59 and 2.92 μM N and 1.83 and 4.85 μM P at 9 and 20 °C, respectively. Although the maximum specific uptake rate (Vmax; Vmax = ρmax/Q0, Q0; minimum cell quota) and Vmax/Ks for nitrate at 9 °C are about 1/2 of those obtained at the optimum temperature (20 °C), they are still higher than those obtained for many other phytoplankton at their optimum temperature conditions for uptake. These results suggest that E. zodiacus utilizes nitrogen efficiently at low water temperature, and it is one of the important factors causing the serious damage to Porphyra thalli by bleaching due of this species. For phosphate, the Ks values of E. zodiacus were higher than those reported for other species; the Vmax and Vmax/Ks values were much lower than those of other diatoms such as Skeletonema costatum (Greville) Cleve. These results suggest that E. zodiacus is disadvantaged compared to other diatom species during competitive utilization of phosphate.