Nitrate and phosphate uptake kinetics of the harmful diatom Coscinodiscus wailesii, a causative organism in the bleaching of aquacultured Porphyra thalli

The large diatom Coscinodiscus wailesii is one of the problematic species which indirectly cause bleaching damage to “Nori” (Porphyra thalli) cultivation through competitive utilization of nutrients during its bloom. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of C. wailesii, Harima-Nada strain. Maximum uptake rates (ρmax), obtained by short-term experiments, were 58.3 and 95.5 pmol cell−1 h−1 for nitrate and 41.9 and 59.1 pmol cell−1 h−1 for phosphate at 9 and 20 °C, respectively. The half saturation constants for uptake (Ks) were 2.91 and 5.08 μM N and 5.62 and 6.67 μM P at 9 and 20 °C, respectively. The ρmax values of C. wailesii, much higher than those of other marine phytoplankton species, suggest that C. wailesii is able to take up large amounts of nutrients from the water column. On the other hand, Vmax/Ks (Vmax; Vmax = ρmax/Q0, Q0; minimum cell quota) values of C. wailesii, which is a better measure to evaluate the competitive ability for nutrient uptake, were low in dominant diatom species. This parameter indicates that C. wailesii is disadvantaged compared to other diatom species in competing for nutrients, and the decreasing nutrient concentrations from winter to spring is an important factor limiting C. wailesii blooming in early spring.