Comparative uptake and assimilation of nitrate, ammonium, and urea by dinoflagellate Karenia mikimotoi and diatom Skeletonema costatum s.l. in the coastal waters of the East China Sea

The nitrogen uptake kinetics and physiological growth of Karenia mikimotoi and Skeletonema costatum sensu lato grown on different N substrates and concentrations were compared in the laboratory. In the presence of three N substrates, both species preferred to take up NH4+. K. mikimotoi and S. costatum s.l. showed the highest substrate affinities for urea and NO3-, respectively. Both species grew well on three N substrates, and the growth parameters were comparable among the different N substrates. However, K. mikimotoi assimilated urea more efficiently than it assimilated either NO3- or NH4+. Different with S. costatum s.l., K. mikimotoi grew slowly and steady and the physiological and growth activities in N-depleted conditions were higher than those in N-replete conditions. Our results suggested that K. mikimotoi shows a greater readiness for uptake and assimilation of urea, and that this species is more competitive in an N-depleted environment when compared with S. costatum s.l.