Effects of nitrogenous nutrition on growth and nitrogen assimilation enzymes of dinoflagellate Akashiwo sanguinea

The dinoflagellate Akashiwo sanguinea is a cosmopolitan species which frequently causes dense blooms in estuaries and coasts around the world, while the mechanism regulating the formation of the algal bloom remains unclear. To further explore the physiological response of A. sanguinea during algal blooms, we studied the effects of various nitrogenous nutrients on growth and nitrogen assimilation enzymes of A. sanguinea in laboratory trials. Our results indicated that A. sanguinea was able to grow well on multiple N substrates within appropriate ranges of concentrations. The dinoflagellate grew faster in NH4+, while it showed greatest affinity for urea. The intracellular enzymes of A. sanguinea were regulated to adapt for the environmental nutrient conditions and growth stages: nitrate reductase (NR) was induced when grown on NO3− while inhibited by NH4+ and urea, glutamine synthetase (GS) expressed constitutively in all N sources, and urease was up-regulated by NO3− and urea but down-regulated by NH4+. Meanwhile, the activities of the N assimilation enzymes decreased with the growth of A. sanguinea, and GS activity showed a significant positive correlation with growth rates. The urease activity of A. sanguinea was extremely high assessed by the kinetics of N assimilation, indicating its competitive advantage for urea over other phytoplankton species. The physiological activity of N assimilation enzymes in A. sanguinea facilitates its utilization of reduced N, and implies its capability of forming blooms in environments where reduced N is the common N source.