Physiological and transcriptomic responses of Heterosigma akashiwo to nitrogen deficiency and urea utilization.

Nitrogen (N) is an essential nutrient for phytoplankton growth. The availability of ambient N and its chemical forms are critically important for the formation of harmful algal blooms (HABs). Although previous studies have explored the transcriptomic responses of Heterosigma akashiwo to N conditions, these analyses were largely descriptive and lacked in-depth characterization of the molecular mechanisms underlying urea utilization and N deficiency responses. To address this gap, we conducted a comprehensive physiological and transcriptomic investigation of this globally distributed HAB species in response to the typical dissolved organic nitrogen urea utilization and N deficiency. Our findings indicate that N deficiency markedly inhibits cell growth and photosynthesis, while leading to increased cell volume and a higher carbon-to-nitrogen (C:N) ratio. Although urea supported the growth of H. akashiwo, it was less effective than nitrate. Under N-deficient conditions, genes associated with N metabolism, glycolysis, the tricarboxylic acid cycle, lipid metabolism, and endocytosis were significantly upregulated, whereas genes related to cell division, photosynthesis and carbon fixation were notably downregulated. In contrast, the utilization of urea by H. akashiwo was characterized by sustained uptake, with evidence suggesting relatively limited intracellular conversion, which subsequently influenced the overall physiological state of the cells. Overall, compared with nitrate, urea utilization by H. akashiwo is less efficient and associated with a mild N‑deficiency‑like physiological state, providing new insights into its adaptive strategies in eutrophic marine environments.