The role of nitrogen (N) fixation in determining the frequency, magnitude, and extent of harmful algal blooms (HABs) has not been well studied. Dolichospermum is a common HAB species that is diazotrophic (capable of N fixation) and thus growth is often considered never to be limited by low combined N sources. However, N fixation is energetically expensive and its cost during bloom formation has not been quantified. Additionally, it is unknown how acclimation to differing nutrient ratios affects growth and cellular carbon (C):N stoichiometry. Here, we test the hypotheses that diazotrophic cyanobacteria are homeostatic for N because of their ability to fix atmospheric N₂ and that previous acclimation to low N environments will result in more fixed N and lower C:N stoichiometry. Briefly, cultures that varied in resource N:phosphorus (P) ranging from 0.01 to 100 (atom), were seeded with Dolichospermum which were previously acclimated to low and high N:P conditions and then sampled temporally for growth and C:N stoichiometry. We found that Dolichospermum was not homeostatic for N and displayed classic signs of N limitation and elevated C:N stoichiometry, highlighting the necessary growth trade-off within cells when expending energy to fix N. Acclimation to N limited conditions caused differences in both C:N and fixed N at various time points in the experiment. These results highlight the importance of environmentally available N to a diazotrophic bloom, as well as how previous growth conditions can influence population growth during blooms experiencing variable N:P.

固氮 (N) 在确定有害藻华 (HAB) 的频率、幅度和程度方面的作用尚未得到很好的研究。Dolichospermum是一种常见的 HAB 物种，具有固氮能力（能够固氮），因此通常认为其生长不会受到低组合氮源的限制。然而，N 固定在能量上非常昂贵，并且其在开花形成期间的成本尚未量化。此外，尚不清楚对不同营养比例的适应如何影响生长和细胞碳 (C):N 化学计量。在这里，我们检验了固氮蓝藻对 N 具有稳态的假设，因为它们能够固定大气中的 N ₂并且先前对低 N 环境的适应将导致更固定的 N 和更低的 C:N 化学计量。简而言之，将资源 N：磷（P）从 0.01 到 100（原子）不等的培养物与之前适应低和高 N：P 条件的Dolichospermum播种，然后对生长和 C：N 化学计量进行时间取样。我们发现DolichospermumN 不是稳态的，并显示出 N 限制和 C:N 化学计量升高的经典迹象，突出了在消耗能量来固定 N 时细胞内必要的生长权衡。适应 N 限制条件导致 C:N 和固定 N 的差异在实验的不同时间点。这些结果强调了环境可利用的 N 对固氮水华的重要性，以及以前的生长条件如何影响经历可变 N:P 的水华期间的人口增长。