Primary productivity in the nutrient-poor subtropical ocean gyres depends on new nitrogen inputs from nitrogen fixers that convert inert dinitrogen gas into bioavailable forms. Temperature and iron (Fe) availability constrain marine nitrogen fixation, and both are changing due to anthropogenic ocean warming. We examined the physiological responses of the globally important marine nitrogen fixer, Crocosphaera watsonii across its full thermal range as a function of iron availability. At the lower end of its thermal range, from 22 to 27°C, Crocosphaera growth, nitrogen fixation, and Nitrogen-specific Iron Use Efficiencies (N-IUEs, mol N fixed hour^–1 mol Fe^–1) increased with temperature. At an optimal growth temperature of 27°C, N-IUEs were 66% higher under iron-limited conditions than iron-replete conditions, indicating that low-iron availability increases metabolic efficiency. However, Crocosphaera growth and function decrease from 27 to 32°C, temperatures that are predicted for an increasing fraction of tropical oceans in the future. Altogether, this suggests that Crocosphaera are well adapted to iron-limited, warm waters, within prescribed limits. A model incorporating these results under the IPCC RCP 8.5 warming scenario predicts that Crocosphaera N-IUEs could increase by a net 47% by 2100, particularly in higher-latitude waters. These results contrast with published responses of another dominant nitrogen fixer (Trichodesmium), with predicted N-IUEs that increase most in low-latitude, tropical waters. These models project that differing responses of Crocosphaera and Trichodesmium N-IUEs to future warming of iron-limited oceans could enhance their current contributions to global marine nitrogen fixation with rates increasing by ∼91 and ∼22%, respectively, thereby shifting their relative importance to marine new production and also intensifying their regional divergence. Thus, interactive temperature and iron effects may profoundly transform existing paradigms of nitrogen biogeochemistry and primary productivity in open ocean regimes.

营养不良的亚热带海洋回旋区的初级生产力取决于固氮剂的新氮输入，这些固氮剂会将惰性二氮气转化为生物可利用的形式。温度和铁（Fe）的可用性限制了海洋固氮，两者都因人为海洋变暖而发生了变化。我们研究了全球重要的海洋固氮剂的生理反应，华鳄在整个热量范围内，取决于铁的可用性。在其22至27°C的温度范围的下限，鳄科生长，固氮和特定氮的铁利用效率（N-IUEs，摩尔固定氮小时^–1摩尔铁^–1）随温度增加。在27°C的最佳生长温度下，铁限制条件下的N-IUEs比铁充足条件下的N-IUEs高66％，这表明低铁利用率可提高代谢效率。然而，鳄科生长和功能从27°C降低到32°C，预计未来热带海洋的比例将增加。总之，这表明鳄科在规定的范围内非常适合铁含量有限的温水。在IPCC RCP 8.5变暖方案下纳入这些结果的模型预测，鳄科到2100年，N-IUE可能会净增加47％，特别是在高纬度水域。这些结果与另一种主要固氮剂（毛线虫），并预测N-IUE在低纬度热带水域增加最多。这些模型预测出不同的响应鳄科 和 毛线虫未来铁限制海洋变暖的N-IUE可能会增加其目前对全球海洋固氮的贡献，比率分别增加约91％和约22％，从而将其相对重要性转移到海洋新产品上，并加剧其区域差异。因此，相互作用的温度和铁效应可能会深刻改变现有的氮生物地球化学范式和开放海洋制度下的初级生产力。