Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N₂) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N₂ fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean. However, even in the North Atlantic where most research on diazotrophs and N₂ fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25–29 °C) Sargasso Sea and relatively nutrient-enriched, cold (13–24 °C) subpolar and eastern American coastal waters. Meanwhile, N₂ fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N₂ fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N₂ fixation rates in the cold coastal region where nitrate concentrations are highest (~200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N₂ fixation.

氮的可用性限制了大型海洋区域的海洋生产力。_{固氮菌可以通过固氮 (N 2} )为海洋环境提供新的氮，从而缓解氮限制。已经假设固氮菌的分布和 N _{2固定通常受全球海洋中的温度、磷和铁的可用性控制。然而，即使在大多数固氮菌和 N 2}固定研究已经发生的北大西洋，环境控制仍然存在争议。在这里，我们使用新开发的正在进行的采样和传感技术以高分辨率测量固氮菌的组成、丰度和活性。我们从Trichodesmium捕捉到固氮群落转变到贫营养的温暖（25-29°C）马尾藻海和相对营养丰富的寒冷（13-24°C）亚极地和美国东部沿海水域之间的UCYN-A。同时，本研究中测得的 N ₂固定率是全球有史以来最高的，尽管温度较低且硝酸盐浓度较高，但在从墨西哥湾流到亚极地和沿海水域的过渡过程中，磷的可用性显着增加。Trichodesmium和 UCYN-A 的转录模式表明亚热带环流中的磷胁迫。在这个横跨北大西洋西部的富含铁的横断面上，我们的研究结果表明，温度是控制固氮群落结构的主要因素，而磷驱动 N ₂固定率。总体而言，在硝酸盐浓度最高（~200 nM）的寒冷沿海地区出现创纪录的 UCYN-A 丰度和峰值 N _{2固定率，挑战了当前驱动固氮菌分布和 N 2}固定的范式。