Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC, 27708, USA.
Department of Geosciences, Princeton University, Princeton, NJ, 08544, USA.
ISME J. 2020 Oct;14(10):2514-2526. doi: 10.1038/s41396-020-0703-6. Epub 2020 Jun 24.
Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N 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 N 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, N 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 N fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N 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 N fixation.
氮素可用性限制了海洋生产力在大片海洋区域的发展。固氮生物可以通过固氮作用将新的氮素供应到海洋环境中，从而缓解氮素限制。固氮生物的分布和固氮作用被认为通常受到全球海洋温度、磷和铁供应的控制。然而，即使在北大西洋，大多数关于固氮生物和固氮作用的研究都集中在这里，环境控制仍然存在争议。在这里，我们使用新开发的现场采样和传感技术，以高分辨率测量固氮生物的组成、丰度和活性。我们发现，从贫营养、温暖（25-29°C）的马尾藻海到相对富营养、寒冷（13-24°C）的亚极地和美国东海岸水域，固氮生物群落发生了从束毛藻到 UCYN-A 的转变。同时，尽管温度较低、硝酸盐浓度较高，但在从墨西哥湾流进入亚极地和沿海水域的过程中，我们测量到的固氮速率是全球有记录以来最高的，并且随着磷供应的增加而显著增加。在亚热带环流中，束毛藻和 UCYN-A 的转录模式都表明存在磷胁迫。在横跨北大西洋西部的这条富含铁的横截线上，我们的结果表明，温度是控制固氮生物群落结构的主要因素，而磷则驱动固氮速率。总的来说，在硝酸盐浓度最高（约 200nM）的寒冷沿海地区，UCYN-A 丰度和固氮速率达到历史最高水平的现象，挑战了目前关于驱动固氮生物和固氮作用分布的范式。
