Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.
Nature. 2012 Aug 16;488(7411):361-4. doi: 10.1038/nature11338.
Biological dinitrogen fixation provides the largest input of nitrogen to the oceans, therefore exerting important control on the ocean's nitrogen inventory and primary productivity. Nitrogen-isotope data from ocean sediments suggest that the marine-nitrogen inventory has been balanced for the past 3,000 years (ref. 4). Producing a balanced marine-nitrogen budget based on direct measurements has proved difficult, however, with nitrogen loss exceeding the gain from dinitrogen fixation by approximately 200 Tg N yr−1 (refs 5, 6). Here we present data from the Atlantic Ocean and show that the most widely used method of measuring oceanic N2-fixation rates underestimates the contribution of N2-fixing microorganisms (diazotrophs) relative to a newly developed method. Using molecular techniques to quantify the abundance of specific clades of diazotrophs in parallel with rates of 15N2 incorporation into particulate organic matter, we suggest that the difference between N2-fixation rates measured with the established method and those measured with the new method can be related to the composition of the diazotrophic community. Our data show that in areas dominated by Trichodesmium, the established method underestimates N2-fixation rates by an average of 62%. We also find that the newly developed method yields N2-fixation rates more than six times higher than those from the established method when unicellular, symbiotic cyanobacteria and γ-proteobacteria dominate the diazotrophic community. On the basis of average areal rates measured over the Atlantic Ocean, we calculated basin-wide N2-fixation rates of 14 ± 1 Tg N yr−1 and 24 ±1 Tg N yr−1 for the established and new methods, respectively. If our findings can be extrapolated to other ocean basins, this suggests that the global marine N2-fixation rate derived from direct measurements may increase from 103 ± 8 Tg N yr−1 to 177 ± 8 Tg N yr−1, and that the contribution of N2 fixers other than Trichodesmium is much more significant than was previously thought.
生物固氮为海洋提供了最大的氮输入，因此对海洋氮储量和初级生产力具有重要的控制作用。海洋沉积物中的氮同位素数据表明，在过去的 3000 年里，海洋氮储量一直处于平衡状态（参考文献 4）。然而，基于直接测量来生成平衡的海洋氮预算一直很困难，因为氮的损失超过了固氮作用的增益，大约为 200TgNyr-1（参考文献 5、6）。在这里，我们展示了来自大西洋的数据，并表明，最广泛使用的测量海洋 N2 固定率的方法低估了固氮微生物（固氮菌）的贡献，相对于新开发的方法而言。我们使用分子技术来定量特定固氮菌类群的丰度，并与 15N2 掺入颗粒有机物的速率平行，我们认为，用已建立的方法测量的固氮率与用新方法测量的固氮率之间的差异可以与固氮菌群的组成有关。我们的数据表明，在以 Trichodesmium 为主的地区，用已建立的方法平均低估了 62%的 N2 固定率。我们还发现，当单细胞共生蓝细菌和γ-变形菌主导固氮菌群时，新开发的方法产生的 N2 固定率比已建立的方法高 6 倍以上。基于大西洋上测量的平均面积速率，我们分别计算出用已建立的和新方法得出的大西洋固氮速率为 14±1TgNyr-1和 24±1TgNyr-1。如果我们的发现可以外推到其他海洋盆地，这表明从直接测量得出的全球海洋 N2 固定率可能从 103±8TgNyr-1增加到 177±8TgNyr-1，并且除了 Trichodesmium 之外的固氮生物的贡献比之前认为的要大得多。
