Diazotrophs are widespread microorganisms that alleviate nitrogen limitation in 60% of our oceans, thereby regulating marine productivity. Yet, the group-specific contribution of diazotrophs to organic matter export has not been quantified, which so far has impeded an accurate assessment of their impact on the biological carbon pump. Here, we examine the fate of five groups of globally-distributed diazotrophs by using an original combination of mesopelagic particle sampling devices across the subtropical South Pacific Ocean. We demonstrate that cyanobacterial and non-cyanobacterial diazotrophs are exported down to 1000 m depth. Surprisingly, group-specific export turnover rates point to a more efficient export of small unicellular cyanobacterial diazotrophs (UCYN) relative to the larger and filamentous Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were recurrently found embedded in large (>50 µm) organic aggregates or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, presumably facilitating their export. Beyond the South Pacific, our data are supported by analysis of the Tara Oceans metagenomes collected in other ocean basins, extending the scope of our results globally. We show that, when diazotrophs are found in the euphotic zone, they are also systematically present in mesopelagic waters, suggesting their transport to the deep ocean. We thus conclude that diazotrophs are a significant part of the carbon sequestered in the deep ocean and, therefore, they need to be accounted in regional and global estimates of export.

固氮菌是一种广泛存在的微生物，可以缓解我们 60% 的海洋中的氮限制，从而调节海洋生产力。然而，固氮菌对有机物输出的特定群体贡献尚未量化，到目前为止，这阻碍了准确评估它们对生物碳泵的影响。在这里，我们通过使用横跨亚热带南太平洋的中层远洋粒子采样装置的原始组合来检查五组全球分布的固氮菌的命运。我们证明蓝藻和非蓝藻重氮菌可以输出到 1000 米深。令人惊讶的是，特定群体的出口周转率表明，相对于较大的丝状毛藻，小型单细胞重氮藻蓝藻 (UCYN) 的出口效率更高. 反复发现含有藻红蛋白的 UCYN-B 和 UCYN-C 样细胞嵌入大的 (>50 µm) 有机聚集体中或组织成由细胞外基质连接的数十至数百个细胞的簇，这可能促进了它们的输出。在南太平洋之外，我们的数据得到了对其他海盆收集的塔拉海洋宏基因组分析的支持，从而将我们的结果范围扩展到全球。我们表明，当在透光区发现固氮菌时，它们也系统地存在于中远洋水域，表明它们被输送到深海。因此，我们得出结论，固氮菌是深海中固碳的重要组成部分，因此，它们需要在区域和全球出口估算中考虑在内。