Biological nitrogen fixation contributes significantly to marine primary productivity. The current view depicts few cyanobacterial diazotrophs as the main marine nitrogen fixers. Here, we used 891 Tara Oceans metagenomes derived from surface waters of five oceans and two seas to generate a manually curated genomic database corresponding to free-living, filamentous, colony-forming, particle-attached, and symbiotic bacterial and archaeal populations. The database provides the genomic content of eight cyanobacterial diazotrophs including a newly discovered population related to known heterocystous symbionts of diatoms, as well as 40 heterotrophic bacterial diazotrophs that considerably expand the known diversity of abundant marine nitrogen fixers. These 48 populations encapsulate 92% of metagenomic signal for known nifH genes in the sunlit ocean, suggesting that the genomic characterization of the most abundant marine diazotrophs may be nearing completion. Newly identified heterotrophic bacterial diazotrophs are widespread, express their nifH genes in situ, and also occur in large planktonic size fractions where they might form aggregates that provide the low-oxygen microenvironments required for nitrogen fixation. Critically, we found heterotrophic bacterial diazotrophs to be more abundant than cyanobacterial diazotrophs in most metagenomes from the open oceans and seas, emphasizing the importance of a wide range of heterotrophic populations in the marine nitrogen balance.

生物固氮对海洋初级生产力有显着贡献。目前的观点认为很少有蓝藻固氮菌是主要的海洋固氮菌。在这里，我们使用了 891 Tara海洋宏基因组来源于五个大洋和两个海洋的地表水，以生成一个手动整理的基因组数据库，对应于自由生活、丝状、菌落形成、颗粒附着和共生的细菌和古细菌种群。该数据库提供了八种蓝藻固氮菌的基因组内容，包括新发现的与已知的硅藻异形共生体相关的种群，以及 40 种异养固氮菌，它们大大扩展了丰富的海洋固氮菌的已知多样性。这 48 个群体封装了 92% 的已知nifH宏基因组信号阳光照射的海洋中的基因，这表明最丰富的海洋固氮菌的基因组特征可能已接近完成。新发现的异养细菌固氮菌分布广泛，在原位表达它们的nifH基因，也存在于大型浮游生物中，它们可能形成聚集体，提供固氮所需的低氧微环境。至关重要的是，我们发现在来自开阔海洋的大多数宏基因组中，异养细菌重氮菌比蓝藻重氮菌更丰富，强调了广泛的异养种群在海洋氮平衡中的重要性。