Nitrogen fixation is the primary source of reactive nitrogen in the ocean. Most ecological models do not predict nitrogen fixation in the Antarctic Ocean because of the low availability of iron and high abundance of nitrogen. Here we extensively examined nitrogen fixation in the Antarctic Ocean, and found substantial nitrogen fixation (maximum: 44.4 nmol N l⁻¹ d⁻¹) near the Antarctic coast, especially around ice-covered regions. The nitrogenase gene (nifH) was detected at all coastal stations, including stations where no nitrogen fixation was found. At the stations where nitrogen fixation was detected, the nitrogen-fixing cyanobacterium UCYN-A (Candidatus ‘Atelocyanobacterium thalassa’) dominated nifH gene expression, and the nifH sequence was identical to that of the major oligotype in tropical and subtropical oceans. Our results suggest that marine nitrogen fixation is a ubiquitous process in the global ocean, and that UCYN-A is the keystone species for making it possible.

固氮是海洋中活性氮的主要来源。由于铁的低可用性和高氮含量，大多数生态模型无法预测南极洲的固氮作用。在这里，我们广泛地研究了南极海洋中的固氮作用，并在南极海岸附近，尤其是在冰雪覆盖的地区附近发现了大量固氮作用（最大值：44.4 nmol N l ^-1  d ^-1）。在所有沿海站点（包括未发现固氮的站点）均检测到了固氮酶基因（nifH）。在检测到固氮的站点上，固氮蓝细菌UCYN-A（Candidatus'Atelocyanobacterium thalassa '）占主导地位。基因表达，并且nifH序列与热带和亚热带海洋中的主要寡聚型相同。我们的结果表明，海洋固氮是全球海洋中普遍存在的过程，而UCYN-A是使其成为可能的关键物种。