Non-cyanobacterial diazotrophs (NCDs) have recently emerged as potentially important contributors to marine nitrogen fixation. One of the most widely distributed NCDs is Gamma-A, yet information about its autecology is still scarce and solely relies on the PCR-based detection of its nitrogenase (nifH) gene in seawater, since previous metagenomic surveys targeting free-living planktonic size fractions (<3 μm) have not detected it. Here, we explore the diversity, biogeography, size-distribution, and nitrogenase gene expression of Gamma-A across four larger planktonic size-fractions (0.8–5, 5–20, 20–180, and 180–2000 μm) using metagenomes and metatranscriptomes from the Tara Oceans. We detected a single variant of a complete Gamma-A nifH gene along with other nitrogenase-related genes (nifKDT) within a metatranscriptomic-based contig of the Marine Atlas of Tara Ocean Unigenes. Gamma-A was detected in tropical and subtropical oceanic regions across all the size-fractions. However, the highest gene and transcript abundances were found in the 0.8–5 and 5–20 μm size-fractions at the surface, whereas abundances at the deep chlorophyll maximum were lower and similar across all size-fractions. The ubiquitous presence of active Gamma-A in large planktonic size-fractions suggests a filamentous or particle-attached lifestyle and places its potential to fix nitrogen in larger planktonic compartments.

非蓝藻固氮菌 (NCD) 最近已成为海洋固氮的潜在重要贡献者。分布最广泛的非传染性疾病之一是 Gamma-A，但关于其自身生态学的信息仍然很少，并且完全依赖于基于 PCR 的海水中固氮酶 ( nifH ) 基因的检测，因为之前的宏基因组调查针对的是自由生活的浮游生物大小分数(<3 μm) 没有检测到它。在这里，我们使用宏基因组探索了 Gamma-A 的多样性、生物地理学、尺寸分布和固氮酶基因表达，跨越四个较大的浮游生物尺寸分数（0.8-5、5-20、20-180 和 180-2000 μm）。来自泰拉洋的元转录组。我们检测到完整 Gamma-A nifH的单个变体基因以及其他固氮酶相关基因 ( nifKDT ) 位于 Tara Ocean Unigenes 海洋地图集的基于宏转录组的重叠群中。Gamma-A 在热带和亚热带海洋区域的所有尺寸部分都被检测到。然而，在表面的 0.8-5 和 5-20 μm 尺寸部分中发现了最高的基因和转录本丰度，而在所有尺寸部分的深层叶绿素最大值处的丰度较低且相似。活性 Gamma-A 在大型浮游生物中无处不在，这表明它是一种丝状或附着颗粒的生活方式，并具有在更大的浮游生物隔室中固氮的潜力。