Aquatic N-fixation is generally associated with the growth and mass development of Cyanobacteria in nitrogen-deprived photic zones. However, sequenced genomes and environmental surveys suggest active aquatic N-fixation also by many non-cyanobacterial groups. Here, we revealed the seasonal variation and genomic diversity of potential N-fixers in a humic bog lake using metagenomic data and nif gene clusters analysis. Groups with diazotrophic operons were functionally divergent and included Cholorobi, Geobacter, Desulfobacterales, Methylococcales, and Acidobacteria. In addition to nifH (a gene that encodes the dinitrogenase reductase component of the molybdenum nitrogenase), we also identified sequences corresponding to vanadium and iron-only nitrogenase genes. Within the Chlorobi population, the nitrogenase (nifH) cluster was included in a well-structured retrotransposon. Furthermore, the presence of light-harvesting photosynthesis genes implies that anoxygenic photosynthesis may fuel nitrogen fixation under the prevailing low-irradiance conditions. The presence of rnf genes (related to the expression of H⁺/Na⁺-translocating ferredoxin: NAD+ oxidoreductase) in Methylococcales and Desulfobacterales suggests that other energy-generating processes may drive the costly N-fixation in the absence of photosynthesis. The highly reducing environment of the anoxic bottom layer of Trout Bog Lake may thus also provide a suitable niche for active N-fixers and primary producers. While future studies on the activity of these potential N-fixers are needed to clarify their role in freshwater nitrogen cycling, the metagenomic data presented here enabled an initial characterization of previously overlooked diazotrophs in freshwater biomes.

水生固氮通常与海藻的生长和大量发育有关。 蓝细菌在缺氮的光区。然而，测序的基因组和环境调查表明，许多非蓝细菌类群也对水生氮具有活性。在这里，我们使用宏基因组学数据和揭示了腐殖质沼泽湖中潜在N-固色剂的季节性变化和基因组多样性。尼夫基因聚类分析。重氮营养操纵子组在功能上有所不同，包括霍洛比， 地细菌， 脱硫细菌， 甲基球菌和 酸菌。此外镍氢（一个编码钼固氮酶二硝基酶还原酶成分的基因），我们还鉴定了对应于钒和仅铁固氮酶基因的序列。内克洛罗比 人口，固氮酶（镍氢）簇被包含在结构良好的逆转座子中。此外，光捕获光合作用基因的存在暗示，在主要的低辐照条件下，产氧光合作用可能促进固氮作用。存在的rnf基因（与H ⁺ / Na ^+-易位铁氧还蛋白的表达有关：NAD +氧化还原酶）甲基球菌 和 脱硫细菌这表明在没有光合作用的情况下，其他产生能量的过程可能会导致昂贵的固氮。鳟鱼沼泽湖缺氧底层的高度还原性环境因此也可以为活跃的固氮剂和初级生产者提供合适的生态位。尽管需要进一步研究这些潜在的N-固色剂的活性，以阐明它们在淡水氮循环中的作用，但此处提供的宏基因组学数据可以初步表征淡水生物群落中先前被忽视的重氮营养菌。