AUTHOR=Fernandez Leyden , Peura Sari , Eiler Alexander , Linz Alexandra M. , McMahon Katherine D. , Bertilsson Stefan 

TITLE=Diazotroph Genomes and Their Seasonal Dynamics in a Stratified Humic Bog Lake

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01500

DOI=10.3389/fmicb.2020.01500

ISSN=1664-302X

ABSTRACT=<p>Aquatic N-fixation is generally associated with the growth and mass development of <italic>Cyanobacteria</italic> 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 <italic>nif</italic> gene clusters analysis. Groups with diazotrophic operons were functionally divergent and included <italic>Cholorobi</italic>, <italic>Geobacter</italic>, <italic>Desulfobacterales</italic>, <italic>Methylococcales</italic>, and <italic>Acidobacteria</italic>. In addition to <italic>nifH</italic> (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 <italic>Chlorobi</italic> population, the nitrogenase (<italic>nifH</italic>) 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 <italic>rnf</italic> genes (related to the expression of H<sup>+</sup>/Na<sup>+</sup>-translocating ferredoxin: NAD+ oxidoreductase) in <italic>Methylococcales</italic> and <italic>Desulfobacterales</italic> 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.</p>