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Research article - Peer-reviewed, 2020

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

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


Aquatic N-fixation is generally associated with the growth and mass development ofCyanobacteriain 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 andnifgene clusters analysis. Groups with diazotrophic operons were functionally divergent and includedCholorobi,Geobacter,Desulfobacterales,Methylococcales, andAcidobacteria. In addition tonifH(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 theChlorobipopulation, 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 ofrnfgenes (related to the expression of H+/Na+-translocating ferredoxin: NAD+ oxidoreductase) inMethylococcalesandDesulfobacteralessuggests 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.


diazotrophs; N-fixation; lake; hypolimnion; nifHgene

Published in

Frontiers in Microbiology
2020, Volume: 11, article number: 1500