Inferring the Metabolic Diversity and Ecological Function of Non-Photosynthetic Diazotrophs in the Context of their Associated Microbial Communities Along a Latitudinal Transect in the Canadian Arctic Gateway

Diazotrophs are a group of microorganisms with the ability to convert N₂ gas into ammonia, thereby contributing to new nitrogen (N) input to the oceanic N inventory. Although diazotrophs are taxonomically and metabolically diverse, they have been broadly separated into cyanobacterial and non-cyanobacterial groups. Classically, the role of cyanobacterial diazotrophs is well established as primary producers in the oligotrophic ocean, where they can escape N limitation. Marine environments at high latitude however are dominated by non-cyanobacterial diazotrophs whose functions in the planktonic community remain unclear. Several well-studied model non-cyanobacterial diazotrophs are metabolically diverse, while genome sequences of diazotrophic isolates from marine environments also point to versatility in substrate utilization and ecological function. Here we combine several approaches ranging from satellite imagery to (meta)genomics to explore the potential metabolic diversity and functional role of non-photosynthetic diazotrophs in the microbial communities of the Canadian Arctic Gateway (CAG). DNA samples were collected at multiple depths during the first leg (July 10 to August 10, 2015) of the Canadian Arctic GEOTRACES (GN02) summer campaign from Québec City to Kugluktuk on board the CCGS Amundsen. Highly multiplexed amplicon sequencing of the nifH gene, a marker gene for diazotrophy, indicated that diazotrophic microbial communities of the CAG waters are different from those observed in the Canadian Arctic Archipelago (CAA). The suite of chemical and biological parameters from the GEOTRACES transect, metagenomic analyses of the microbial communities along the latitudinal transect (55°N to 73°N), as well as the genomes of recently isolated strains, will be used to infer the functional role of diazotrophs within these microbial communities. In particular, we have focused on the Pseudomonas stutzeri, a dominant and widely distributed gamma-proteobacterium in the CAG.