Primary productivity is limited by the availability of fixed nitrogen in large regions of theoceans. Dinitrogen fixation, the only biological input pathway into the marine N cycle, is anenergetically expensive biochemical process that reduces N2 gas into NH3, a form of fixednitrogen that is readily incorporated into biomolecules. The nitrogen fixers, or diazotrophs,are a selected group of prokaryotic microorganisms that can carry out this biochemicalprocess. Historically, marine nitrogen fixation was thought to be a process carried outprimarily by cyanobacteria and important mainlyin the tropical and subtropical oligotrophicwaters. Recent realization concerning the wide diversity of marine microbes harboring thenitrogenase enzyme indicates that we do notfully understand theroles of the diversediazotrophs that populate the ocean.
In the context of the Ocean Frontier Institute located atDalhousie University, the microbial community structure and function in Northwest Atlantic(NWA) have been assessed through next-generation sequencing of hypervariable regions of16S and 18S rRNA genes, nifH gene and metagenomics at existing time-series stations since2014. The nifH gene, a marker gene for diazotrophy, has shown that both cyanobacterial andnon-cyanobacterial diazotrophs are members of the microbial communities in our NWAmicrobial observatories. The lecture will focus on the microbial community structure in theNWA, with a specific attention to the diazotrophs. In particular, the potential metabolicpathways identified from the genome annotationof a novel bacterial isolate, belonging to aclade of gamma-proteobacteria widely distributed in the Tara expedition database, will bediscussed in a global context.