Award: OCE-1733610

Nitrogen is a key element in all organisms. In the ocean, the availability of nitrogen is often limiting the plankton primary production, but certain micro-organisms can alleviate this shortage by a biological mechanism: nitrogen (N2) fixation. The aim of this study was - through a variety of methods assessments - to investigate the importance of N2 fixation at depths below the sunlit surface layers of the ocean, and to elucidate the overall importance of N2 fixing microbes (diazotrophs) that are not cyanobacteria. A specific focus of this part of the collaborative project was to assess the community composition of these microbes in the water column by determining their relative abundances based on DNA sequencing, by assessing their gene expression activity, and by examining the abundances of the most prevalent community members with additional molecular quantification methods (quantitative Polymerase Chain Reaction). The project involved collection of samples in the North Atlantic Ocean water column, and bioinformatic analyses of these samples in the context of samples collected and analyzed from other ocean regions. The results show wide presence of putative diazotroph sequences from surface ocean extending to several hundred meters deep in the ocean where energy from sunlight does not penetrate. In this study, the deep communities appeared to generally exhibit low to non-detectable N2 fixation activity. The relative roles of the non-cyanobacterial groups potentially contributing to N2 fixation in the ocean appear to vary, with most apparently exhibiting a low N2 fixation activity. Yet, some of the non-cyanobacterial members of the microbial communities that are containing a key gene required for N2 fixation, show persistent depth-specific stratification both in surface (sunlit) and the deep (non-sunlit) layers, suggesting the distributions are not random but likely controlled by biological and/or chemical and physical factors. Last Modified: 06/25/2019 Submitted by: Pia Moisander