Rhizosolenia mat diatoms associate with nitrogen-fixing microbes.

Some Rhizosolenia diatoms living in oligotrophic marine ecosystems are known to form large, conspicuous mats and are thought to be sources of new nitrogen to surface waters via vertical migration to the nitracline where subsurface nitrate is accessed for growth. These vertically migrating Rhizosolenia mats are chronically under sampled, and both the diatom species comprising the mats and the associated microbiome have not been characterized using modern molecular techniques. Here we present the first DNA-based analysis of Rhizosolenia mats collected in the North Pacific Subtropical Gyre. Using sequencing of 18S rRNA and nifH genes (a proxy for N2 fixation capacity), we report on the molecular diversity of mat-forming Rhizosolenia species, which include two newly sequenced clades, and an assemblage of associated N2-fixing microorganisms that is distinct from the non-mat associated water column assemblage. Our findings advance knowledge of oligotrophic diatom diversity and challenge prevailing views of their nitrogen sources, suggesting these mats may obtain nitrogen through association-based N2 fixation. Further work is needed to understand the nature of these associations, and whether Rhizosolenia mat communities are a significant unrecognized source of N2-fixation-derived new nitrogen to the oligotrophic surface waters.