Novel pennate diatom symbionts support high N₂ fixation rates

Diazotrophy is the most important nitrogen source in the oligotrophic surface ocean, but the organisms involved and their contributions are incompletely understood due to limited observations. Only diazotrophic organisms possess the nifH gene to reduce dinitrogen to ammonium, but their distribution and activity can only be quantified through sampling and experiments during research cruises. Some recent studies document small diatoms with symbionts able to fix nitrogen, a new source of biologically available nitrogen in addition to the well-known cyanobacterial species such as Trichodesmium or symbionts of haptophytes (UCYN-A) and diatoms (Diatom-Diazotroph Associations, or DDAs). Here, we document a very active symbiosis between small pennate diatoms such as Mastogloia and Haslea with rhizobial and cyanobacterial symbionts in waters of the Western tropical North Atlantic influenced by the Amazon River plume. We used NanoSIMS analysis of ¹⁵N₂ tracer experiments to quantify high rates of nitrogen fixation in generally abundant, symbiont-bearing pennate diatoms. This newly described symbiosis may contribute a previously unquantified flux of biologically available nitrogen to oceanic systems. Pennate diatoms and their symbionts may close a key gap in our understanding of the supply of nutrients to the ocean and provide a previously unknown biological sink for carbon dioxide.