# Novel pennate diatom symbionts support high N2 fixation rates

## TL;DR

A new type of diatom with nitrogen-fixing symbionts may significantly contribute to nitrogen availability in the ocean.

## Contribution

Discovery of active nitrogen fixation in pennate diatoms symbiotic with rhizobial and cyanobacterial partners in the Amazon River plume region.

## Key findings

- Pennate diatoms like Mastogloia and Haslea host rhizobial and cyanobacterial symbionts capable of high N2 fixation rates.
- NanoSIMS analysis confirmed significant nitrogen fixation in these diatom-symbiont associations.
- This symbiosis may represent a new and unquantified source of biologically available nitrogen in oligotrophic oceans.

## Abstract

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.