A nitrogen-fixing seagrass symbionts

Terrestrial plants that grow in N-poor habitats are often associated with symbiotic N₂-fixing bacteria. Seagrasses emerged about 100 million years ago from terrestrial flowering plants that migrated back to the sea, but thus far symbiont microbes have not been described. Here, Mohr and colleagues report the discovery and characterization of the first seagrass symbiont, ‘Candidatus Celerinatantimonas neptuna’, which inhabits the root tissue of the seagrass Posidonia oceanica, found in the N-limited Mediterranean Sea. They demonstrate that ‘Ca. C. neptuna’ is capable of N₂ fixation, providing its host with ammonia and amino acids in exchange for sugars. The authors also identify relatives of ‘Ca. C. neptuna’ in coastal waters around the globe, suggesting that this may be a widespread mechanism enabling seagrasses to thrive and likely was important for the transition of flowering plants to invade N-poor marine habitats.