Overlooked and widespread pennate diatom-diazotroph symbioses in the sea

Nitrogen depletion in the ocean provides a favourable niche for nitrogen-fixing cyanobacteria, which can form symbioses with eukaryotic algae. This study reports the discovery of two distinct marine pennate diatom-diazotroph symbioses, which had previously only been observed in freshwater environments and represent an overlooked but widespread source of bioavailable nitrogen in marine habitats. Persistent nitrogen depletion in sunlit open ocean waters provides a favorable ecological niche for nitrogen-fixing (diazotrophic) cyanobacteria, some of which associate symbiotically with eukaryotic algae. All known marine examples of these symbioses have involved either centric diatom or haptophyte hosts. We report here the discovery and characterization of two distinct marine pennate diatom-diazotroph symbioses, which until now had only been observed in freshwater environments. Rhopalodiaceae diatoms Epithemia pelagica sp. nov. and Epithemia catenata sp. nov. were isolated repeatedly from the subtropical North Pacific Ocean, and analysis of sequence libraries reveals a global distribution. These symbioses likely escaped attention because the endosymbionts lack fluorescent photopigments, have nifH gene sequences similar to those of free-living unicellular cyanobacteria, and are lost in nitrogen-replete medium. Marine Rhopalodiaceae-diazotroph symbioses are a previously overlooked but widespread source of bioavailable nitrogen in marine habitats and provide new, easily cultured model organisms for the study of organelle evolution.

Automated summary

This study reports the discovery of two distinct marine pennate diatom-diazotroph symbioses, which had previously only been observed in freshwater environments and represent an overlooked but widespread source of bioavailable nitrogen in marine habitats.