This startling study proved the universal symbiosis of diatom genus Epithemia with N2-fixing intracellular cyanobacteria by providing evidence for marine Epithemia species able to fix nitrogen, similar to their freshwater allies. Oceanographers have known about diatom-cyanobacteria symbioses in waters around Hawaiʻi for many years, and finally successfully isolated and cultured two new and unusual diatom species.

I was excited to help with the taxonomic descriptions of these novel marine Epithemia species, each harboring one or two N2-fixing coccoid cyanobacteria. Cyanobacterial endosymbionts in these marine diatoms form a monophyletic clade with the endosymbionts of freshwater E. turgida but each reside on distinct phylogenetic branch. One of the species, E. pelagica is unicellular and shows typical morphology for Rhopalodiales, while the other species - E. catenata was a surprise.

Epithemia catenata cells are nearly symmetrical and joined in chains. The frustules are translucent without visible striation and with central slightly sigmoid fibulate raphe. Extensive phylogenetic analyses show greater support for the inclusion of E. catenata within the genus Epithemia, than for its exclusion from the genus and the Rhopalodiales. The phylogenetic placement of this novel and morphologically unusual chain-forming N2-fixing diatom opens many exciting avenues of further research, especially when genome sequencing data for more rhopalodiacean genera become available. The cultures are also valuable as model systems for studying symbiosis.

Diazotrophy is a critical process, particularly within the open ocean, which is poor in nutrients. Nucleotide sequences of these two species were confirmed from ocean waters around the world, including the northern Pacific, northern and southern Atlantic, and Indian oceans. They were also found in the seas near China, Philippines, and Japan.

Top image: Three cells of E. pelagica (SEM, uncleaned material). Image by Chris Schvarcz.
Lower left image:
Raphe of E. catena (transmission electron micrograph [TEM], cleaned material). Image by Tina Weatherby.
Lower right image:
Chain of three cells of E. catena (SEM, uncleaned material). Image by Chris Schvarcz.

Schvarcz, C. R., Wilson, S. T., Caffin, M., Stancheva, R., Li, Q., Turk-Kubo, K. A., White, A. E., Karl, D. M., Zehr, J. P. and Steward G. F. 2022. Overlooked and widespread pennate diatom-diazotroph symbioses in the sea. Nature Communications.