The genome sequence of the heterocyst-forming cyanobacterial symbiont of the diatom Rhizosolenia. Richelia intracellularis RC01

Heterocyst-forming cyanobacteria form associations with a wide variety of eukaryotic hosts, including marine diatoms. The relationships of Nostoc spp. and Anabaena spp. with multicellular plants are well-studied, but little is known about diatom-diazotroph associations (DDAs). The symbiont of the marine diatom Hemiaulus hauckii was recently shown to have a streamlined genome, especially within nitrogen metabolism pathways. Richelia is also commonly found in oligotrophic open ocean environments associated with the diatom Rhizosolenia clevei. We obtained a draft genome of R. intracellularis from cells separated from the host diatom R. clevei by flow cytometry sorting. The genome of R. intracellularis has low percent coding (64%) relative to the sequenced of heterocyst-forming cyanobacteria (averages of 80% and 41%, respectively), but not as low as the H. hauckii symbiont (56%). Additionally, the R. clevei symbiont lacks several genes encoding for key nitrogen acquisition proteins, like the H. hauckii symbiont. However, a gene encoding glutamate synthase is present in the R. clevei symbiont genome. Hypotheses regarding the symbiont evolution, interactions between host and symbiont, and the implications for cycling of nitrogen and carbon in the open ocean are considered.