Genome of Richelia intracellularis HM01, the endosymbiotic cyanobacterium of host diatom Hemiaulus membranaceus.. Richelia intracellularis HM01

Symbioses between diatoms and heterocyst-forming N2-fixing cyanobacteria (termed diatom-diazotroph associations, DDAs) are widespread in oligotrophic open ocean environments, and are an important source of new production. However, little is known about the nature of these associations. We sequenced the genomes of Richelia intracellularis HH01, a cyanobacterium that is found within the frustule of the diatom Hemiaulus hauckii, and R. intracellularis HM01, endosymbiont of H. membranaceus. The identity level found between the coding sequences of the H. hauckii and H. membranaceus symbionts supports previous studies on partial hetR gene sequences, and the specificity of symbionts for each host taxa. The synteny and overall similarity of these two genomes suggests that the mechanisms of symbiosis and lifestyles are also similar, and provides evidence for a single symbiont infection in a Hemiaulus ancestor, and the host and symbiont have co-evolved.