Genome of Richelia intracellularis HH01, the endosymbiotic cyanobacterium of the diatom host Hemiaulus hauckii. Richelia intracellularis HH01

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 genome of Richelia intracellularis HH01, a cyanobacterium that is found within the frustule of the diatom Hemiaulus hauckii. The genome size (3.2 Mb), GC content (34%), percent coding (56%), and transporter count (187) are all reduced relative to previously sequenced heterocystous cyanobacteria, and strongly suggest the cyanobacterial symbiosis is obligate. The R. intracellularis HH01 genome lacks the genes for ammonium transporters, nitrate or nitrite reductase, or urease, greatly limiting the forms of nitrogen available for growth. Furthermore, the absence of a gene to encode glutamate synthase (GOGAT) alters the pathway for assimilating N2-derived ammonium. R. intracellularis HH01 has evolved a modified nitrogen metabolic pathway selective for enhanced N2-fixation and efficient nitrogen transfer to the host diatom.