Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle

Members of the diatom family Rhopalodiaceae possess a cyanobacterial endosymbiont called a “spheroid body.” The spheroid body evolved much more recently than did mitochondria or plastids and is predicted to fix nitrogen. Here we present what is, to our knowledge, the first completely sequenced spheroid body genome from a rhopalodiacean diatom. Comparative analyses revealed that the endosymbiont is metabolically reduced, confirming its status as an obligate endosymbiont. The genome possesses genes for nitrogen fixation, and, to our surprise, no essential genes for photosynthesis. Thus, the spheroid body is, to our knowledge, the first known example of a nonphotosynthetic cyanobacterium, free-living or symbiotic. Rhopalodiacean diatoms have the potential to provide unique insight into the evolution of bacterial endosymbionts and their hosts.