cyanobacterium endosymbiont of benthic diatoms
Diatoms in the genus Epithemia host endosymbiotic cyanobacteria called spheroid bodies (SBs) that are in the process of becoming nitrogen-fixing organelles. By providing biologically usable N, SBs allow their diatom hosts to live in freshwater environments around the world that lack a preferred N source such as nitrate. Over ~12 million years of coevolution with its host, the SB genome has been reduced by half compared to its closest free-living relative. This large-scale genome reduction is a defining trait in long-lasting endosymbiotic relationships. We sequenced the complete spheroid body genomes from isolates of diatoms in the genus Epithemia to expand the diversity of available SB genomes and further our understanding of the evolutionary dynamics of the diatom-SB relationship.