Genomic deletions disrupt nitrogen metabolismpathways of a cyanobacterial diatom symbiont

摘要

Diatoms with symbiotic N2-fixing cyanobacteria are often abundant in the oligotrophic openocean gyres. The most abundant cyanobacterial symbionts form heterocysts (specializedcells for N2 fixation) and provide nitrogen (N) to their hosts, but their morphology, cellularlocations and abundances differ depending on the host. Here we show that the location of thesymbiont and its dependency on the host are linked to the evolution of the symbiont genome.The genome of Richelia (found inside the siliceous frustule of Hemiaulus) is reduced andlacks ammonium transporters, nitrateitrite reductases and glutamine:2-oxoglutarateaminotransferase. In contrast, the genome of the closely related Calothrix (found outsidethe frustule of Chaetoceros) is more similar to those of free-living heterocyst-formingcyanobacteria. The genome of Richelia is an example of metabolic streamlining that hasimplications for the evolution of N2-fixing symbiosis and potentially for manipulatingplant–cyanobacterial interactions.