Epithemia and
Rhopalodia, two (or one) genera of rhopalodiacean diatoms, have nitrogen-fixing endosymbionts called spheroid bodies (SBs) or
diazoplasts in addition to the regular chloroplast and mitochondria. They are cyanobacteria that have undergone various degrees of gene loss in adaptation to their new dependent lifestyle. The SB of
E. turgida has undergone more rapid evolution and more gene loss compared to the SB of
E. gibberula. The diazoplasts have lost most genes involved in photosynthesis. They have complete
OPP, C3 glycolysis, AA synthesis, and glycogen utilization pathways, but their C6 glycolysis and TCA cycles are interrupted by gene deletions. They retain
thylakoid membranes. They perform nitrogen fixation during both day and night, relying on the catabolism of host-provided C3 and C6 sugars. Unlike other organelles (including the relatively recent cases of nitroplasts), the diazoplasts do not have a high degree of functional gene transfer to the host's nuclear genome and has minimal reliance on host-imported proteins. Nevertheless, they display typical organelle traits of metabolic dependence on the host and coordinated division. This arrangement may suggest a possible route for engineering nitrogen fixation into crops.
Relatives The closest relatives of the SBs are within the family
Aphanothecaceae. The closest known relatives of the SBs are the unnamed unicellular cyanobacterium SU2 and
Rippkaea orientalis PCC 8801. The SBs are closely related to the
nitroplasts, which had entered into a separate endosymbiosis with
Braarudosphaera bigelowii about 90 million years ago.
GTDB annotates the SB of
E. gibberula as
Rippkaea sp003574135, a species-level cluster bearing a placeholder name. Additional assignments include
Ri. sp000829235 for the
E. turgida SB,
Ri. sp029919255 for the
E. clementina SB, and
Ri. sp947331815 for the
E. pelagica SB. ==Species==