Epithemia

Epithemia is a genus of diatoms belonging to the family Rhopalodiaceae. The genus has cosmopolitan distribution and are found in freshwater and marine ecosystems. Recent studies have proposed that the genus Rhopalodia should be recategorized to join Epithemia based on phylogenetic evidence, although this change in nomenclature has been disputed or ignored.
Members of this genus have endosymbionts that fix nitrogen called spheroid bodies, that are derived from cyanobacteria. Because of their nitrogen fixing endosymbionts, they can be a possible indicator of eutrophication, because Epithemia abundance decreased with increased ambient inorganic N concentrations.

Endosymbiosis

Epithemia and Rhopalodia, two genera of rhopalodiacean diatoms, have nitrogen-fixing endosymbioants called spheroid bodies 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 faster evolution and more gene loss compared to the SB of E. gibberula.
The diazoplasts have no photosynthetic genes at all. They have complete OPP, C3 glycolysis, AA synthesis, and glycogen utilization pathways, but see their C6 glycolysis and TCA cycle interrupted by gene deletions. They retain thylakoid membranes. They perform nitrogen fixation in both daytime and nighttime, relying on catabolizing host-provided C3 and C6 sugars.
Unlike other organelles, the diazoplast does 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 way to more easily engineer nitrogen fixation into crops.

Relatives

The closest relatives of the SBs are various members of the genus Cyanothece, especially strain ATCC 51142. Based on fossil records, the symbiosis happened in a common ancestor of species that carry it about 35 million years ago. The SBs are closely related to the nitroplasts, which had separately entered into an 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. The closest named to the SBs is Rippkaea orientalis.

Species

Those marked with a * were previously in the genus Rhopalodia.

Epithemia adnata Brébisson, 1838
Epithemia alpestris
Epithemia alpestris
Epithemia anasthasiae
Epithemia argus Kützing 1844
Epithemia catenata
Epithemia constricta W. Smith *
Epithemia gibba *
Epithemia gibberula *
Epithemia musculus *
Epithemia pelagica
Epithemia reicheltii Schmidt et al. 1904
Epithemia smithii Carruthers 1864
Epithemia sorex
''Epithemia turgida''

Phylogeny

Key to clade labels:

Epithemia s.l.. Also the last common ancestor node of all diazoplast-carrying species.
Freshwater clade
Traditional Epithemia s.s.
Freshwater Rhopalodia

What is traditionally known as Rhopalodia is paraphyletic to Epithemia. A merge is performed by Ruck et al. to maintain monophyly. An alternative would be to constrict Rhopalodia to the freshwater clade, moving the grade of "marine Rhopalodia" to incertae sedis. In this new view, Rhopalodia sensu stricto would be sister to Epithemia.
Traditional Epithemia is sister to Tetralunata according to the scientists who proposed Tetralunata. Larger Epithemia is sister to Protokeelia, as far as the only species sampled in Ruck et al. - Protokeelia bassonii - is concerned. There is considerable uncertainty regarding the placement of Auricula.