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.