Epithemia is a genus of diatoms belonging to the family Rhopalodiaceae. Species in this genus have a cosmopolitan distribution and are found in freshwater and marine ecosystems. Recent studies have proposed that the genus Rhopalodia should be merged into Epithemia based on phylogenetic evidence, although this taxonomic change 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 may serve as indicators of eutrophication, because Epithemia abundance decreases with increased ambient inorganic N concentrations.

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