Epithemia is a genus of diatoms belonging to the family Rhopalodiaceae.[1] The genus has cosmopolitan distribution and are found in freshwater and marine ecosystems.[2][1] Recent studies have proposed that the genus Rhopalodia should be recategorized to join Epithemia based on phylogenetic evidence,[3] although this change in nomenclature has been disputed[4] or ignored.[5]
Members of this genus have endosymbionts that fix nitrogen called spheroid bodies, that are derived from cyanobacteria.[6][7] Because of their nitrogen fixing endosymbionts, they can be a possible indicator of eutrophication, because Epithemia abundance decreased with increased ambient inorganic N concentrations.[8]
Endosymbiosis
Epithemia and Rhopalodia, two (or one) genera of rhopalodiacean diatoms, have nitrogen-fixing endosymbioants called spheroid bodies (SBs) or diazoplasts[a] 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.[9]
The diazoplasts have no photosynthetic genes at all.[10] 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.[11]
Unlike other organelles (including the relatively recent cases of nitroplasts), 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.[12]
Relatives
The closest relatives of the SBs are various members of the (overly broad, since divided)[13] 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.[7] The SBs are closely related to the nitroplasts,[14] which had separately entered into an endosymbiosis with Braarudosphaera bigelowii about 90 million years ago.[15]
GTDB annotates the SB of E. gibberula as Rippkaea sp003574135, a species-level cluster bearing a placeholder name.[16] 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.[b] The closest named to the SBs is Rippkaea orientalis.[17]
Epithemia s.l. (post-merge sensu Ruck et al. 2016). 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. (2016) to maintain monophyly. An alternative would be to constrict Rhopalodia to the freshwater clade (which includes the type species R. gigga), moving the grade of "marine Rhopalodia" to incertae sedis. In this new view, Rhopalodia sensu stricto would be sister to Epithemia.[3]
Traditional (pre-merge) Epithemia is sister to Tetralunata according to the scientists who proposed Tetralunata.[3] Larger (post-merge) Epithemia is sister to Protokeelia, as far as the only species sampled in Ruck et al. (2016) –Protokeelia bassonii– is concerned. There is considerable uncertainty regarding the placement of Auricula.[3]
References
↑Contrary to what the -plast suffix may imply, thee organelle are not derived from the original plastid.
↑Mapping from provisional names of diazoplasts to published names of Epithemia hosts is obtained as follows: the associated NCBI genome assembly records is queried based on the GTDB genome IDs (e.g. GCF_003574135.1) and the host species name is found from the linked publication or BioSample metadata.
↑Stancheva, Rosalina; Sheath, Robert G.; Read, Betsy A.; McArthur, Kimberly D.; Schroepfer, Chrystal; Kociolek, J. Patrick; Fetscher, A. Elizabeth (2013-12-01). "Nitrogen-fixing cyanobacteria (free-living and diatom endosymbionts): their use in southern California stream bioassessment". Hydrobiologia. 720 (1): 111–127. Bibcode:2013HyBio.720..111S. doi:10.1007/s10750-013-1630-6. ISSN1573-5117. S2CID254550477.
↑Nakayama, Takuro; Inagaki, Yuji (12 October 2017). "Genomic divergence within non-photosynthetic cyanobacterial endosymbionts in rhopalodiacean diatoms". Scientific Reports. 7 (1). doi:10.1038/s41598-017-13578-8.
↑Nakayama, T; Kamikawa, R; Tanifuji, G; Kashiyama, Y; Ohkouchi, N; Archibald, JM; Inagaki, Y (5 August 2014). "Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle". Proceedings of the National Academy of Sciences of the United States of America. 111 (31): 11407–12. doi:10.1073/pnas.1405222111. PMID25049384.
↑Moulin, SLY; Frail, S; Braukmann, T; Doenier, J; Steele-Ogus, M; Marks, JC; Mills, MM; Yeh, E (January 2024). "The endosymbiont of Epithemia clementina is specialized for nitrogen fixation within a photosynthetic eukaryote". ISME communications. 4 (1) ycae055. doi:10.1093/ismeco/ycae055. PMID38707843.
↑Frail, S; Steele-Ogus, M; Doenier, J; Moulin, SLY; Braukmann, T; Xu, S; Yeh, E (19 August 2025). "Genomes of nitrogen-fixing eukaryotes reveal an alternate path for organellogenesis". Proceedings of the National Academy of Sciences of the United States of America. 122 (33) e2507237122. doi:10.1073/pnas.2507237122. PMID40794833.
↑Mareš, J; Johansen, JR; Hauer, T; Zima J, Jr; Ventura, S; Cuzman, O; Tiribilli, B; Kaštovský, J (June 2019). "Taxonomic resolution of the genus Cyanothece (Chroococcales, Cyanobacteria), with a treatment on Gloeothece and three new genera, Crocosphaera, Rippkaea, and Zehria". Journal of phycology. 55 (3): 578–610. doi:10.1111/jpy.12853. PMID30830691.
↑Cornejo‐Castillo, Francisco M.; Muñoz‐Marín, Maria del Carmen; Turk‐Kubo, Kendra A.; Royo‐Llonch, Marta; Farnelid, Hanna; Acinas, Silvia G.; Zehr, Jonathan P. (January 2019). "UCYN‐A3, a newly characterized open ocean sublineage of the symbiotic N 2 ‐fixing cyanobacterium Candidatus Atelocyanobacterium thalassa". Environmental Microbiology. 21 (1): 111–124. doi:10.1111/1462-2920.14429. PMID30255541.
↑Moulin, SLY; Frail, S; Braukmann, T; Doenier, J; Steele-Ogus, M; Marks, JC; Mills, MM; Yeh, E (January 2024). "The endosymbiont of Epithemia clementina is specialized for nitrogen fixation within a photosynthetic eukaryote". ISME communications. 4 (1) ycae055. doi:10.1093/ismeco/ycae055. PMID38707843.
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