Editing Endosymbiont (section)

==== Phytoplankton ====
{{Further|Microbial loop}}

In marine environments,<ref name="Villareal-1994">{{Cite journal|vauthors=Villareal T  |date=1994|title=Widespread occurrence of the Hemiaulus-cyanobacterial symbiosis in the southwest North Atlantic Ocean |journal=Bulletin of Marine Science|volume=54|pages=1–7}}</ref><ref name="Carpenter-1999">{{Cite journal |vauthors=Carpenter EJ, Montoya JP, Burns J, Mulholland MR, Subramaniam A, Capone DG |date=20 August 1999 |title=Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean|journal=Marine Ecology Progress Series |volume=185 |pages=273–283 |doi=10.3354/meps185273|bibcode=1999MEPS..185..273C|doi-access=free |hdl=1853/43100 |hdl-access=free }}</ref><ref name="Foster-2007">{{Cite journal|vauthors=Foster RA, Subramaniam A, Mahaffey C, Carpenter EJ, Capone DG, Zehr JP |s2cid=53504106 |date=March 2007 |title=Influence of the Amazon River plume on distributions of free-living and symbiotic cyanobacteria in the western tropical north Atlantic Ocean|journal=Limnology and Oceanography |volume=52|issue=2|pages=517–532|doi=10.4319/lo.2007.52.2.0517|bibcode=2007LimOc..52..517F |doi-access=free}}</ref><ref>{{cite journal |vauthors=Subramaniam A, Yager PL, Carpenter EJ, Mahaffey C, Björkman K, Cooley S, Kustka AB, Montoya JP, Sañudo-Wilhelmy SA, Shipe R, Capone DG |display-authors=6 |title=Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=105 |issue=30 |pages=10460–10465 |date=July 2008 |pmid=18647838 |pmc=2480616 |doi=10.1073/pnas.0710279105 |doi-access=free }}</ref> endosymbiont relationships are especially prevalent in [[Trophic state index|oligotrophic]] or nutrient-poor regions of the ocean like that of the North Atlantic.<ref name="Villareal-1994" /><ref name="Goebel-2010">{{cite journal |vauthors=Goebel NL, Turk KA, Achilles KM, Paerl R, Hewson I, Morrison AE, Montoya JP, Edwards CA, Zehr JP |display-authors=6 |title=Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N₂ fixation in the tropical Atlantic Ocean |journal=Environmental Microbiology |volume=12 |issue=12 |pages=3272–3289 |date=December 2010 |pmid=20678117 |doi=10.1111/j.1462-2920.2010.02303.x |bibcode=2010EnvMi..12.3272G }}</ref><ref name="Carpenter-1999" /><ref name="Foster-2007" /> In such waters, cell growth of larger [[phytoplankton]] such as [[diatom]]s is limited by (insufficient) [[nitrate]] concentrations.<ref name="Foster-2011">{{cite journal |vauthors=Foster RA, Kuypers MM, Vagner T, Paerl RW, Musat N, Zehr JP |title=Nitrogen fixation and transfer in open ocean diatom-cyanobacterial symbioses |journal=The ISME Journal |volume=5 |issue=9 |pages=1484–1493 |date=September 2011 |pmid=21451586 |pmc=3160684 |doi=10.1038/ismej.2011.26 |bibcode=2011ISMEJ...5.1484F }}</ref> Endosymbiotic bacteria fix nitrogen for their hosts and in turn receive organic carbon from photosynthesis.<ref name="Goebel-2010" /> These symbioses play an important role in global [[carbon cycle|carbon cycling]].<ref>{{Cite journal|vauthors=Scharek R, Tupas LM, Karl DM |date=11 June 1999|title=Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station Aloha |journal=Marine Ecology Progress Series |volume=182|pages=55–67|doi=10.3354/meps182055|bibcode=1999MEPS..182...55S|doi-access=free|hdl=10261/184131|hdl-access=free}}</ref><ref name="Carpenter-1999" /><ref name="Foster-2007" />

One known symbiosis between the diatom ''[[Cyanobiont#Diatoms|Hemialus]]'' spp. and the cyanobacterium ''[[Richelia intracellularis]]'' has been reported in North Atlantic, Mediterranean, and Pacific waters.<ref name="Villareal-1994" /><ref name="Carpenter-1999" /><ref>{{cite journal |vauthors=Zeev EB, Yogev T, Man-Aharonovich D, Kress N, Herut B, Béjà O, Berman-Frank I |title=Seasonal dynamics of the endosymbiotic, nitrogen-fixing cyanobacterium Richelia intracellularis in the eastern Mediterranean Sea |journal=The ISME Journal |volume=2 |issue=9 |pages=911–923 |date=September 2008 |pmid=18580972 |doi=10.1038/ismej.2008.56 |doi-access=free |bibcode=2008ISMEJ...2..911Z }}</ref> ''Richelia'' is found within the [[diatom frustule]] of ''Hemiaulus'' spp., and has a reduced genome.<ref name="Hilton-2013">{{cite journal |vauthors=Hilton JA, Foster RA, Tripp HJ, Carter BJ, Zehr JP, Villareal TA |title=Genomic deletions disrupt nitrogen metabolism pathways of a cyanobacterial diatom symbiont |journal=Nature Communications |volume=4 |issue=1 |pages=1767 |date=23 April 2013 |pmid=23612308 |pmc=3667715 |doi=10.1038/ncomms2748 |bibcode=2013NatCo...4.1767H }}</ref> A 2011 study measured nitrogen fixation by the [[cyanobacteria]]l host ''Richelia intracellularis'' well above intracellular requirements, and found the cyanobacterium was likely fixing nitrogen for its host.<ref name="Foster-2011" /> Additionally, both host and symbiont cell growth were much greater than free-living ''Richelia intracellularis'' or symbiont-free ''Hemiaulus'' spp.<ref name="Foster-2011" /> The ''Hemaiulus''-''Richelia'' symbiosis is not obligatory, especially in nitrogen-replete areas.<ref name="Villareal-1994" />

''Richelia intracellularis'' is also found in ''Rhizosolenia'' spp., a diatom found in oligotrophic oceans.<ref name="Goebel-2010" /><ref name="Foster-2011" /><ref name="Foster-2007" /> Compared to the ''Hemaiulus'' host, the endosymbiosis with ''Rhizosolenia'' is much more consistent, and ''Richelia intracellularis'' is generally found in ''Rhizosolenia''.<ref name="Villareal-1994" /> There are some asymbiotic (occurs without an endosymbiont) Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions.<ref name="Villareal-1989">{{Cite journal|vauthors=Villareal TA |date=December 1989 |title=Division cycles in the nitrogen-fixingRhizosolenia(Bacillariophyceae)-Richelia(Nostocaceae) symbiosis |journal=British Phycological Journal |volume=24 |issue=4 |pages=357–365 |doi=10.1080/00071618900650371|doi-access=free }}</ref> Cell division for both the diatom host and cyanobacterial symbiont can be uncoupled and mechanisms for passing bacterial symbionts to daughter cells during cell division are still relatively unknown.<ref name="Villareal-1989" />

Other endosymbiosis with nitrogen fixers in open oceans include ''[[Calothrix]]'' in ''[[Chaetoceros]]'' spp. and UNCY-A in [[prymnesiophyte]] microalga.<ref name="Zehr-2015">{{cite journal |vauthors=Zehr JP |title=EVOLUTION. How single cells work together |journal=Science |volume=349 |issue=6253 |pages=1163–1164 |date=September 2015 |pmid=26359387 |doi=10.1126/science.aac9752 |s2cid=206641230 }}</ref> The ''Chaetoceros''-''Calothrix'' endosymbiosis is hypothesized to be more recent, as the ''Calothrix'' genome is generally intact. While other species like that of the UNCY-A symbiont and Richelia have reduced genomes.<ref name="Hilton-2013" /> This reduction in genome size occurs within nitrogen metabolism pathways indicating endosymbiont species are generating nitrogen for their hosts and losing the ability to use this nitrogen independently.<ref name="Hilton-2013" /> This endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles (above).<ref name="Zehr-2015" />