Editing Endosymbiont (section)

=== Invertebrates ===
The best-studied examples of endosymbiosis are in [[invertebrate]]s. These symbioses affect organisms with global impact, including ''[[Symbiodinium]]'' (corals), or ''[[Wolbachia]]'' (insects). Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.<ref>{{Cite journal |last1=Eleftherianos |first1=Ioannis |last2=Atri |first2=Jaishri |last3=Accetta |first3=Julia |last4=Castillo |first4=Julio C. |date=2013 |title=Endosymbiotic bacteria in insects: guardians of the immune system? |journal=Frontiers in Physiology |volume=4 |page=46 |doi=10.3389/fphys.2013.00046 |issn=1664-042X |pmc=3597943 |pmid=23508299 |doi-access=free }}</ref>

==== Insects ====
[[File:Cospeciation (5 processes) - with key.png|thumb|right|Diagram of cospeciation, where parasites or endosymbionts speciate or branch alongside their hosts. This process is more common in hosts with primary endosymbionts.]]

Scientists classify insect endosymbionts as Primary or Secondary. Primary endosymbionts (P-endosymbionts) have been associated with their [[insect]] hosts for millions of years (from ten to several hundred million years). They form obligate associations and display [[cospeciation]] with their insect hosts. Secondary endosymbionts more recently associated with their hosts, may be horizontally transferred, live in the [[hemolymph]] of the insects (not specialized bacteriocytes, see below), and are not obligate.<ref>{{cite book |vauthors=Baumann P, Moran NA, Baumann L |chapter=Bacteriocyte-associated endosymbionts of insects |veditors=Dworkin M |title=The prokaryotes |publisher=Springer |location=New York |date=2000 |chapter-url=http://link.springer.de/link/service/books/10125/ }}</ref>

===== Primary =====
Among primary endosymbionts of insects, the best-studied are the pea [[aphid]] (''[[Acyrthosiphon pisum]]'') and its endosymbiont ''[[Buchnera (proteobacteria)|Buchnera]] sp.'' APS,<ref>{{cite journal |vauthors=Douglas AE |title=Nutritional interactions in insect-microbial symbioses: aphids and their symbiotic bacteria Buchnera |journal=Annual Review of Entomology |volume=43 |pages=17–37 |date=January 1998 |pmid=15012383 |doi=10.1146/annurev.ento.43.1.17 |s2cid=29594533 }}</ref><ref name="pmid109930772"/> the [[tsetse fly]] ''Glossina morsitans morsitans'' and its endosymbiont ''[[Wigglesworthia glossinidia brevipalpis]]'' and the endosymbiotic [[protists]] in lower [[termite]]s. As with endosymbiosis in other insects, the symbiosis is obligate. Nutritionally enhanced diets allow symbiont-free specimens to survive, but they are unhealthy, and at best survive only a few generations.<ref>{{Cite web |title=Figure 5—figure supplement 2. KEGG metabolic reconstructions based on the intact genes present in the Acromyrmex, Solenopsis, Apis mellifera and Anopheles gambiae genomes, together constituting the urea cycle. |url=https://elifesciences.org/articles/39209/figures#fig5s2 |doi=10.7554/elife.39209.022 |doi-access=free }}</ref>

In some insect groups, these endosymbionts live in specialized insect cells called [[bacteriocyte]]s (also called ''mycetocytes''), and are maternally transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in the [[Egg (biology)|egg]], as in ''Buchnera''; in others like ''Wigglesworthia'', they are transmitted via milk to the embryo. In termites, the endosymbionts reside within the hindguts and are transmitted through [[trophallaxis]] among colony members.<ref>{{Cite journal |last=Nalepa |first=Christine A. |date=2020 |title=Origin of Mutualism Between Termites and Flagellated Gut Protists: Transition From Horizontal to Vertical Transmission |journal=Frontiers in Ecology and Evolution |volume=8 |doi=10.3389/fevo.2020.00014 |issn=2296-701X |doi-access=free }}</ref>

Primary endosymbionts are thought to help the host either by providing essential nutrients or by metabolizing insect waste products into safer forms. For example, the putative primary role of ''Buchnera'' is to synthesize [[essential amino acid]]s that the aphid cannot acquire from its diet of plant sap. The primary role of ''Wigglesworthia'' is to synthesize [[vitamin]]s that the tsetse fly does not get from the [[blood]] that it eats. In lower termites, the endosymbiotic protists play a major role in the digestion of lignocellulosic materials that constitute a bulk of the termites' diet.

Bacteria benefit from the reduced exposure to [[predator]]s and competition from other bacterial species, the ample supply of nutrients and relative environmental stability inside the host.

Primary endosymbionts of insects have among the smallest of known bacterial genomes and have [[genome reduction|lost many genes]] commonly found in closely related bacteria. One theory claimed that some of these genes are not needed in the environment of the host insect cell. A complementary theory suggests that the relatively small numbers of bacteria inside each insect decrease the efficiency of natural selection in 'purging' deleterious mutations and small mutations from the population, resulting in a loss of genes over many millions of years. Research in which a parallel [[phylogeny]] of bacteria and insects was inferred supports the assumption hat primary endosymbionts are transferred only vertically.<ref>{{cite journal |vauthors=Wernegreen JJ |title=Endosymbiosis: lessons in conflict resolution |journal=PLOS Biology |volume=2 |issue=3 |pages=E68 |date=March 2004 |pmid=15024418 |pmc=368163 |doi=10.1371/journal.pbio.0020068 |df=dmy |doi-access=free }}</ref><ref>{{cite journal |vauthors=Moran NA |title=Accelerated evolution and Muller's rachet in endosymbiotic bacteria |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=93 |issue=7 |pages=2873–2878 |date=April 1996 |pmid=8610134 |pmc=39726 |doi=10.1073/pnas.93.7.2873 |doi-access=free |bibcode=1996PNAS...93.2873M }}</ref>

Attacking obligate bacterial endosymbionts may present a way to control their hosts, many of which are pests or human disease carriers. For example, aphids are crop pests and the tsetse fly carries the organism ''[[Trypanosoma brucei]]'' that causes African [[African trypanosomiasis|sleeping sickness]].<ref>{{cite journal |vauthors=Aksoy S, Maudlin I, Dale C, Robinson AS, O'Neill SL |title=Prospects for control of African trypanosomiasis by tsetse vector manipulation |journal=Trends in Parasitology |volume=17 |issue=1 |pages=29–35 |date=January 2001 |pmid=11137738 |doi=10.1016/S1471-4922(00)01850-X }}</ref> Studying insect endosymbionts can aid understanding the origins of symbioses in general, as a proxy for understanding endosymbiosis in other species.

The best-studied ant endosymbionts are ''[[Blochmannia]]'' bacteria, which are the primary endosymbiont of ''[[Camponotus]]'' ants. In 2018 a new ant-associated symbiont, ''Candidatus Westeberhardia Cardiocondylae,'' was discovered in ''[[Cardiocondyla]]''. It is reported to be a primary symbiont.<ref>{{cite journal |display-authors=6 |vauthors=Klein A, Schrader L, Gil R, Manzano-Marín A, Flórez L, Wheeler D, Werren JH, Latorre A, Heinze J, Kaltenpoth M, Moya A, Oettler J |date=February 2016 |title=A novel intracellular mutualistic bacterium in the invasive ant Cardiocondyla obscurior |journal=The ISME Journal |volume=10 |issue=2 |pages=376–388 |bibcode=2016ISMEJ..10..376K |doi=10.1038/ismej.2015.119 |pmc=4737929 |pmid=26172209 |doi-access=free}}</ref>

===== Secondary =====
[[File:HEMI Aphididae Aphidius attacking pea aphid.png|thumb|right|Pea aphids are commonly infested by parasitic wasps. Their secondary endosymbionts attack the infesting parasitoid wasp larvae promoting the survival of both the aphid host and its endosymbionts.]]

The pea aphid (''[[Acyrthosiphon pisum]]'') contains at least three secondary endosymbionts, ''[[Hamiltonella defensa]]'', ''[[Regiella insecticola]]'', and ''[[Serratia symbiotica]]''. ''Hamiltonella defensa'' defends its aphid host from parasitoid wasps.<ref name="pmid18029301">{{cite journal |vauthors=Oliver KM, Campos J, Moran NA, Hunter MS |title=Population dynamics of defensive symbionts in aphids |journal=Proceedings. Biological Sciences |volume=275 |issue=1632 |pages=293–299 |date=February 2008 |pmid=18029301 |pmc=2593717 |doi=10.1098/rspb.2007.1192 }}</ref> This symbiosis replaces lost elements of the insect's immune response.<ref name="pmid20186266">{{cite journal |title=Genome sequence of the pea aphid Acyrthosiphon pisum |journal=PLOS Biology |volume=8 |issue=2 |pages=e1000313 |date=February 2010 |pmid=20186266 |pmc=2826372 |doi=10.1371/journal.pbio.1000313 |author1=International Aphid Genomics Consortium |doi-access=free }}</ref>

One of the best-understood defensive symbionts is the spiral bacteria ''[[Spiroplasma poulsonii]]''. ''Spiroplasma sp.'' can be reproductive manipulators, but also defensive symbionts of ''[[Drosophila]]'' flies. In ''[[Drosophila neotestacea]]'', ''S. poulsonii'' has spread across North America owing to its ability to defend its fly host against [[nematode]] parasites.<ref>{{cite journal |vauthors=Jaenike J, Unckless R, Cockburn SN, Boelio LM, Perlman SJ |title=Adaptation via symbiosis: recent spread of a Drosophila defensive symbiont |journal=Science |volume=329 |issue=5988 |pages=212–215 |date=July 2010 |pmid=20616278 |doi=10.1126/science.1188235 |s2cid=206526012 |bibcode=2010Sci...329..212J }}</ref> This defence is mediated by toxins called "[[ribosome]]-inactivating [[proteins]]" that attack the molecular machinery of invading parasites.<ref>{{cite journal |vauthors=Hamilton PT, Peng F, Boulanger MJ, Perlman SJ |title=A ribosome-inactivating protein in a Drosophila defensive symbiont |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=113 |issue=2 |pages=350–355 |date=January 2016 |pmid=26712000 |pmc=4720295 |doi=10.1073/pnas.1518648113 |doi-access=free |bibcode=2016PNAS..113..350H }}</ref><ref>{{cite journal |vauthors=Ballinger MJ, Perlman SJ |title=Generality of toxins in defensive symbiosis: Ribosome-inactivating proteins and defense against parasitic wasps in Drosophila |journal=PLOS Pathogens |volume=13 |issue=7 |pages=e1006431 |date=July 2017 |pmid=28683136 |pmc=5500355 |doi=10.1371/journal.ppat.1006431 |doi-access=free }}</ref> These toxins represent one of the first understood examples of a defensive symbiosis with a mechanistic understanding for defensive symbiosis between an insect endosymbiont and its host.<ref name="Ballinger-2017">{{cite journal |vauthors=Ballinger MJ, Perlman SJ |date=July 2017 |title=Generality of toxins in defensive symbiosis: Ribosome-inactivating proteins and defense against parasitic wasps in Drosophila |journal=PLOS Pathogens |volume=13 |issue=7 |pages=e1006431 |doi=10.1371/journal.ppat.1006431 |pmc=5500355 |pmid=28683136 |doi-access=free }}</ref>

''[[Sodalis glossinidius]]'' is a secondary endosymbiont of tsetse flies that lives inter- and intracellularly in various host tissues, including the midgut and hemolymph. Phylogenetic studies do not report a correlation between evolution of ''[[Sodalis (genus)|Sodalis]]'' and tsetse.<ref>Aksoy, S., Pourhosseini, A. & Chow, A. 1995. Mycetome endosymbionts of tsetse flies constitute a distinct lineage related to Enterobacteriaceae. Insect Mol Biol. '''4''', 15–22.</ref> Unlike ''Wigglesworthia,'' ''Sodalis'' has been cultured ''in vitro''.<ref name="pmid3662675">{{cite journal |vauthors=Welburn SC, Maudlin I, Ellis DS |title=In vitro cultivation of rickettsia-like-organisms from Glossina spp |journal=Annals of Tropical Medicine and Parasitology |volume=81 |issue=3 |pages=331–335 |date=June 1987 |pmid=3662675 |doi=10.1080/00034983.1987.11812127 }}</ref>

''[[Cardinium]]'' and many other insects have secondary endosymbionts.<ref name="pmid15189221">{{cite journal |vauthors=Zchori-Fein E, Perlman SJ |title=Distribution of the bacterial symbiont Cardinium in arthropods |journal=Molecular Ecology |volume=13 |issue=7 |pages=2009–2016 |date=July 2004 |pmid=15189221 |doi=10.1111/j.1365-294X.2004.02203.x |bibcode=2004MolEc..13.2009Z |s2cid=24361903 }}</ref><ref name="pmid12415315">{{cite journal |vauthors=Wernegreen JJ |title=Genome evolution in bacterial endosymbionts of insects |journal=Nature Reviews. Genetics |volume=3 |issue=11 |pages=850–861 |date=November 2002 |pmid=12415315 |doi=10.1038/nrg931 |s2cid=29136336 }}</ref>

==== Marine ====
Extracellular endosymbionts are represented in all four extant classes of [[Echinodermata]] ([[Crinoidea]], [[Ophiuroidea]], [[Echinoidea]], and [[Holothuroidea]]). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) but [[phylogenetic]] analysis indicates that these symbionts belong to the class [[Alphaproteobacteria]], relating them to ''[[Rhizobium]]'' and ''[[Thiobacillus]]''. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and widely distributed among the Echinoderms.<ref>{{cite journal |vauthors=Burnett WJ, McKenzie JD |title=Subcuticular bacteria from the brittle star Ophiactis balli (Echinodermata: Ophiuroidea) represent a new lineage of extracellular marine symbionts in the alpha subdivision of the class Proteobacteria |journal=Applied and Environmental Microbiology |volume=63 |issue=5 |pages=1721–1724 |date=May 1997 |pmid=9143108 |pmc=168468 |doi=10.1128/AEM.63.5.1721-1724.1997 |bibcode=1997ApEnM..63.1721B }}</ref>

Some marine [[oligochaeta]] (e.g., ''[[Olavius algarvensis]]'' and ''[[Inanidrilus|Inanidrillus]] spp.'') have obligate extracellular endosymbionts that fill the entire body of their host. These marine worms are nutritionally dependent on their symbiotic [[chemoautotroph]]ic bacteria lacking any digestive or excretory system (no gut, mouth, or [[nephridia]]).<ref>{{cite journal |vauthors=Dubilier N, Mülders C, Ferdelman T, de Beer D, Pernthaler A, Klein M, Wagner M, Erséus C, Thiermann F, Krieger J, Giere O, Amann R |display-authors=6 |title=Endosymbiotic sulphate-reducing and sulphide-oxidizing bacteria in an oligochaete worm |journal=Nature |volume=411 |issue=6835 |pages=298–302 |date=May 2001 |pmid=11357130 |doi=10.1038/35077067 |s2cid=4420931 |bibcode=2001Natur.411..298D }}</ref>

The sea slug ''[[Elysia chlorotica]]'s'' endosymbiont is the [[algae]] ''[[Vaucheria litorea]].'' The [[jellyfish]] ''[[Mastigias]]'' have a similar relationship with an algae. ''[[Elysia chlorotica]]'' forms this relationship intracellularly with the algae's chloroplasts. These chloroplasts retain their photosynthetic capabilities and structures for several months after entering the slug's cells.<ref>{{cite journal |vauthors=Mujer CV, Andrews DL, Manhart JR, Pierce SK, Rumpho ME |title=Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=93 |issue=22 |pages=12333–12338 |date=October 1996 |pmid=8901581 |pmc=37991 |doi=10.1073/pnas.93.22.12333 |bibcode=1996PNAS...9312333M |doi-access=free }}</ref>

''[[Trichoplax]]'' have two bacterial endosymbionts. Ruthmannia lives inside the animal's digestive cells. Grellia lives permanently inside the [[endoplasmic reticulum]] (ER), the first known symbiont to do so.<ref>{{Cite web|url=https://phys.org/news/2019-06-deceptively-simple-minute-marine-animals.html|title=Deceptively simple: Minute marine animals live in a sophisticated symbiosis with bacteria|first=Max Planck|last=Society|website=phys.org}}</ref>

''[[Paracatenula]]'' is a [[flatworm]] which have lived in symbiosis with an endosymbiotic bacteria for 500 million years. The bacteria produce numerous small, droplet-like vesicles that provide the host with needed nutrients.<ref>{{Cite web|url=https://phys.org/news/2019-04-bacterium-entire-flatworm.html|title=How a bacterium feeds an entire flatworm|first=Max Planck|last=Society|website=phys.org}}</ref>

===== Dinoflagellates =====
[[Dinoflagellate]] endosymbionts of the genus ''[[Symbiodinium]]'', commonly known as [[zooxanthella]]e, are found in [[corals]], [[mollusk]]s (esp. [[giant clam]]s, the ''Tridacna''), [[sea sponge|sponges]], and the unicellular [[foraminifera]]. These endosymbionts capture sunlight and provide their hosts with energy via [[carbonate]] deposition.<ref name=Baker2003>{{cite journal |author=Baker AC |s2cid=35278104 |title=Flexibility and Specificity in Coral-Algal Symbiosis: Diversity, Ecology, and Biogeography of Symbiodinium |journal=Annual Review of Ecology, Evolution, and Systematics |volume=34 |pages=661–89 |date=November 2003 |doi=10.1146/annurev.ecolsys.34.011802.132417 }}</ref>

Previously thought to be a single species, molecular [[phylogenetic]] evidence reported diversity in ''Symbiodinium''. In some cases, the host requires a specific ''Symbiodinium'' [[clade]]. More often, however, the distribution is ecological, with symbionts switching among hosts with ease. When reefs become environmentally stressed, this distribution is related to the observed pattern of [[coral bleaching]] and recovery. Thus, the distribution of ''Symbiodinium'' on coral reefs and its role in coral bleaching is an important in coral reef ecology.<ref name=Baker2003/>

==== 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" />