Heterocyst

2025-06-29T08:51:07Z

45.249.86.254:

{{Short description|Nitrogen-fixing cells}}
[[File:Microphotographs of heterocystous cyanobacteria.png|thumb|upright=1.7| {{center|'''Microphotographs of heterocystous cyanobacteria''' A–F: ''[[Nostoc commune]]'' G–H: ''[[Nostoc|Nostoc calcicola]]'' I–M: ''Tolypothrix distorta'' N–R: ''[[Scytonema|Scytonema hyalinum]]'' Scale bar {{=}} 10 μm. Abbreviations: hc - heterocyst, ak - akinete, hm - hormogonium, nd - necridia}}]]

'''Heterocysts''' or '''heterocytes''' are specialized [[nitrogen fixation|nitrogen-fixing]] cells formed during [[nitrogen starvation]] by some [[filamentous cyanobacteria]], such as ''[[Nostoc]]'', ''[[Cylindrospermum]]'', and ''[[Anabaena]]''.<ref>{{Cite web | author= Basic Biology | date= 18 March 2016 | title= Bacteria | url= https://basicbiology.net/micro/microorganisms/bacteria}}</ref> They fix [[nitrogen]] from dinitrogen (N2) in the air using the [[enzyme]] [[nitrogenase]], in order to provide the cells in the filament with nitrogen for biosynthesis.<ref>{{cite book|last=Wolk|first=C.P. |author2=Ernst, A. |author3=Elhai, J.|title=The Molecular Biology of Cyanobacteria |chapter=Heterocyst Metabolism and Development |year=1994|pages=769–823|doi=10.1007/978-94-011-0227-8_27 |isbn=978-0-7923-3273-2 }}</ref>

Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment. The heterocysts' unique structure and [[physiology]] require a global change in [[gene expression]]. For example, heterocysts:

* produce three additional [[cell wall]]s, including one of [[glycolipid]] that forms a hydrophobic barrier to oxygen and carbon dioxide
* produce nitrogenase and other proteins involved in nitrogen fixation
* degrade [[photosystem II]], which produces oxygen
* up-regulate [[glycolysis|glycolytic]] enzymes
* produce proteins that scavenge any remaining oxygen
* contain polar plugs composed of [[cyanophycin]] which slows down cell-to-cell diffusion

Cyanobacteria usually obtain a fixed carbon (carbohydrate) by [[photosynthesis]]. The lack of water-splitting in [[photosystem]] II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with [[carbohydrate]]s, which is thought to be [[sucrose]]. The fixed carbon and nitrogen sources are exchanged through channels between the cells in the filament. Heterocysts maintain [[photosystem I]], allowing them to generate [[Adenosine triphosphate|ATP]] by [[photophosphorylation|cyclic photophosphorylation]].

Single heterocysts develop about every 9-15 cells, producing a one-dimensional pattern along the filament. The interval between heterocysts remains approximately constant even though the cells in the filament are dividing. The bacterial filament can be seen as a multicellular organism with two distinct yet interdependent cell types. Such behavior is highly unusual in [[prokaryote]]s and may have been the first example of multicellular patterning in [[evolution]]. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called [[akinete]]s or motile cells called [[hormogonia]], making them the most [[phenotype|phenotypically]] versatile of all prokaryotes.

==Gene expression==
[[File:Anabæna inæqualis.jpg|thumb|upright=0.7| {{center|Illustration of ''[[Anabaena|Anabaena inaequalis]]'', where heterocysts are labeled with letter ''h''}}]]

In low nitrogen environments, heterocyst [[Cellular differentiation|differentiation]] is triggered by the [[Transcriptional regulation|transcriptional regulator]] NtcA. NtcA influences heterocyst differentiation by signaling proteins involved in the process of heterocyst differentiation. For instance, NtcA controls the [[Gene expression|expression]] of several genes, including HetR, which is crucial for heterocyst differentiation<ref>{{cite journal|last1=Herrero|first1=Antonia|last2=Muro-Pastor|first2=Alicia M.|last3=Flores|first3=Enrique|title=Nitrogen Control in Cyanobacteria|journal=Journal of Bacteriology|date=15 January 2001|volume=183|issue=2|pages=411–425|doi=10.1128/JB.183.2.411-425.2001|language=en|issn=0021-9193|pmc=94895|pmid=11133933}}</ref> as it up-regulates other genes such as hetR, patS, and hepA by binding to their [[Promoter (genetics)|promoter]] and thus acting as a [[transcription factor]]. It is also worthy to note that the [[Gene expression|expression]] of ntcA and HetR are dependent on each other and their presence promotes heterocyst differentiation even in the presence of nitrogen. It has also been recently found that other genes such as PatA and hetP regulate heterocyst differentiation.<ref>{{cite journal|last1=Higa|first1=Kelly C.|last2=Callahan|first2=Sean M.|title=Ectopic expression of hetP can partially bypass the need for hetR in heterocyst differentiation by Anabaena sp. strain PCC 7120|journal=Molecular Microbiology|date=1 August 2010|volume=77|issue=3|pages=562–574|doi=10.1111/j.1365-2958.2010.07257.x|pmid=20545862|language=en|issn=1365-2958|doi-access=}}</ref> PatA patterns the heterocysts along the filaments, and it is also important for [[cell division]]. PatS influences the heterocyst patterning by inhibiting heterocyst differentiation when a group of differentiating cells come together to form a pro- heterocyst (immature heterocyst).<ref>{{cite journal|last1=Orozco|first1=Christine C.|last2=Risser|first2=Douglas D.|last3=Callahan|first3=Sean M.|title=Epistasis Analysis of Four Genes from Anabaena sp. Strain PCC 7120 Suggests a Connection between PatA and PatS in Heterocyst Pattern Formation|journal=Journal of Bacteriology|date=2006|volume=188|issue=5|pages=1808–1816|doi=10.1128/JB.188.5.1808-1816.2006|issn=0021-9193|pmc=1426565|pmid=16484191}}</ref> Heterocyst maintenance is dependent on an enzyme called hetN. Heterocyst formation is inhibited by the presence of a [[Nitrogen fixation|fixed nitrogen]] source, such as [[ammonium]] or [[nitrate]].<ref name=":0">{{cite book|url=http://www.dbbe.fcen.uba.ar/contenido/objetos/PhycologyLee.pdf|title=Phycology|last1=lee|first1=Robert Edward|access-date=9 October 2017}}</ref>

==Heterocyst formation==
The following sequences take place in formation of heterocysts from a vegetative cell:
*The cell enlarges.
*Granular inclusions decrease.
*Photosynthetic lammel reorients.
*The wall finally becomes triple-layered. These three layers develop outside the cell's outer layer.
**The middle layer is homogeneous.
**The inner layer is laminated.
*The senescent heterocyst undergoes vacuolation and finally breaks off from the filament causing fragmentation. These fragments are called hormogonia (singular [[hormogonium]]) and undergo asexual reproduction.

The cyanobacteria that form heterocysts are divided into the orders [[Nostocales]] and [[Stigonematales]], which form simple and branching filaments respectively. Together they form a [[monophyletic]] group, with very low [[gene pool|genetic variability]].

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== Symbiotic relationships ==
[[File:Figure2a.pdf|thumb|upright=1.9| {{center|'''Division of labor in cyanobacteria'''}} Some cells within clonal filaments differentiate into heterocysts (large, round cell, right). Heterocysts abandon oxygen-producing photosynthesis in order to fix nitrogen with the oxygen-sensitive enzyme nitrogenase. Vegetative and heterocyst cells divide labor by exchanging sugars and nitrogen.]]

The bacteria may also enter a [[Symbiosis|symbiotic relationship]] with certain plants. In such a relationship, the bacteria do not respond to the availability of nitrogen, but to signals produced by the plant for heterocyst differentiation. Up to 60% of the cells can become heterocysts, providing fixed nitrogen to the plant in return for fixed carbon.<ref name=":0" /> The signal produced by the plant and the stage of heterocyst differentiation it affects is unknown. Presumably, the symbiotic signal generated by the plant acts before NtcA activation as hetR is required for symbiotic heterocyst differentiation. For the symbiotic association with the plant, ntcA is needed as bacteria with mutated ntcA cannot infect plants.<ref>{{cite journal|title=Regulation of Cellular Differentiation in Filamentous Cyanobacteria in Free-Living and Plant-Associated Symbiotic Growth States|journal=Microbiology and Molecular Biology Reviews|language=en|volume=66|issue=1|doi=10.1128/MMBR.66.1.94-121.2002|pmc=120779|pmid=11875129|year=2002|pages=94–121; table of contents | last1 = Meeks | first1 = JC | last2 = Elhai | first2 = J}}</ref>

=== ''Anabaena-Azolla'' ===
A notable [[Symbiosis|symbiotic]] relationship is that of ''[[Anabaena|Anabaena azollae]]''{{efn|Currently ''[https://lpsn.dsmz.de/species/trichormus-azollae Trichormus azollae]'' {{au|(Strasburger 1884) Komárek and Anagnostidis 1989}}. Synonyms: ''Nostoc azollae'' {{au|Strasburger 1883}}, ''Anabaena azollae'' {{au|Strasburger 1884}}, ''Desikacharya azollae'' {{au|(Strasburger 1884) Saraf et al. 2019}}.}} [[cyanobacteria]] with ''[[Azolla]]'' plants. ''Anabaena'' reside on the stems and within leaves of ''Azolla'' plants.<ref name=":02">{{Cite journal|last1=van Hove|first1=C.|last2=Lejeune|first2=A.|date=2002|title=The Azolla: Anabaena Symbiosis|journal=Biology and Environment: Proceedings of the Royal Irish Academy|volume=102B|issue=1|pages=23–26|doi=10.1353/bae.2002.0036|jstor=20500136|s2cid=245843704}}</ref> The ''Azolla'' plant undergoes [[photosynthesis]] and provides fixed [[carbon]] for the ''Anabaena'' to use as an energy source for [[dinitrogenase]]s in the heterocyst cells.<ref name=":02" /> In return, the heterocysts are able to provide the vegetative cells and the ''Azolla'' plant with fixed nitrogen in the form of [[ammonia]] which supports growth of both organisms.<ref name=":02" /><ref name=":1">{{Cite journal|last1=Vaishampayan|first1=A.|last2=Sinha|first2=R. P.|last3=Häder|first3=D.-P.|last4=Dey|first4=T.|last5=Gupta|first5=A. K.|last6=Bhan|first6=U.|last7=Rao|first7=A. L.|date=2001|title=Cyanobacterial Biofertilizers in Rice Agriculture|journal=Botanical Review|volume=67|issue=4|pages=453–516|jstor=4354403|doi=10.1007/bf02857893|bibcode=2001BotRv..67..453V |s2cid=20058464}}</ref>

This symbiotic relationship is exploited by humans in agriculture. In Asia, ''Azolla'' plants containing ''Anabaena'' species are used as [[biofertilizer]] where nitrogen is limiting<ref name=":02" /> as well as in [[animal feed]].<ref name=":1" /> Different strains of ''Azolla-Anabaena'' are suited for different environments and may lead to differences in crop production.<ref>{{Cite journal|last1=Bocchi|first1=Stefano|last2=Malgioglio|first2=Antonino|date=2010|title=Azolla-Anabaenaas a Biofertilizer for Rice Paddy Fields in the Po Valley, a Temperate Rice Area in Northern Italy|journal=International Journal of Agronomy|language=en|volume=2010|pages=1–5|doi=10.1155/2010/152158|issn=1687-8159|url=https://air.unimi.it/bitstream/2434/149583/2/152158.pdf|doi-access=free}}</ref> [[Rice]] crops grown with ''Azolla-Anabaena'' as biofertilizer have been shown to result in a much greater quantity and quality of produce compared to crops without the cyanobacteria.<ref name=":1" /><ref>{{Cite journal|last1=Singh|first1=S.|last2=Prasad|first2=R.|last3=Singh|first3=B. V.|last4=Goyal|first4=S. K.|last5=Sharma|first5=S. N.|date=1990-06-01|title=Effect of green manuring, blue-green algae and neem-cake-coated urea on wetland rice (Oryza sativa L.)|journal=Biology and Fertility of Soils|language=en|volume=9|issue=3|pages=235–238|doi=10.1007/bf00336232|bibcode=1990BioFS...9..235S |s2cid=38989291|issn=0178-2762}}</ref> ''Azolla-Anabaena'' plants are grown before and after rice crops are planted.<ref name=":1" /> As the ''Azolla-Anabaena'' plants grow, they accumulate fixed nitrogen due to the actions of the nitrogenase enzymes and organic carbon from photosynthesis by the ''Azolla'' plants and ''Anabaena'' vegetative cells.<ref name=":1" /> When the ''Azolla-Anabaena'' plants die and decompose, they release high amounts of fixed nitrogen, [[phosphorus]], organic carbon, and many other nutrients into the soil, providing a rich environment ideal for the growth of rice crops.<ref name=":1" />

The ''Anabaena''-''Azolla'' relationship has also been explored as a possible method of removing [[pollutant]]s from the environment, a process known as [[phytoremediation]].<ref name=":2">{{Cite journal|last1=Bennicelli|first1=R.|last2=Stępniewska|first2=Z.|last3=Banach|first3=A.|last4=Szajnocha|first4=K.|last5=Ostrowski|first5=J.|date=2004-04-01|title=The ability of Azolla caroliniana to remove heavy metals (Hg(II), Cr(III), Cr(VI)) from municipal waste water|journal=Chemosphere|volume=55|issue=1|pages=141–146|doi=10.1016/j.chemosphere.2003.11.015|pmid=14720557|bibcode=2004Chmsp..55..141B}}</ref> ''Anabaena sp.'' together with ''Azolla'' ''caroliniana'' has been shown to be successful in removing [[uranium]], a toxic pollutant caused by [[mining]], as well as the [[heavy metals]] [[Mercury (element)|mercury(II)]], [[Chromium|chromium(III)]], and [[chromium(VI)]] from contaminated waste water.<ref name=":2" /><ref>{{Cite journal|last1=Pan|first1=Changchun|last2=Hu|first2=Nan|last3=Ding|first3=Dexin|last4=Hu|first4=Jinsong|last5=Li|first5=Guangyue|last6=Wang|first6=Yongdong|date=2016-01-01|title=An experimental study on the synergistic effects between Azolla and Anabaena in removal of uranium from solutions by Azolla–anabaena symbiotic system|journal=Journal of Radioanalytical and Nuclear Chemistry|language=en|volume=307|issue=1|pages=385–394|doi=10.1007/s10967-015-4161-y|bibcode=2016JRNC..307..385P |s2cid=82545272|issn=0236-5731}}</ref>

<gallery mode=packed style=float:left heights=170px>
File:Azolla caroliniana0.jpg| ''Azolla caroliniana'' plant
File:Anabaena circinalis.jpg| ''[[Anabaena circinalis]]'' filament
File:Simplefilaments022 Cylindrospermum.jpg| ''[[Cylindrospermum]]'' filament
</gallery>

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== References ==
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[[Category:Cyanobacterial cells]]
[[Category:Nitrogen cycle]]

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Heterocyst