Heterocyst

Heterocysts or heterocytes are specialized <a href="/facts/Nitrogen_fixation/TeLoBpWS">nitrogen-fixing</a> cells formed during <a href="/facts/Nitrogen_starvation/YoCAyNQf">nitrogen starvation</a> by some <a href="/facts/Filamentous_cyanobacteria/A3tFsyrP">filamentous cyanobacteria</a>, such as <a href="/facts/Nostoc/WoDSJKYR">Nostoc</a>, <a href="/facts/Cylindrospermum/tmfVlbQf">Cylindrospermum</a>, and <a href="/facts/Anabaena/pVSXdnFd">Anabaena</a>. They fix <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> from dinitrogen (N2) in the air using the <a href="/facts/Enzyme/DSI1Fh7y">enzyme</a> <a href="/facts/Nitrogenase/gIhxgCaL">nitrogenase</a>, in order to provide the cells in the filament with nitrogen for biosynthesis. 
Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment. The heterocysts' unique structure and <a href="/facts/Physiology/fh25y2pQ">physiology</a> require a global change in <a href="/facts/Gene_expression/alCPohLR">gene expression</a>. For example, heterocysts:

<ul><li>produce three additional <a href="/facts/Cell_wall/DYXJVrnw">cell walls</a>, including one of <a href="/facts/Glycolipid/QUR36mXd">glycolipid</a> that forms a hydrophobic barrier to oxygen and carbon dioxide</li>
<li>produce nitrogenase and other proteins involved in nitrogen fixation</li>
<li>degrade <a href="/facts/Photosystem_II/9tmDPU50">photosystem II</a>, which produces oxygen</li>
<li>up-regulate <a href="/facts/Glycolysis/a5ApxLoG">glycolytic</a> enzymes</li>
<li>produce proteins that scavenge any remaining oxygen</li>
<li>contain polar plugs composed of <a href="/facts/Cyanophycin/fu4WcnTY">cyanophycin</a> which slows down cell-to-cell diffusion</li></ul>
Cyanobacteria usually obtain a fixed carbon (carbohydrate) by <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>. The lack of water-splitting in <a href="/facts/Photosystem/80xW3Cpm">photosystem</a> II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with <a href="/facts/Carbohydrate/Geuuzxsf">carbohydrates</a>, which is thought to be <a href="/facts/Sucrose/7y1ke94L">sucrose</a>. The fixed carbon and nitrogen sources are exchanged through channels between the cells in the filament. Heterocysts maintain <a href="/facts/Photosystem_I/z9hRH0kt">photosystem I</a>, allowing them to generate <a href="/facts/Adenosine_triphosphate/nB9bwgcU">ATP</a> by <a href="/facts/Photophosphorylation/vMcaVqpy">cyclic photophosphorylation</a>.
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 <a href="/facts/Prokaryote/oYyvaDDH">prokaryotes</a> and may have been the first example of multicellular patterning in <a href="/facts/Evolution/qE5amP6a">evolution</a>. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called <a href="/facts/Akinete/RjyL9o4c">akinetes</a> or motile cells called <a href="/facts/Hormogonia/Rup0Wxqh">hormogonia</a>, making them the most <a href="/facts/Phenotype/vxWXKGLj">phenotypically</a> versatile of all prokaryotes.

Heterocyst open-in-new

Heterocyst