Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc, Cylindrospermum, and Anabaena.
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:
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produce three additional , including one of glycolipid that forms a hydrophobic barrier to oxygen and carbon dioxide
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produce nitrogenase and other proteins involved in nitrogen fixation
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degrade photosystem II, which produces oxygen
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up-regulate glycolysis enzymes
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produce proteins that scavenge any remaining oxygen
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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 , 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 ATP by 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 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 or motile cells called hormogonia, making them the most phenotype versatile of all prokaryotes.
Gene expression
In low nitrogen environments, heterocyst differentiation is triggered by the transcriptional regulator NtcA. NtcA influences heterocyst differentiation by signaling proteins involved in the process of heterocyst differentiation. For instance, NtcA controls the
Gene expression of several genes, including HetR, which is crucial for heterocyst differentiation
as it up-regulates other genes such as hetR, patS, and hepA by binding to their promoter and thus acting as a transcription factor. It is also worthy to note that the
Gene 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.
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).
Heterocyst maintenance is dependent on an enzyme called hetN. Heterocyst formation is inhibited by the presence of a fixed nitrogen source, such as
ammonium or
nitrate.
Heterocyst formation
The following sequences take place in formation of heterocysts from a vegetative cell:
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The cell enlarges.
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Granular inclusions decrease.
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Photosynthetic lammel reorients.
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The wall finally becomes triple-layered. These three layers develop outside the cell's outer layer.
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The middle layer is homogeneous.
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The inner layer is laminated.
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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.
Symbiotic relationships
The bacteria may also enter a
Symbiosis 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.
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.
Anabaena-Azolla
A notable
Symbiosis relationship is that of
Anabaena cyanobacteria with
Azolla plants.
Anabaena reside on the stems and within leaves of
Azolla plants.
The
Azolla plant undergoes
photosynthesis and provides fixed
carbon for the
Anabaena to use as an energy source for
in the heterocyst cells.
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.
This symbiotic relationship is exploited by humans in agriculture. In Asia, Azolla plants containing Anabaena species are used as biofertilizer where nitrogen is limiting
The Anabaena- Azolla relationship has also been explored as a possible method of removing from the environment, a process known as phytoremediation.
File:Azolla caroliniana0.jpg| Azolla caroliniana plant
File:Anabaena circinalis.jpg| Anabaena circinalis filament
File:Simplefilaments022 Cylindrospermum.jpg| Cylindrospermum filament
