![Microphotographs of heterocystous cyanobacteria](https://upload.wikimedia.org/wikipedia/commons/a/a5/Microphotographs_of_heterocystous_cyanobacteria.png)
Heterocysts or heterocytes are specialized
nitrogen-fixing
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atm ...
cells formed during
nitrogen starvation by some
filamentous cyanobacteria
Cyanobacterial morphology refers to the form or shape of cyanobacteria. Cyanobacteria are a large and diverse phylum of bacteria defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis.
Cyanobacteria ...
, such as ''
Nostoc punctiforme
''Nostoc punctiforme'' is a species of filamentous cyanobacterium. Under non-limiting nutritional environmental conditions, its filaments are composed of photosynthetic vegetative cells; upon nutrient limitation, some of these cells undergo dif ...
'', ''
Cylindrospermum
''Cylindrospermum'' is a genus of filamentous cyanobacteria found in terrestrial and aquatic environments. In terrestrial ecosystems, ''Cylindrospermum'' is found in soils, and in aquatic ones, it commonly grows as part of the periphyton
Per ...
stagnale'', and ''
Anabaena
''Anabaena'' is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanob ...
sphaerica''. They fix
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
from dinitrogen (N
2) in the air using the
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
nitrogenase
Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...
, 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
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
require a global change in
gene expression. For example, heterocysts:
* produce three additional
cell walls, including one of
glycolipid that forms a hydrophobic barrier to oxygen
* produce nitrogenase and other proteins involved in nitrogen fixation
* degrade
photosystem II, which produces oxygen
* up-regulate
glycolytic enzymes
* produce proteins that scavenge any remaining oxygen
* contain polar plugs composed of
cyanophycin
Cyanophycin, also known as CGP (cyanophycin granule polypeptide) or multi-L-arginyl-poly (L-aspartic acid), is a non-protein, non-ribosomally produced amino acid polymer composed of an aspartic acid backbone and arginine side groups.
Cyanophycin ...
which slows down cell-to-cell diffusion
Cyanobacteria usually obtain a fixed carbon (carbohydrate) by
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
. The lack of water-splitting in
photosystem
Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems ...
II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
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
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that us ...
, allowing them to generate
ATP by
cyclic photophosphorylation In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of ...
.
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
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s and may have been the first example of multicellular patterning in
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called
akinete
An akinete is an enveloped, thick-walled, non-motile, dormant cell formed by filamentous, heterocyst-forming cyanobacteria under the order Nostocales and Stigonematales. Akinetes are resistant to cold and desiccation. They also accumulate and sto ...
s or motile cells called
hormogonia
Hormogonia are motile filaments of cells formed by some cyanobacteria in the order Nostocales and Stigonematales. They are formed during vegetative reproduction in unicellular, filamentous cyanobacteria, and some may contain heterocysts and ...
, making them the most
phenotyptically versatile of all prokaryotes.
Gene Expression
![Anabæna inæqualis](https://upload.wikimedia.org/wikipedia/commons/f/f0/Anab%C3%A6na_in%C3%A6qualis.jpg)
In low nitrogen environments, heterocyst
differentiation is triggered by the
transcriptional regulator
In molecular biology and genetics, transcriptional regulation is the means by which a cell regulates the conversion of DNA to RNA ( transcription), thereby orchestrating gene activity. A single gene can be regulated in a range of ways, from a ...
NtcA. NtcA influences heterocyst differentiation by signaling proteins involved in the process of heterocyst differentiation. For instance, NtcA controls the
expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
of several genes including HetR which is crucial for heterocyst differentiation. It is crucial as it up-regulates other genes such as hetR, patS, hepA by binding to their
promoter and thus acting as a
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...
. It is also worthy to note that the
expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
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, hetP regulate heterocyst differentiation. PatA patterns the heterocysts along the filaments, and it is also important for
cell division
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
. 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:
*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
Hormogonia are motile filaments of cells formed by some cyanobacteria in the order Nostocales and Stigonematales. They are formed during vegetative reproduction in unicellular, filamentous cyanobacteria, and some may contain heterocysts and a ...
) and undergo asexual reproduction.
The cyanobacteria that form heterocysts are divided into the orders
Nostocales
The Nostocales are an order of cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly f ...
and
Stigonematales
The Rivulariaceae are a family of cyanobacteria within the Nostocales in which the filaments (trichomes) are tapered from wider at the base to narrower at the tip.
The type species is '' Rivularia haematites'' (Dc) C. A. Agardh.
Genera
, Wo ...
, which form simple and branching filaments respectively. Together they form a
monophyletic group, with very low
genetic variability
Genetic variability is either the presence of, or the generation of, genetic differences.
It is defined as "the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally i ...
.
Symbiotic relationships
The bacteria may also enter a
symbiotic relationship
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
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 heterocyst, 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 the bacteria with mutated ntcA can't infect plants.
''Anabaena-Azolla''
A notable
symbiotic relationship is that of ''
Anabaena
''Anabaena'' is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanob ...
''
cyanobacteria with ''
Azolla
''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like other typical ferns but more rese ...
'' plants. ''Anabaena'' reside on the stems and within leaves of ''Azolla'' plants.
The ''Azolla'' plant undergoes
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
and provides fixed
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
for the ''Anabaena'' to use as an energy source for
dinitrogenases 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
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
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
A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of prim ...
where nitrogen is limiting
as well as in
animal feed.
Different strains of ''Azolla-Anabaena'' are suited for different environments and may lead to differences in crop production.
Rice
Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...
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.
''Azolla-Anabaena'' plants are grown before and after rice crops are planted.
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.
When the ''Azolla-Anabaena'' plants die and decompose, they release high amounts of fixed nitrogen,
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, organic carbon, and many other nutrients into the soil, providing a rich environment ideal for the growth of rice crops.
The ''Anabaena''-''Azolla'' relationship has also been explored as a possible method of removing
pollutant
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
s from the environment, a process known as
phytoremediation
Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronom ...
.
''Anabaena sp.'' together with ''Azolla'' ''caroliniana'' has been shown to be successful in removing
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
, a toxic pollutant caused by
mining
Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...
, as well as the
heavy metals mercury (II),
chromium(III), and
chromium(VI) from contaminated waste water.
File:Azolla caroliniana0.jpg, ''Azolla caroliniana'' plant
File:Anabaena circinalis.jpg, ''Anabaena circinalis
''Anabaena circinalis'' is a species of Gram-negative, photosynthetic cyanobacteria common to freshwater environments throughout the world. Much of the scientific interest in '' A. circinalis '' owes to its production of several potentially harm ...
'' filament
File:Simplefilaments022 Cylindrospermum.jpg, ''Cylindrospermum
''Cylindrospermum'' is a genus of filamentous cyanobacteria found in terrestrial and aquatic environments. In terrestrial ecosystems, ''Cylindrospermum'' is found in soils, and in aquatic ones, it commonly grows as part of the periphyton
Per ...
'' filament
{{clear
References
Nitrogen cycle
Cyanobacteria