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Heterocysts or heterocytes are specialized
nitrogen-fixing Nitrogen fixation is a chemical process by which molecular nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific st ...
cells formed during nitrogen starvation by some
filamentous cyanobacteria Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical nu ...
, such as ''
Nostoc punctiforme ''Nostoc punctiforme'' is a species of filamentous cyanobacterium Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers ...
'', ''
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 on aqua ...
stagnale'', and ''
Anabaena ''Anabaena'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...
sphaerica''. They fix
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
from dinitrogen (N2) in the air using the
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria). These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the onl ...

nitrogenase
, 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 Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
require a global change in
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
. For example, heterocysts: * produce three additional
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
s, including one of
glycolipid Glycolipids are lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physio ...

glycolipid
that forms a hydrophobic barrier to oxygen * produce nitrogenase and other proteins involved in nitrogen fixation * degrade
photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ...

photosystem II
, which produces oxygen * up-regulate
glycolytic Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of on ...

glycolytic
enzymes * produce proteins that scavenge any remaining oxygen * contain polar plugs composed of
cyanophycin Cyanophycin, also known as CGP (cyanophycin granule polypeptide Peptides (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ) ...

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 Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
. The lack of water-splitting in
photosystem Photosystems are functional and structural units of protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', A ...
II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s, which is thought to be
sucrose Sucrose is a type of sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosacchari ...

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 Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

photosystem I
, allowing them to generate
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

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
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s and may have been the first example of multicellular patterning in
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

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 An akinete is an enveloped, thick-walled, non-motile, dormant cell (biology), cell formed by filamentous, heterocyst-forming cyanobacteria under the order Nostocales and Stigonematales. Akinetes are resistant to cold and desiccation. They also accu ...
s or motile cells called
hormogonia Hormogonia are motile filaments ...
, making them the most
phenotyptically
phenotyptically
versatile of all prokaryotes.


Gene Expression

In low nitrogen environments, heterocyst
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
is triggered by the
transcriptional regulator In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, m ...
NtcA. NtcA influences heterocyst differentiation by signaling proteins involved in the process of heterocyst differentiation. For instance, NtcA controls the
expression
expression
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 Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
. It is also worthy to note that the
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, 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 (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

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 The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

ammonium
or
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the format ...

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) and undergo asexual reproduction. The cyanobacteria that form heterocysts are divided into the orders
Nostocales The Nostocales are an order of cyanobacteria containing most of its species. It includes filamentous forms, both simple or branched, and both those occurring as single strands or multiple strands within a sheath. Some members show a decrease in ...
and Stigonematales, which form simple and branching filaments respectively. Together they form a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

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 The genotype of an organism is its complete set of genetic material. Genotype can also b ...
.


Symbiotic Relationships

The bacteria may also enter a
symbiotic relationship Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

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 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 Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

symbiotic
relationship is that of ''
Anabaena ''Anabaena'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...
''
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
with ''
Azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of ...

Azolla
'' 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 Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
and provides fixed
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
for the ''Anabaena'' to use as an energy source for s 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 a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

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 '' A biofertilizer (also bio-fertilizer) 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 suppl ...
where nitrogen is limiting as well as in
animal feed Animal feed is food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that emb ...
. Different strains of ''Azolla-Anabaena'' are suited for different environments and may lead to differences in crop production.
Rice Rice is the seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was relea ...

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. ''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 In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
, 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 is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal s ...
s from the environment, a process known as
phytoremediation Phytoremediation technologies use living plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * ...
. ''Anabaena sp.'' together with ''Azolla'' ''caroliniana'' has been shown to be successful in removing
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
, a toxic pollutant caused by
mining Mining is the extraction of valuable mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occu ...

mining
, as well as the
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as ...
, , 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 harmfu ...

Anabaena circinalis
'' 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 on aqua ...
'' filament
{{clear


References

Nitrogen cycle Cyanobacteria