Heterocyst

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Microphotographs of heterocystous cyanobacteria A-F: Nostoc commune G-H: Nostoc calcicola I-M: Tolypothrix distorta N-R: Scytonema hyalinum Scale bar = 10 um , hc, heterocyst, ak, akinete, hm, hormogonium, nd, necridia Microphotographs of heterocystous cyanobacteria.png
Microphotographs of heterocystous cyanobacteria
A–F: Nostoc commune G–H: Nostoc calcicola
I–M: Tolypothrix distorta N–R: Scytonema hyalinum
Scale bar = 10 µm , hc, heterocyst, ak, akinete, hm, hormogonium, nd, necridia

Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme , Cylindrospermum stagnale, and Anabaena sphaerica. [1] 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. [2]

Contents

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:

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 carbohydrates, 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 prokaryotes 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 akinetes or motile cells called hormogonia, making them the most phenotyptically versatile of all prokaryotes.

Gene Expression

Illustration of Anabaena inaequalis, where heterocysts are labeled with letter h Anabaena inaequalis.jpg
Illustration of Anabaena inaequalis , where heterocysts are labeled with letter h

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 expression of several genes including HetR which is crucial for heterocyst differentiation. [3] 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. It is also worthy to note that the 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. [4] 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). [5] 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. [6]

Heterocyst formation

The following sequences take place in formation of heterocysts from a vegetative cell:

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 genetic variability.

Symbiotic relationships

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. Figure2a.pdf
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 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. [6] 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. [7]

Anabaena-Azolla

A notable symbiotic relationship is that of Anabaena cyanobacteria with Azolla plants. Anabaena reside on the stems and within leaves of Azolla plants. [8] The Azolla plant undergoes photosynthesis and provides fixed carbon for the Anabaena to use as an energy source for dinitrogenases in the heterocyst cells. [8] 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. [8] [9]

This symbiotic relationship is exploited by humans in agriculture. In Asia, Azolla plants containing Anabaena species are used as biofertilizer where nitrogen is limiting [8] as well as in animal feed. [9] Different strains of Azolla-Anabaena are suited for different environments and may lead to differences in crop production. [10] 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. [9] [11] Azolla-Anabaena plants are grown before and after rice crops are planted. [9] 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. [9] 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. [9]

The Anabaena-Azolla relationship has also been explored as a possible method of removing pollutants from the environment, a process known as phytoremediation. [12] Anabaena sp. together with Azollacaroliniana has been shown to be successful in removing uranium, a toxic pollutant caused by mining, as well as the heavy metals mercury (II), chromium(III), and chromium(VI) from contaminated waste water. [12] [13]

Related Research Articles

Nitrogen fixation is a chemical process by which molecular nitrogen (N
2), with a strong triple covalent bond, in the air is converted into ammonia (NH
3) or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbially mediated process that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif).

Photosynthesis Biological process to convert light into chemical energy

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 in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs, "light", and sunthesis, "putting together". In most cases, oxygen is also released as a waste product that stores three times more chemical energy than the carbohydrates. Most plants, algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.

Cyanobacteria Phylum of photosynthesising prokaryotes

Cyanobacteria, also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name cyanobacteria refers to their color, giving them their other name, "blue-green algae", though modern botanists restrict the term algae to eukaryotes and do not apply it to cyanobacteria, which are prokaryotes. They appear to have originated in freshwater or a terrestrial environment. Sericytochromatia, the proposed name of the paraphyletic and most basal group, is the ancestor of both the non-photosynthetic group Melainabacteria and the photosynthetic cyanobacteria, also called Oxyphotobacteria.

Hormogonium Motile filament of cells formed by some cyanobacteria

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 akinetes.

<i>Trichodesmium</i> Genus of bacteria

Trichodesmium, also called sea sawdust, is a genus of filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters. Trichodesmium is a diazotroph; that is, it fixes atmospheric nitrogen into ammonium, a nutrient used by other organisms. Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. Trichodesmium is the only known diazotroph able to fix nitrogen in daylight under aerobic conditions without the use of heterocysts.

Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.

<i>Anabaena</i> Genus of bacteria

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 cyanobacteria that produce neurotoxins, which are harmful to local wildlife, as well as farm animals and pets. Production of these neurotoxins is assumed to be an input into its symbiotic relationships, protecting the plant from grazing pressure.

Nostocaceae Family of bacteria

The Nostocaceae are a family of cyanobacteria that forms filament-shaped colonies enclosed in mucus or a gelatinous sheath. Some genera in this family are found primarily in fresh water, while others are found primarily in salt water. Other genera may be found in both fresh and salt water. Most benthic algae of the order Nostocales belong to this family.

<i>Anabaena circinalis</i> Species of bacterium

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 harmful cyanotoxins, ranging in potency from irritating to lethal. Under favorable conditions for growth, A. circinalis forms large algae-like blooms, potentially harming the flora and fauna of an area.

<i>Azolla cristata</i> Species of aquatic plant

Azolla cristata , the Carolina mosquitofern, Carolina azolla or water velvet, is a species of Azolla native to the Americas, in eastern North America from southern Ontario southward, and from the east coast west to Wisconsin and Texas, and in the Caribbean, and in Central and South America from southeastern Mexico (Chiapas) south to northern Argentina and Uruguay.

<i>Aphanizomenon</i> Genus of bacteria

Aphanizomenon is a genus of cyanobacteria that inhabits freshwater lakes and can cause dense blooms. They are unicellular organisms that consolidate into linear (non-branching) chains called trichomes. Parallel trichomes can then further unite into aggregates called rafts. Since Aphanizomenon is a genus in the cyanobacteria phylum. Bacteria in the Cyanobacteria phylum are known for using photosynthesis to create energy and therefore use sunlight as their energy source. Aphanizomenon bacteria also play a big role in the Nitrogen cycle since they can perform nitrogen fixation. Studies on the species Aphanizomenon flos-aquae have shown that it can regulate buoyancy through light-induced changes in turgor pressure. It is also able to move by means of gliding, though the specific mechanism by which this is possible is not yet known.

Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a symbiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions, such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the genus Nostoc. With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.

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 primary nutrients to the host plant. Biofertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. The micro-organisms in biofertilizers restore the soil's natural nutrient cycle and build soil organic matter. Through the use of biofertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Biofertilizers can be expected to reduce the use of synthetic fertilizers and pesticides, but they are not yet able to replace their use. Since they play several roles, a preferred scientific term for such beneficial bacteria is "plant-growth promoting rhizobacteria" (PGPR).

Anabaena variabilis is a species of filamentous cyanobacterium. This species of the genus Anabaena and the domain Eubacteria is capable of photosynthesis. This species is heterotrophic, meaning that it may grow without light in the presence of fructose. It also can convert atmospheric dinitrogen to ammonia via nitrogen fixation.

Cylindrospermopsis raciborskii is a freshwater cyanobacterium.

Aulosira is a genus of cyanobacteria found in a variety of environmental niches that forms colonies composed of filaments of moniliform cells.

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 differentiation into heterocysts, akinetes or hormogonia.

<i>Crocosphaera watsonii</i> Species of bacterium

Crocosphaera watsonii is an isolate of a species of unicellular, diazotrophic marine cyanobacteria which represent less than 0.1% of the marine microbial population. They thrive in offshore, open-ocean oligotrophic regions where the waters are warmer than 24 degrees Celsius. Crocosphaera watsonii cell density can exceed 1,000 cells per milliliter within the euphotic zone; however, their growth may be limited by the concentration of phosphorus. Crocosphaera watsonii are able to contribute to the oceanic carbon and nitrogen budgets in tropical oceans due to their size, abundance, and rapid growth rate. Crocosphaera watsonii are unicellular nitrogen fixers that fix atmospheric nitrogen to ammonia during the night and contribute to new nitrogen in the oceans. They are a major source of nitrogen to open-ocean systems. Nitrogen fixation is important in the oceans as it not only allows phytoplankton to continue growing when nitrogen and ammonium are in very low supply but it also replenishes other forms of nitrogen, thus fertilizing the ocean and allowing more phytoplankton growth.

Richelia is a genus of nitrogen-fixing filamentous heterocystous cyanobacteria. It contains the single species Richelia intracellularis. They exist as both free-living organisms as well as symbionts within potentially up to 13 diatoms distributed throughout the global ocean. As a symbiont, Richelia can associate epiphytically and as endosymbionts within the periplasmic space between the cell membrane and cell wall of diatoms.

Cyanobacterial morphology Tha

Cyanobacteria are a large and diverse phylum of bacteria defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis.

References

  1. ↑ Basic Biology (18 March 2016). "Bacteria".
  2. ↑ Wolk, C.P.; Ernst, A.; Elhai, J. (1994). Heterocyst metabolism and development. The Molecular Biology of Cyanobacteria. pp. 769–823. doi:10.1007/978-94-011-0227-8_27. ISBN   978-0-7923-3273-2.
  3. ↑ Herrero, Antonia; Muro-Pastor, Alicia M.; Flores, Enrique (15 January 2001). "Nitrogen Control in Cyanobacteria". Journal of Bacteriology. 183 (2): 411–425. doi:10.1128/JB.183.2.411-425.2001. ISSN   0021-9193. PMC   94895 . PMID   11133933.
  4. ↑ Higa, Kelly C.; Callahan, Sean M. (1 August 2010). "Ectopic expression of hetP can partially bypass the need for hetR in heterocyst differentiation by Anabaena sp. strain PCC 7120". Molecular Microbiology. 77 (3): 562–574. doi: 10.1111/j.1365-2958.2010.07257.x . ISSN   1365-2958. PMID   20545862.
  5. ↑ Orozco, Christine C.; Risser, Douglas D.; Callahan, Sean M. (2006). "Epistasis Analysis of Four Genes from Anabaena sp. Strain PCC 7120 Suggests a Connection between PatA and PatS in Heterocyst Pattern Formation". Journal of Bacteriology. 188 (5): 1808–1816. doi:10.1128/JB.188.5.1808-1816.2006. ISSN   0021-9193. PMC   1426565 . PMID   16484191.
  6. 1 2 lee, Robert Edward. Phycology (PDF). Retrieved 9 October 2017.
  7. ↑ Meeks, JC; Elhai, J (2002). "Regulation of Cellular Differentiation in Filamentous Cyanobacteria in Free-Living and Plant-Associated Symbiotic Growth States". Microbiology and Molecular Biology Reviews. 66 (1): 94–121, table of contents. doi:10.1128/MMBR.66.1.94-121.2002. PMC   120779 . PMID   11875129.
  8. 1 2 3 4 van Hove, C.; Lejeune, A. (2002). "The Azolla: Anabaena Symbiosis". Biology and Environment: Proceedings of the Royal Irish Academy. 102B (1): 23–26. doi:10.1353/bae.2002.0036. JSTOR   20500136. S2CID   245843704.
  9. 1 2 3 4 5 6 Vaishampayan, A.; Sinha, R. P.; Häder, D.-P.; Dey, T.; Gupta, A. K.; Bhan, U.; Rao, A. L. (2001). "Cyanobacterial Biofertilizers in Rice Agriculture". Botanical Review. 67 (4): 453–516. doi:10.1007/bf02857893. JSTOR   4354403. S2CID   20058464.
  10. ↑ Bocchi, Stefano; Malgioglio, Antonino (2010). "Azolla-Anabaenaas a Biofertilizer for Rice Paddy Fields in the Po Valley, a Temperate Rice Area in Northern Italy" (PDF). International Journal of Agronomy. 2010: 1–5. doi: 10.1155/2010/152158 . ISSN   1687-8159.
  11. ↑ Singh, S.; Prasad, R.; Singh, B. V.; Goyal, S. K.; Sharma, S. N. (1990-06-01). "Effect of green manuring, blue-green algae and neem-cake-coated urea on wetland rice (Oryza sativa L.)". Biology and Fertility of Soils. 9 (3): 235–238. doi:10.1007/bf00336232. ISSN   0178-2762. S2CID   38989291.
  12. 1 2 Bennicelli, R.; Stępniewska, Z.; Banach, A.; Szajnocha, K.; Ostrowski, J. (2004-04-01). "The ability of Azolla caroliniana to remove heavy metals (Hg(II), Cr(III), Cr(VI)) from municipal waste water". Chemosphere. 55 (1): 141–146. Bibcode:2004Chmsp..55..141B. doi:10.1016/j.chemosphere.2003.11.015. PMID   14720557.
  13. ↑ Pan, Changchun; Hu, Nan; Ding, Dexin; Hu, Jinsong; Li, Guangyue; Wang, Yongdong (2016-01-01). "An experimental study on the synergistic effects between Azolla and Anabaena in removal of uranium from solutions by Azolla–anabaena symbiotic system". Journal of Radioanalytical and Nuclear Chemistry. 307 (1): 385–394. doi:10.1007/s10967-015-4161-y. ISSN   0236-5731. S2CID   82545272.
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