Richelia
Richelia | |
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Scientific classification | |
Domain: | Bacteria |
Phylum: | Cyanobacteria |
Class: | Cyanophyceae |
Order: | Nostocales |
Family: | Nostocaceae |
Genus: | Richelia J.Schmidt |
Species: | R. intracellularis
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Binomial name | |
Richelia intracellularis C.H.Ostenfeld ex J.Schmidt, 1901
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Richelia is a genus of
Morphology
Richelia are made up of filaments called
Symbiosis
Nitrogen fixation and symbiosis
Host specificity
Richelia's host specificity and location within a host has been linked to the symbiont genome evolution. Even for taxonomically and morphologically related organisms, preference for diatom hosts and locations within a host differ.[7] These differences usually depend on which host a symbiont typically resides in. For example, in the Hemiaulus and Richelia symbiosis, Richelia resides inside the siliceous frustule of Hemiaulus. Richelia lacks principal nitrogen metabolism enzymes and transporters, such as ammonium transporters, nitrate and nitrite reductases as well as glutamate synthase. It also has a reduced genome, likely following the genome streamlining theory. Hemiaulus has genes that code for all of these enzymes and transporters while lacking the nitrogen fixation genes present in Richelia. This allows the host to complement its symbiont and vice versa, resulting in host specificity that follows host and symbiont genome evolution.[7]
Coordination of gene expression
Day-night cycles potentially play a role in coordination of resource exchange and cell division between a diazotroph and its diatom host. Photosynthesis, nitrogen fixation, and resource acquisition related genes show day-night fluctuations in their expression pattern in Richelia. Nitrogen uptake, metabolism, and carbon transport gene expression in diatom hosts seem to be synchronized with nitrogen fixation gene expression in Richelia, suggesting a coordinated exchange of nitrogen and carbon. Symbiont-host cell physiology is thought to be coordinated and strongly dependent on each other, especially with regard to the time of the day.[8]
Taxonomy
The genus name of Richelia is in honour of Andreas du Plessis de Richelieu (1852–1932), who was a Danish naval officer and businessman who became a Siamese admiral and minister of the Royal Thai Navy.[9]
The genus was
Species associations
While studies have identified Richelia in up to 13 species, there is a debate as to how many of those identifications were accurate.[10]
The diatom-Richelia symbiotic relationships that are confirmed and most well-known are as follows:
- Rhizosolenia (Diatom)-Richelia symbiosis which occurs inside the Diatom cell. It is located outside the plasma membrane in the periplasmic space[11][12]
- Hemiaulus (Diatom)-Richelia symbiosis which also happens inside the Diatom cell. The exact location is currently unknown.[12] Hemiaulus-Richelia symbiosis is often associated with Trichodesmium, especially in transitional waters.[13]
- Another form of symbiosis was found between Chaetoceros (Diatom) and Richelia, but it appears to be rare.[14]
Life cycle
Within diatom hosts
Richelia are most commonly found and best understood within host diatoms. For most of its life cycle within diatoms, the orientation of Richelia cells remains unchanged[15] with the orientation of the terminal heterocyst cell fixed towards the closest diatom valve.[4][15] This orientation only changes during separation and migration of the Richelia trichomes.[15] This separation and migration is presumed to occur synchronously with growth and division of the host diatom as it produces daughter cells, in order to provide new daughter cells with symbionts. While transfer of the Richelia trichome to daughter cells can occur before division, this method will eventually end as it limits vegetative growth due to the progressive reduction in the size of the host diatom. Within diatoms that are dead or dying, some Richelia cells have enlarged and rounded vegetative cells, some begin to disintegrate and die with their host, and some emerge from a trichome-shaped opening in the diatom frustule and presumably become free-living Richelia .[4]
Free-living
While Richelia cells can exist as free-living organisms in the
Distribution
Cyanobacteria in the genus Richelia are primarily found in symbiotic association with diatoms in nitrogen-limited regions of the ocean.[2] This distribution pattern is attributed to the symbiotic relationship that Richelia forms with different species of diatoms in which they provide diatoms with nitrogen that is otherwise limiting for growth.[2] Similar to other diazotrophs, Richelia cells are in low abundances in productive equatorial regions due to nutrient upwelling and in high abundances in non-productive subtropical areas where low concentrations of nitrate limit the growth of diatoms.[16]
Quantitative analyses of the distribution of Richelia is an emerging field of study.[16] Thus far, many observations have been subject to criticism due to issues of misidentifying hosts and the associated diazotrophs, and demonstrating symbiotic relationships overall.[17]
Global ocean
Richelia are found throughout the Pacific Ocean, the Atlantic Ocean, the Amazon River plume, and the Mediterranean Sea.[16][18] They follow similar distributions to other diazotrophs including cyanobacteria in the genus Trichodesmium, Candidatus Atelocyanobacterium thalassa (formerly known as UCYN-A), and UCYN-B, which are in high abundance throughout much of the tropical oceans, although the relative abundances of the different taxa varies.[16] The abundance of Richelia cells also varies based on different environmental conditions across regions.[16] Richelia, when compared to other diazotrophs, show lower abundances at deeper depths.[16] Warm, silicate-rich conditions, such as those found in the Amazon River plume, allow for high Richelia growth rates.[16] Richelia cells also decrease in abundance as inorganic nitrogen increases because they are at a competitive disadvantage when nitrate concentrations are high.[16] However, unlike other diazotrophs, Richelia cells do not decrease in abundance when phosphate levels are high.[16] The abundances of Richelia cells also depend on the availability of iron, due to the iron requirements of the enzyme nitrogenase that is needed to fix di-nitrogen gas.[16] Grazing is also a factor that may affect the abundances of diazotrophs throughout different regions in the global ocean.[19]
Mediterranean Sea
Richelia is an
Western/Southwestern Pacific Ocean
Richelia have been found as epiphytes to Chaetoceros compressus and to Rhizosolenia clevei in the Western Pacific Ocean. It is hypothesized that Richelia filaments can detach from Rhizosolenia clevei and subsequently become symbionts to Chaetoceros compressus. This is suggested as the Richelia and Chaetoceros compressus symbiosis has been found to follow occurrences of the Richelia and Rhizosolenia clevei symbiosis.[17]
Kuroshio Current
The distribution of Richelia in the
Sulu Sea
Symbiosis between Richelia and Chaetoceros compressus has also been observed in the Southern Sulu Sea. This is due to the lower than 0.1 μM nitrogen concentrations in surface waters causing nitrogen limiting conditions.[17]
Indian Ocean
Richelia have been found as epiphytes to Chaetoceros compressus in the Indian Ocean.[17]
Western Tropical Atlantic Ocean
Nitrogen fixation and cyanobacteria-diatom symbiosis occur in the freshwater layer of the Amazon River plume due to low surface nitrate conditions. In these nitrogen limited areas, Richelia can be found in symbiosis with Rhizosolenia clevei and Hemiaulus spp. Richelia symbiosis with H. hauckii is found predominantly in this region with depth as well as throughout the surface. The abundance of the symbiosis between Richelia and H. hauckii is higher further northwest from the Amazon River outflow. A positive correlation can be found between salinity and abundances of Richelia symbioses.[17]
References
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