''Richelia'' is a genus of

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. Atmos ...

filamentous The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

heterocyst 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''. They fix nitrogen fr ...

ous and

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 forms the basis of cyanobacteria's common name, blu ...

. It contains the single species ''Richelia intracellularis''. They exist as both free-living organisms as well as

symbionts 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 ...

within potentially up to 13

diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

s distributed throughout the global ocean. As a

symbiont 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 ...

, ''Richelia'' can associate

epiphytically An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

and as

endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...

s within the

periplasm The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the ''periplasmic space'' in gram-negative bacteria. Using cryo-electron microscopy it has been found that ...

ic space between the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

and

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

Morphology

''Richelia'' are made up of filaments called

trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a pla ...

s, which are fine hair-like structures that grow out of a myriad of plant species, though their presence as free-living organisms in the

marine environment Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...

is rare. The number of

s ''Richelia'' have in each

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

varies. The

s serve the purpose of

nitrogen fixation 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. Atmo ...

as well as nutrient exchange with

s. The location of ''Richelia'' within their various

is not fully known, though it is commonly assumed to be within the diatom's

periplasmic space The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the ''periplasmic space'' in gram-negative bacteria. Using cryo-electron microscopy it has been found that ...

between the

plasmalemma The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

and the

frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on d ...

. ''Richelia''’s

s are made up of two cell types:

Heterocyst 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''. They fix nitrogen fr ...

and Vegetative. The

is a terminal single cell within which

occurs, while the rest of the

is made up of vegetative cells within which

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 ...

occurs. Some ''Richelia'' are made up of many vegetative cells and a terminal

, while others only contain a terminal

. The heterocyst is characterized by a thick glycolipid layer which minimizes oxygen's ability to interfere with

. This is important to ''Richelia''’s function as oxygen can bind to

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

and inhibit the

nitrogen fixing Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the Atmosphere of Earth, air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but al ...

abilities. The

does not divide, while the vegetative cells do. ''Richelia'' also remain photosynthetically active while within their

s, a behaviour that is somewhat uncommon for similar

Symbiosis

Nitrogen fixation and symbiosis

Nitrogen fixation 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. Atmo ...

is an important biological process in

marine ecosystem Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 70% of the surf ...

s. In many regions of the world's oceans the availability of inorganic nitrogen such as

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

and

ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...

limits the rate of photosynthesis (

primary productivity In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...

). Hence, organisms that form symbiotic relationships with other organisms, often

, to fix nitrogen can be at an advantage. In many cyanobacteria, nitrogen fixation is carried out in specialized cells called

s. Cyanobacteria in the genus ''Richelia'' are an example of

are capable of fixing nitrogen gas into organic forms of nitrogen. The organic nitrogen can then be transferred from the cyanobacteria to the

s with which they have 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 ...

. Evidence of this nitrogen transfer has been observed multiple times, and this relationship has benefits for both the ''Richelia'' cells, which exist inside the

, and the

itself. For example, the growth of cyanobacteria inside the

is increased, releasing carbon dioxide through respiration that can be used by the

. The

benefits from enhanced growth as a result of the nitrogen fixed by the

. The presence of this

symbiosis 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 ...

can allow

s to persist through nitrogen limiting conditions.

Host specificity

''Richelias

host specificity In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

and location within a host has been linked to the

genome evolution Genome evolution is the process by which a genome changes in structure (sequence) or size over time. The study of genome evolution involves multiple fields such as structural analysis of the genome, the study of genomic parasites, gene and ancient ...

. Even for

taxonomically In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...

and morphologically related organisms, preference for

s and locations within a

differ. These differences usually depend on which

typically resides in. For example, in the ''Hemiaulus'' and ''Richelia''

, ''Richelia'' resides inside the siliceous

of ''Hemiaulus''. ''Richelia'' lacks principal

nitrogen metabolism The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biologi ...

enzymes and transporters, such as ammonium transporters,

and

nitrite reductase Nitrite reductase refers to any of several classes of enzymes that catalyze the reduction of nitrite. There are two classes of NIR's. A multi haem enzyme reduces NO2− to a variety of products. Copper containing enzymes carry out a single elect ...

s as well as glutamate synthase. It also has a reduced genome, likely following the genome streamlining theory. ''Hemiaulus'' has

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s that code for all of these enzymes and transporters while lacking the

s present in ''Richelia''. This allows the

to complement its

and vice versa, resulting in

that follows

and

Coordination of gene expression

Day-night cycles potentially play a role in coordination of resource exchange and

cell division Cell division is the process by which a parent cell (biology), 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 eukar ...

between a

diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms that d ...

and its

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 resource acquisition related

s show day-night fluctuations in their expression pattern in ''Richelia''.

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 ...

uptake,

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

, and carbon transport

expression in

s seem to be synchronized with

expression in ''Richelia'', suggesting a coordinated exchange of

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 ...

and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

. Symbiont-host

cell physiology Cell physiology is the biological study of the activities that take place in a cell to keep it alive. The term ''physiology'' refers to normal functions in a living organism. Animal cells, plant cells and microorganism cells show similarities in th ...

is thought to be coordinated and strongly dependent on each other, especially with regard to the time of the day.

Taxonomy

The genus name of ''Richelia'' is in honour of

Andreas du Plessis de Richelieu Andreas du Plessis de Richelieu (24 February 1852 – 25 March 1932) was a Danish naval officer and businessman who became a Siamese admiral and minister of the navy. He was granted the Thai noble title Phraya Chonlayutthayothin ( th, พระ ...

(1852–1932), who was a

Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...

naval officer and businessman who became a Siamese admiral and minister of the

Royal Thai Navy The Royal Thai Navy ( Abrv: RTN, ทร.; th, กองทัพเรือไทย, ) is the naval warfare force of Thailand. Established in 1906, it was modernised by the Admiral Prince Abhakara Kiartiwongse (1880–1923) who is known as ...

. The genus was

circumscribed In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

by Ernst Johannes Schmidt in Vidensk. Meddel. Dansk Naturhist. Foren. Kjøbenhavn 1901 on pages 146 and 149 in 1901.

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. The

-''Richelia''

symbiotic relationships 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 ...

that are confirmed and most well-known are as follows: * ''Rhizosolenia'' (

Diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

)-''Richelia''

which occurs inside the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. It is located outside the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

in the

* ''Hemiaulus'' (

)-''Richelia''

which also happens inside the

. The exact location is currently unknown. ''Hemiaulus-Richelia''

is often associated with ''

Trichodesmium ''Trichodesmium'', also called sea sawdust, is a genus of Filamentation, filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first des ...

'', especially in transitional waters. * Another form of

was found between

Chaetoceros ''Chaetoceros'' is probably the largest genus of marine planktonic diatoms with approximately 400 species described, although many of these descriptions are no longer valid. It is often very difficult to distinguish between different ''Chaet ...

'' (

) and ''Richelia'', but it appears to be rare.

Life cycle

Within diatom hosts

''Richelia'' are most commonly found and best understood within

s. For most of its life cycle within

s, the orientation of ''Richelia'' cells remains unchanged with the orientation of the terminal

cell fixed towards the closest

valve. This orientation only changes during separation and migration of the ''Richelia''

s. This separation and migration is presumed to occur synchronously with growth and division of the

as it produces daughter cells, in order to provide new daughter cells with

. While transfer of the ''Richelia''

to daughter cells can occur before

division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting ...

, this method will eventually end as it limits vegetative growth due to the progressive reduction in the size of the

. Within

s that are dead or dying, some ''Richelia'' cells have enlarged and rounded vegetative cells, some begin to disintegrate and die with their

, and some emerge from a

-shaped opening in the

and presumably become free-living ''Richelia'' .

Free-living

While ''Richelia'' cells can exist as free-living organisms in the

, it is rarely observed. Additionally, how

s without

are colonized by free-living ''Richelia'' is unknown; however, a number of mechanisms have been hypothesized, including ''Richelia'' cells entering non-colonized diatoms directly. Also hypothesized is that ''Richelia'' cells may be affiliated with auxospore cells, or may enter

s during

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

when the

is transported to the

auxospore In certain species of diatoms, auxospores are specialised cells that are produced at key stages in their cell cycle or life history. Auxospores typically play a role in growth processes, sexual reproduction or dormancy.Hoek, C. van den, Mann, ...

. ''Richelia'' cells may also colonize

s during instances of vegetative cell enlargement.

Distribution

Cyanobacteria in the genus ''Richelia'' are primarily found in

symbiotic 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 ...

association with

s in nitrogen-limited regions of the ocean. This distribution pattern is attributed to the

that ''Richelia'' forms with different species of

s in which they provide

s with nitrogen that is otherwise limiting for growth. Similar to other

s, ''Richelia'' cells are in low abundances in productive equatorial regions due to nutrient

upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nutr ...

and in high abundances in non-productive subtropical areas where low concentrations of nitrate limit the growth of diatoms. Quantitative analyses of the distribution of ''Richelia'' is an emerging field of study. Thus far, many observations have been subject to criticism due to issues of misidentifying hosts and the associated

s, and demonstrating symbiotic relationships overall.

Global ocean

''Richelia'' are found throughout the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

, the

Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of t ...

plume, and the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

. They follow similar distributions to other

s including cyanobacteria in the genus ''

'', ''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. The abundance of ''Richelia'' cells also varies based on different environmental conditions across regions. ''Richelia'', when compared to other

s, show lower abundances at deeper depths. Warm, silicate-rich conditions, such as those found in the

plume, allow for high ''Richelia'' growth rates. ''Richelia'' cells also decrease in abundance as

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

nitrogen increases because they are at a competitive disadvantage when nitrate concentrations are high. However, unlike other

s, ''Richelia'' cells do not decrease in abundance when phosphate levels are high. The abundances of ''Richelia'' cells also depend on the availability of iron, due to the iron requirements of 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. A ...

that is needed to fix di-nitrogen gas.

Grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and other ...

is also a factor that may affect the abundances of

s throughout different regions in the global ocean.

Mediterranean Sea

''Richelia'' is an

with

s such as ''Rhizosolenia'' spp. and ''Hemiaulus'' spp. ''Richelia'' is found to be highly correlated with the presence of ''Hemiaulus'' spp., and sporadically correlated with the presence of ''Rhizosolenia'' spp. The highest counts of ''Richelia'' as sampled in 2006 in the Eastern

are 50

s L⁻¹ in June and October in

coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

al regions, and 50

s L⁻¹ in June and November in

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

regions. These peaks occur during a deepening of the

mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...

depth at each region. ''Richelia'' and ''Hemiaulus hauckii'' are found together in both

al and

regions year-round in diurnal and

nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...

sampling, and it is suggested that this

pair has an

evolutionary Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...

advantage over other host options for ''Richelia''. Because the Eastern

has

oligotrophic An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of ...

conditions due to a large transport of nutrients out to the

North Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe an ...

through the

Strait of Gibraltar The Strait of Gibraltar ( ar, مضيق جبل طارق, Maḍīq Jabal Ṭāriq; es, Estrecho de Gibraltar, Archaic: Pillars of Hercules), also known as the Straits of Gibraltar, is a narrow strait that connects the Atlantic Ocean to the Medi ...

, ''Richelia'' provides important nitrogen fixation capabilities for

s they form

relationships with. Free living ''Richelia'' are not considered to be present in the Eastern

based on the current sampling experiment results available. In the Eastern

water columns, ''Richelia'' is the primary diurnal organism with an

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 the nifH gene. A case of

allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

is observed between

al and

water columns in the Eastern

. These two regions have different

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

s of nifH-expressing cells of ''Richelia'', hypothesized to be due to the restriction of the two regions from each other by a

hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...

barrier caused by the sloping of the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

Western/Southwestern Pacific Ocean

''Richelia'' have been found as

epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

s to ''

compressus'' and to ''Rhizosolenia clevei'' in the Western

. It is hypothesized that ''Richelia'' filaments can detach from ''Rhizosolenia clevei'' and subsequently become

to ''

compressus''. This is suggested as the ''Richelia'' and ''

'' ''compressus''

has been found to follow occurrences of the ''Richelia'' and ''Rhizosolenia clevei''

Kuroshio Current

The distribution of ''Richelia'' in the

Kuroshio Current The , also known as the Black or or the is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...

varies based on the section of the current and the time of year. Physical and

hydrographical Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary ...

conditions vary throughout the year in the current and create changes to the growth of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

l and

colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...

. Conditions in May limit growth to a more narrow region of the current than in July. The region has low concentrations of nitrate throughout both Spring and Summer, with July seeing the least nitrate levels in surface waters. The number of ''Richelia'' filaments per colony of ''

compressus'' ranges from 4 to 9 during May to November, reaching a maximum in July. The maximum abundance of the ''Richelia'' and ''

'' ''compressus''

occurs in July, at 10 colonies L⁻¹. The maximum abundance of the ''Richelia'' and ''Rhizosolenia clevei''

also occurs in July, at 30 colonies L⁻¹.

Sulu Sea

Symbiosis 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 ...

between ''Richelia'' and ''

'' ''compressus'' has also been observed in the Southern

Sulu Sea The Sulu Sea ( fil, Dagat Sulu; Tausug: ''Dagat sin Sūg''; Chavacano: ''Mar de Sulu''; Cebuano: ''Dagat sa Sulu''; Hiligaynon: ''Dagat sang Sulu''; Karay-a: ''Dagat kang Sulu''; Cuyonon: ''Dagat i'ang Sulu''; ms, Laut Sulu) is a body o ...

. This is due to the lower than 0.1 μM nitrogen concentrations in surface waters causing nitrogen limiting conditions.

Indian Ocean

''Richelia'' have been found as

s to ''

compressus'' in the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

Western Tropical Atlantic Ocean

and

occur in the freshwater layer of the

plume due to low surface nitrate conditions. In these nitrogen limited areas, ''Richelia'' can be found in

with ''Rhizosolenia clevei'' and ''Hemiaulus spp''. ''Richelia''

with ''H. hauckii'' is found predominantly in this region with depth as well as throughout the surface. The abundance of the

between ''Richelia'' and ''H. hauckii'' is higher further northwest from the

outflow. A positive correlation can be found between

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

and abundances of ''Richelia'' symbioses.

References

{{Taxonbar, from1=Q27438682, from2=Q67346059 Nostocaceae Cyanobacteria genera Monotypic bacteria genera