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85-696: Although it is shorter in length, its volume is larger than that of the Vorderrhein at their confluence near Reichenau , after which the river is called the Alpine Rhine , which flows out into Lake Constance ( Bodensee ). Flowing from the village Hinterrhein near the San Bernardino Pass through the Rheinwald valley, the river flows into a gorge called Rofla Gorge ( Roflaschlucht ). In this gorge, an equal-sized tributary,

170-499: A stream . It equals the product of average flow velocity (with dimension of length per time, in m/h or ft/h) and the cross-sectional area (in m or ft ). It includes any suspended solids (e.g. sediment), dissolved chemicals like CaCO 3 (aq), or biologic material (e.g. diatoms ) in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow , whereas an engineer operating

255-463: A urea cycle , a feature that they share with animals , although this cycle is used to different metabolic ends in diatoms. The family Rhopalodiaceae also possess a cyanobacterial endosymbiont called a spheroid body. This endosymbiont has lost its photosynthetic properties, but has kept its ability to perform nitrogen fixation , allowing the diatom to fix atmospheric nitrogen . Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among

340-452: A better understanding of cell wall formation processes, establishing fundamental knowledge which can be used to create models that contextualise current findings and clarify how the process works. The process of building a mineral-based cell wall inside the cell, then exporting it outside, is a massive event that must involve large numbers of genes and their protein products. The act of building and exocytosing this large structural object in

425-436: A brown jelly-like material called "brown snot" or "rock snot". This diatom is native to Europe and is an invasive species both in the antipodes and in parts of North America . The problem is most frequently recorded from Australia and New Zealand . When conditions turn unfavourable, usually upon depletion of nutrients, diatom cells typically increase in sinking rate and exit the upper mixed layer ("bust"). This sinking

510-425: A brown, slippery coating on submerged stones and sticks, and may be seen to "stream" with river current. The surface mud of a pond, ditch, or lagoon will almost always yield some diatoms. Living diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. Cladophora is frequently covered with Cocconeis , an elliptically shaped diatom; Vaucheria

595-452: A disproportionately important role in the export of carbon from oceanic surface waters (see also the biological pump ). Significantly, they also play a key role in the regulation of the biogeochemical cycle of silicon in the modern ocean. Diatoms are ecologically successful, and occur in virtually every environment that contains water – not only oceans, seas, lakes, and streams, but also soil and wetlands. The use of silicon by diatoms

680-426: A diverse background in order to identify residues that differentiate function in the silica deposition process. Additionally, the same study found that a number of the regions were conserved within species, likely the base structure of silica transport. These silica transport proteins are unique to diatoms, with no homologs found in other species, such as sponges or rice. The divergence of these silica transport genes

765-454: A functioning urea cycle. This result was significant, since prior to this, the urea cycle was thought to have originated with the metazoans which appeared several hundreds of millions of years before the diatoms. Their study demonstrated that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionarily linked in such a way that animals and plants are not. While often overlooked in photosynthetic organisms,

850-450: A more complete understanding of cell wall synthesis. Most centric and araphid pennate diatoms are nonmotile , and their relatively dense cell walls cause them to readily sink. Planktonic forms in open water usually rely on turbulent mixing of the upper layers of the oceanic waters by the wind to keep them suspended in sunlit surface waters. Many planktonic diatoms have also evolved features that slow their sinking rate, such as spines or

935-428: A much larger cell, which then returns to size-diminishing divisions. The exact mechanism of transferring silica absorbed by the diatom to the cell wall is unknown. Much of the sequencing of diatom genes comes from the search for the mechanism of silica uptake and deposition in nano-scale patterns in the frustule . The most success in this area has come from two species, Thalassiosira pseudonana , which has become

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1020-473: A particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, and as a dynamite stabilizer. Diatoms are protists that form massive annual spring and fall blooms in aquatic environments and are estimated to be responsible for about half of photosynthesis in the global oceans. This predictable annual bloom dynamic fuels higher trophic levels and initiates delivery of carbon into

1105-432: A reservoir system may equate it with outflow , contrasted with inflow . A discharge is a measure of the quantity of any fluid flow over unit time. The quantity may be either volume or mass. Thus the water discharge of a tap (faucet) can be measured with a measuring jug and a stopwatch. Here the discharge might be 1 litre per 15 seconds, equivalent to 67 ml/second or 4 litres/minute. This is an average measure. For measuring

1190-480: A short time period, synched with cell cycle progression, necessitates substantial physical movements within the cell as well as dedication of a significant proportion of the cell's biosynthetic capacities. The first characterisations of the biochemical processes and components involved in diatom silicification were made in the late 1990s. These were followed by insights into how higher order assembly of silica structures might occur. More recent reports describe

1275-529: A slow recession . Because the peak flow also corresponds to the maximum water level reached during the event, it is of interest in flood studies. Analysis of the relationship between precipitation intensity and duration and the response of the stream discharge are aided by the concept of the unit hydrograph , which represents the response of stream discharge over time to the application of a hypothetical "unit" amount and duration of rainfall (e.g., half an inch over one hour). The amount of precipitation correlates to

1360-422: A transitional stage between centric and raphid pennate diatoms, diatoms with a raphe. Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by using hydrolytic enzymes to break down the organic algal material. Diatoms are a widespread group and can be found in the oceans , in fresh water , in soils , and on damp surfaces. They are one of

1445-428: A unique silica cell wall known as a frustule made up of two valves called thecae , that typically overlap one another. The biogenic silica composing the cell wall is synthesised intracellularly by the polymerisation of silicic acid monomers . This material is then extruded to the cell exterior and added to the wall. In most species, when a diatom divides to produce two daughter cells, each cell keeps one of

1530-466: A valve and a girdle band that can easily slide underneath each other and expand to increase cell content over the diatoms progression. The cytoplasm of the centric diatom is located along the inner surface of the shell and provides a hollow lining around the large vacuole located in the center of the cell. This large, central vacuole is filled by a fluid known as "cell sap" which is similar to seawater but varies with specific ion content. The cytoplasmic layer

1615-471: Is remineralized through respiration. Thus, diatoms are one of the main players in this biological carbon pump, which is arguably the most important biological mechanism in the Earth System allowing CO 2 to be removed from the carbon cycle for very long period. A feature of diatoms is the urea cycle , which links them evolutionarily to animals. In 2011, Allen et al. established that diatoms have

1700-533: Is a specimen of extant genus Hemiaulus in Late Jurassic aged amber from Thailand. Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatomaceous earth (diatomite) is a collection of diatom shells found in the Earth's crust. They are soft, silica-containing sedimentary rocks which are easily crumbled into a fine powder and typically have

1785-597: Is also indicative of the structure of the protein evolving from two repeated units composed of five membrane bound segments, which indicates either gene duplication or dimerization . The silica deposition that takes place from the membrane bound vesicle in diatoms has been hypothesized to be a result of the activity of silaffins and long chain polyamines. This Silica Deposition Vesicle (SDV) has been characterized as an acidic compartment fused with Golgi-derived vesicles. These two protein structures have been shown to create sheets of patterned silica in-vivo with irregular pores on

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1870-401: Is asexual by binary fission , during which the diatom divides into two parts, producing two "new" diatoms with identical genes. Each new organism receives one of the two frustules – one larger, the other smaller – possessed by the parent, which is now called the epitheca ; and is used to construct a second, smaller frustule, the hypotheca . The diatom that received the larger frustule becomes

1955-405: Is believed by many researchers to be the key to this ecological success. Raven (1983) noted that, relative to organic cell walls , silica frustules require less energy to synthesize (approximately 8% of a comparable organic wall), potentially a significant saving on the overall cell energy budget. In a now classic study, Egge and Aksnes (1992) found that diatom dominance of mesocosm communities

2040-419: Is home to several organelles, like the chloroplasts and mitochondria. Before the centric diatom begins to expand, its nucleus is at the center of one of the valves and begins to move towards the center of the cytoplasmic layer before division is complete. Centric diatoms have a variety of shapes and sizes, depending on from which axis the shell extends, and if spines are present. Diatom cells are contained within

2125-469: Is induced by either a loss of buoyancy control, the synthesis of mucilage that sticks diatoms cells together, or the production of heavy resting spores . Sinking out of the upper mixed layer removes diatoms from conditions unfavourable to growth, including grazer populations and higher temperatures (which would otherwise increase cell metabolism ). Cells reaching deeper water or the shallow seafloor can then rest until conditions become more favourable again. In

2210-442: Is more readily assimilated). More generally, notwithstanding these possible advantages conferred by their use of silicon, diatoms typically have higher growth rates than other algae of the same corresponding size. Diatoms can be obtained from multiple sources. Marine diatoms can be collected by direct water sampling, and benthic forms can be secured by scraping barnacles , oyster and other shells. Diatoms are frequently present as

2295-560: Is not clear, but it is speculated that it may be related to communication, camouflage, thermal exchange and/or UV protection. Diatoms build intricate hard but porous cell walls called frustules composed primarily of silica . This siliceous wall can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations; all of which can be used to delineate genera and species. The cell itself consists of two halves, each containing an essentially flat plate, or valve, and marginal connecting, or girdle band. One half,

2380-422: Is often covered with small forms. Since diatoms form an important part of the food of molluscs , tunicates , and fishes , the alimentary tracts of these animals often yield forms that are not easily secured in other ways. Diatoms can be made to emerge by filling a jar with water and mud, wrapping it in black paper and letting direct sunlight fall on the surface of the water. Within a day, the diatoms will come to

2465-541: Is that they have lost their cell wall of silica, making them the only known shell-less diatoms. The study of diatoms is a branch of phycology . Diatoms are classified as eukaryotes , organisms with a nuclear envelope -bound cell nucleus , that separates them from the prokaryotes archaea and bacteria . Diatoms are a type of plankton called phytoplankton , the most common of the plankton types. Diatoms also grow attached to benthic substrates, floating debris, and on macrophytes . They comprise an integral component of

2550-528: Is the triple divide between the rivers Rhine , Danube and Po . To learn about the importance of travel along the Hinterrhein, a multiday trekking route is signposted along the river from Thusis to Splügen , where it turns south to Splügen Pass , a historically important transit route to Italy. Nowadays there is no traffic across the Splügen pass in winter, whereas the route across its age-old rival,

2635-416: Is the sum of processes within the hydrologic cycle that increase the water levels of bodies of water. Most precipitation occurs directly over bodies of water such as the oceans, or on land as surface runoff . A portion of runoff enters streams and rivers, and another portion soaks into the ground as groundwater seepage . The rest soaks into the ground as infiltration, some of which infiltrates deep into

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2720-522: Is their restrictive and bipartite silica cell wall that causes them to progressively shrink during asexual cell division. At a critically small cell size and under certain conditions, auxosporulation restitutes cell size and prevents clonal death. The entire lifecycles of only a few diatoms have been described and rarely have sexual events been captured in the environment. Most eukaryotes are capable of sexual reproduction involving meiosis . Sexual reproduction appears to be an obligatory phase in

2805-404: Is typically expressed in units of cubic meters per second (m³/s) or cubic feet per second (cfs). The catchment of a river above a certain location is determined by the surface area of all land which drains toward the river from above that point. The river's discharge at that location depends on the rainfall on the catchment or drainage area and the inflow or outflow of groundwater to or from

2890-399: Is usually limited to a gliding motion. In centric diatoms, the small male gametes have one flagellum while the female gametes are large and non-motile ( oogamous ). Conversely, in pennate diatoms both gametes lack flagella ( isogamous ). Certain araphid species, that is pennate diatoms without a raphe (seam), have been documented as anisogamous and are, therefore, considered to represent

2975-589: The Avers Rhine , adds waters from the deep Val Ferrera and the very remote alp of Avers and its side valley Valle di Lei , located on Italian territory. Below the Rofla Gorge, the valley widens into a section called Schams . The Hinterrhein then reaches Andeer , before passing through another gorge, the Viamala , before arriving at Thusis . Now, another tributary of slightly bigger volume reaches

3060-791: The Hinterrhein , the Albula , coming from the Albula Pass , which is also the name of a railway line that has become a UNESCO world heritage . The two main tributaries of the Albula are the Landwasser , flowing through Davos , and the Gelgia from the Julier pass area. After passing Rothenbrunnen through a valley called Domleschg (a valley with an incredible number of castles, showing

3145-535: The Rhine river in Europe is 2,200 cubic metres per second (78,000 cu ft/s) or 190,000,000 cubic metres (150,000 acre⋅ft) per day. Because of the difficulties of measurement, a stream gauge is often used at a fixed location on the stream or river. A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow

3230-522: The San Bernardino Pass , was given a long road tunnel in 1967 that made it possible to keep the road open all year. This Graubünden location article is a stub . You can help Misplaced Pages by expanding it . This article related to a river in Switzerland is a stub . You can help Misplaced Pages by expanding it . Discharge (hydrology) In hydrology , discharge is the volumetric flow rate (volume per time, in units of m /h or ft /h) of

3315-575: The biological carbon pump and influence the ocean carbon cycle . The anthropogenic CO 2 emission to the atmosphere (mainly generated by fossil fuel burning and deforestation) is nearly 11 gigatonne carbon (GtC) per year, of which almost 2.5 GtC is taken up by the surface ocean. In surface seawater ( pH 8.1–8.4), bicarbonate ( HCO 3 ) and carbonate ions ( CO 3 ) constitute nearly 90 and <10% of dissolved inorganic carbon (DIC) respectively, while dissolved CO 2 (CO 2 aqueous) contributes <1%. Despite this low level of CO 2 in

3400-399: The carotenoid fucoxanthin . Individuals usually lack flagella , but they are present in male gametes of the centric diatoms and have the usual heterokont structure, including the hairs ( mastigonemes ) characteristic in other groups. Diatoms are often referred as "jewels of the sea" or "living opals" due to their optical properties. The biological function of this structural coloration

3485-403: The hypotheca , is slightly smaller than the other half, the epitheca . Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical. Their distinguishing feature is a hard mineral shell or frustule composed of opal (hydrated, polymerized silicic acid). Diatoms are divided into two groups that are distinguished by the shape of

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3570-443: The mitochondria also play critical roles in energy balance. Two nitrogen-related pathways are relevant and they may also change under ammonium ( NH 4 ) nutrition compared with nitrate ( NO 3 ) nutrition. First, in diatoms, and likely some other algae, there is a urea cycle. The long-known function of the urea cycle in animals is to excrete excess nitrogen produced by amino acid Catabolism ; like photorespiration ,

3655-406: The periphyton community. Another classification divides plankton into eight types based on size: in this scheme, diatoms are classed as microalgae. Several systems for classifying the individual diatom species exist. Fossil evidence suggests that diatoms originated during or before the early Jurassic period, which was about 150 to 200 million years ago. The oldest fossil evidence for diatoms

3740-458: The spring ), their competitive edge and rapid growth rate enables them to dominate phytoplankton communities ("boom" or "bloom"). As such they are often classed as opportunistic r-strategists (i.e. those organisms whose ecology is defined by a high growth rate, r ). The freshwater diatom Didymosphenia geminata , commonly known as Didymo, causes severe environmental degradation in water-courses where it blooms, producing large quantities of

3825-422: The zygote . The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore. A new diatom cell of maximum size, the initial cell, forms within the auxospore thus beginning a new generation. Resting spores may also be formed as a response to unfavourable environmental conditions with germination occurring when conditions improve. A defining characteristic of all diatoms

3910-653: The "salt"). Unknowingly, the viewer's confusion captured the essence of diatoms—mineral utilizing plants. It is not clear when it was determined that diatom cell walls are made of silica, but in 1939 a seminal reference characterized the material as silicic acid in a "subcolloidal" state Identification of the main chemical component of the cell wall spurred investigations into how it was made. These investigations have involved, and been propelled by, diverse approaches including, microscopy, chemistry, biochemistry, material characterisation , molecular biology , 'omics , and transgenic approaches. The results from this work have given

3995-561: The "take-over" remains unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the displacement of siliceous sponges from the shelves, suggests that this takeover began in the Cretaceous (146 Ma to 66 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (66 Ma to present). The diagram depicts some mechanisms by which marine diatoms contribute to

4080-425: The ability to grow in colonial chains. These adaptations increase their surface area to volume ratio and drag , allowing them to stay suspended in the water column longer. Individual cells may regulate buoyancy via an ionic pump. Some pennate diatoms are capable of a type of locomotion called "gliding", which allows them to move across surfaces via adhesive mucilage secreted through a seamlike structure called

4165-399: The area, stream modifications such as dams and irrigation diversions, as well as evaporation and evapotranspiration from the area's land and plant surfaces. In storm hydrology, an important consideration is the stream's discharge hydrograph, a record of how the discharge varies over time after a precipitation event. The stream rises to a peak flow after each precipitation event, then falls in

4250-540: The coasts via rivers, and from below via seafloor sediment recycling, weathering, and hydrothermal activity . Although diatoms may have existed since the Triassic , the timing of their ascendancy and "take-over" of the silicon cycle occurred more recently. Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle. Subsequently,

4335-485: The corresponding discharge from the rating curve. If a continuous level-recording device is located at a rated cross-section, the stream's discharge may be continuously determined. Larger flows (higher discharges) can transport more sediment and larger particles downstream than smaller flows due to their greater force. Larger flows can also erode stream banks and damage public infrastructure. G. H. Dury and M. J. Bradshaw are two geographers who devised models showing

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4420-417: The cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges , the former as zooplankton , the latter as sedentary filter-feeders primarily on the continental shelves . Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms. However, the precise timing of

4505-498: The deep ocean biome. Diatoms have complex life history strategies that are presumed to have contributed to their rapid genetic diversification into ~200,000 species  that are distributed between the two major diatom groups: centrics and pennates. Diatoms are generally 20 to 200 micrometers in size, with a few larger species. Their yellowish-brown chloroplasts , the site of photosynthesis, are typical of heterokonts , having four cell membranes and containing pigments such as

4590-528: The diatom (spring) bloom is typically ended by a shortage of silicon. Unlike other minerals, the requirement for silicon is unique to diatoms and it is not regenerated in the plankton ecosystem as efficiently as, for instance, nitrogen or phosphorus nutrients. This can be seen in maps of surface nutrient concentrations – as nutrients decline along gradients, silicon is usually the first to be exhausted (followed normally by nitrogen then phosphorus). Because of this bloom-and-bust cycle, diatoms are believed to play

4675-418: The discharge of a river is based on a simplified form of the continuity equation . The equation implies that for any incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the stream's cross-sectional area (A) and its mean velocity ( u ¯ {\displaystyle {\bar {u}}} ), and is written as: where For example, the average discharge of

4760-633: The discharge of a river we need a different method and the most common is the 'area-velocity' method. The area is the cross sectional area across a river and the average velocity across that section needs to be measured for a unit time, commonly a minute. Measurement of cross sectional area and average velocity, although simple in concept, are frequently non-trivial to determine. The units that are typically used to express discharge in streams or rivers include m /s (cubic meters per second), ft /s (cubic feet per second or cfs) and/or acre-feet per day. A commonly applied methodology for measuring, and estimating,

4845-450: The dominant components of phytoplankton in nutrient-rich coastal waters and during oceanic spring blooms, since they can divide more rapidly than other groups of phytoplankton. Most live pelagically in open water, although some live as surface films at the water-sediment interface ( benthic ), or even under damp atmospheric conditions. They are especially important in oceans, where a 2003 study found that they contribute an estimated 45% of

4930-496: The frustule: the centric diatoms and the pennate diatoms . Pennate diatoms are bilaterally symmetric. Each one of their valves have openings that are slits along the raphes and their shells are typically elongated parallel to these raphes. They generate cell movement through cytoplasm that streams along the raphes, always moving along solid surfaces. Centric diatoms are radially symmetric. They are composed of upper and lower valves – epitheca and hypotheca – each consisting of

5015-441: The genera Hemiaulus , Rhizosolenia and Chaetoceros . Dinotoms are diatoms that have become endosymbionts inside dinoflagellates . Research on the dinoflagellates Durinskia baltica and Glenodinium foliaceum has shown that the endosymbiont event happened so recently, evolutionarily speaking, that their organelles and genome are still intact with minimal to no gene loss. The main difference between these and free living diatoms

5100-464: The ground to replenish aquifers. Diatom A diatom ( Neo-Latin diatoma ) is any member of a large group comprising several genera of algae , specifically microalgae , found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass : they generate about 20 to 50 percent of the oxygen produced on the planet each year, take in over 6.7 billion tonnes of silicon each year from

5185-458: The identification of novel components involved in higher order processes, the dynamics documented through real-time imaging, and the genetic manipulation of silica structure. The approaches established in these recent works provide practical avenues to not only identify the components involved in silica cell wall formation but to elucidate their interactions and spatio-temporal dynamics. This type of holistic understanding will be necessary to achieve

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5270-697: The importance of transit in the area) the Hinterrhein is again left alone from civilisation in the floodplain Isla Bella near Rhäzüns , before it joins the Anterior Rhine ( Vorderrhein ) at Reichenau , near Bonaduz . The drainage basin of the Hinterrhein lies mostly within the canton of Grisons, with one tributary, the Reno di Lei rising in Lombardy, Italy ( Sondrio ). The nearby Lunghin Pass

5355-424: The level of the stream is described by a rating curve . Average velocities and the cross-sectional area of the stream are measured for a given stream level. The velocity and the area give the discharge for that level. After measurements are made for several different levels, a rating table or rating curve may be developed. Once rated, the discharge in the stream may be determined by measuring the level, and determining

5440-399: The life cycle of diatoms, particularly as cell size decreases with successive vegetative divisions. Sexual reproduction involves production of gametes and the fusion of gametes to form a zygote in which maximal cell size is restored. The signaling that triggers the sexual phase is favored when cells accumulate together, so that the distance between them is reduced and the contacts and/or

5525-453: The maximum life span of individual cells is about six days. Diatoms have two distinct shapes: a few ( centric diatoms ) are radially symmetric, while most ( pennate diatoms ) are broadly bilaterally symmetric. The unique feature of diatoms is that they are surrounded by a cell wall made of silica (hydrated silicon dioxide ), called a frustule . These frustules produce structural coloration , prompting them to be described as "jewels of

5610-479: The model species, as the whole genome was sequenced and methods for genetic control were established, and Cylindrotheca fusiformis , in which the important silica deposition proteins silaffins were first discovered. Silaffins, sets of polycationic peptides , were found in C. fusiformis cell walls and can generate intricate silica structures. These structures demonstrated pores of sizes characteristic to diatom patterns. When T. pseudonana underwent genome analysis it

5695-470: The ocean and its slow diffusion rate in water, diatoms fix 10–20 GtC annually via photosynthesis thanks to their carbon dioxide concentrating mechanisms , allowing them to sustain marine food chains . In addition, 0.1–1% of this organic material produced in the euphotic layer sinks down as particles, thus transferring the surface carbon toward the deep ocean and sequestering atmospheric CO 2 for thousands of years or longer. The remaining organic matter

5780-588: The open ocean, many sinking cells are lost to the deep, but refuge populations can persist near the thermocline . Ultimately, diatom cells in these resting populations re-enter the upper mixed layer when vertical mixing entrains them. In most circumstances, this mixing also replenishes nutrients in the upper mixed layer, setting the scene for the next round of diatom blooms. In the open ocean (away from areas of continuous upwelling ), this cycle of bloom, bust, then return to pre-bloom conditions typically occurs over an annual cycle, with diatoms only being prevalent during

5865-463: The perception of chemical cues is facilitated. An exploration of the genomes of five diatoms and one diatom transcriptome led to the identification of 42 genes potentially involved in meiosis. Thus a meiotic toolkit appears to be conserved in these six diatom species, indicating a central role of meiosis in diatoms as in other eukaryotes. Diatoms are mostly non-motile ; however, sperm found in some species can be flagellated , though motility

5950-423: The raphe. In order for a diatom cell to glide, it must have a solid substrate for the mucilage to adhere to. Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures; mucilage pads, stalks or tubes; amorphous masses of mucilage; or by threads of chitin (polysaccharide), which are secreted through strutted processes of the cell. Reproduction among these organisms

6035-583: The relationship between discharge and other variables in a river. The Bradshaw model described how pebble size and other variables change from source to mouth; while Dury considered the relationships between discharge and variables such as stream slope and friction. These follow from the ideas presented by Leopold, Wolman and Miller in Fluvial Processes in Geomorphology . and on land use affecting river discharge and bedload supply. Inflow

6120-614: The same size as its parent, but the diatom that received the smaller frustule remains smaller than its parent. This causes the average cell size of this diatom population to decrease. It has been observed, however, that certain taxa have the ability to divide without causing a reduction in cell size. Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and auxospore formation must occur. Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form

6205-678: The scale of diatom frustules . One hypothesis as to how these proteins work to create complex structure is that residues are conserved within the SDV's, which is unfortunately difficult to identify or observe due to the limited number of diverse sequences available. Though the exact mechanism of the highly uniform deposition of silica is as yet unknown, the Thalassiosira pseudonana genes linked to silaffins are being looked to as targets for genetic control of nanoscale silica deposition. The ability of diatoms to make silica-based cell walls has been

6290-468: The sea" and "living opals". Movement in diatoms primarily occurs passively as a result of both ocean currents and wind-induced water turbulence ; however, male gametes of centric diatoms have flagella , permitting active movement to seek female gametes. Similar to plants , diatoms convert light energy to chemical energy by photosynthesis , but their chloroplasts were acquired in different ways. Unusually for autotrophic organisms, diatoms possess

6375-400: The spring and early summer. In some locations, however, an autumn bloom may occur, caused by the breakdown of summer stratification and the entrainment of nutrients while light levels are still sufficient for growth. Since vertical mixing is increasing, and light levels are falling as winter approaches, these blooms are smaller and shorter-lived than their spring equivalents. In the open ocean,

6460-465: The subject of fascination for centuries. It started with a microscopic observation by an anonymous English country nobleman in 1703, who observed an object that looked like a chain of regular parallelograms and debated whether it was just crystals of salt, or a plant. The viewer decided that it was a plant because the parallelograms didn't separate upon agitation, nor did they vary in appearance when dried or subjected to warm water (in an attempt to dissolve

6545-401: The top in a scum and can be isolated. The diagram shows the major fluxes of silicon in the current ocean. Most biogenic silica in the ocean ( silica produced by biological activity ) comes from diatoms. Diatoms extract dissolved silicic acid from surface waters as they grow, and return it to the water column when they die. Inputs of silicon arrive from above via aeolian dust , from

6630-461: The total oceanic primary production of organic material. However, a more recent 2016 study estimates that the number is closer to 20%. Spatial distribution of marine phytoplankton species is restricted both horizontally and vertically. Planktonic diatoms in freshwater and marine environments typically exhibit a " boom and bust " (or " bloom and bust") lifestyle. When conditions in the upper mixed layer (nutrients and light) are favourable (as at

6715-400: The two-halves and grows a smaller half within it. As a result, after each division cycle, the average size of diatom cells in the population gets smaller. Once such cells reach a certain minimum size, rather than simply divide, they reverse this decline by forming an auxospore , usually through meiosis and sexual reproduction, but exceptions exist. The auxospore expands in size to give rise to

6800-436: The urea cycle had long been considered a waste pathway. However, in diatoms the urea cycle appears to play a role in exchange of nutrients between the mitochondria and the cytoplasm , and potentially the plastid   and may help to regulate ammonium metabolism. Because of this cycle, marine diatoms, in contrast to chlorophytes , also have acquired a mitochondrial urea transporter and, in fact, based on bioinformatics ,

6885-420: The volume of water (depending on the area of the catchment) that subsequently flows out of the river. Using the unit hydrograph method, actual historical rainfalls can be modeled mathematically to confirm characteristics of historical floods, and hypothetical "design storms" can be created for comparison to observed stream responses. The relationship between the discharge in the stream at a given cross-section and

6970-531: The waters in which they live, and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile (800 m) deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara , much of it from the Bodélé Depression , which

7055-399: Was directly related to the availability of silicic acid – when concentrations were greater than 2 μmol m , they found that diatoms typically represented more than 70% of the phytoplankton community. Other researchers have suggested that the biogenic silica in diatom cell walls acts as an effective pH buffering agent , facilitating the conversion of bicarbonate to dissolved CO 2 (which

7140-505: Was found that it encoded a urea cycle , including a higher number of polyamines than most genomes, as well as three distinct silica transport genes. In a phylogenetic study on silica transport genes from 8 diverse groups of diatoms, silica transport was found to generally group with species. This study also found structural differences between the silica transporters of pennate (bilateral symmetry) and centric (radial symmetry) diatoms. The sequences compared in this study were used to create

7225-421: Was once made up of a system of fresh-water lakes. Diatoms are unicellular organisms : they occur either as solitary cells or in colonies , which can take the shape of ribbons, fans, zigzags, or stars. Individual cells range in size from 2 to 2000 micrometers. In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles approximately every 24 hours by asexual multiple fission ;

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