Chalk is a soft, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3, it forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells shed from micro-organisms called coccolithophores. Flint is common as bands parallel to the bedding or as nodules embedded in chalk, it is derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is deposited around larger fossils such as Echinoidea which may be silicified. Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones; this indicates stable conditions over tens of millions of years. Chalk has greater resistance to weathering and slumping than the clays with which it is associated, thus forming tall, steep cliffs where chalk ridges meet the sea.
Chalk hills, known as chalk downland form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water through dry seasons. Chalk is mined from chalk deposits both above underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks; some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty. The Chalk Group is a European stratigraphic unit, it forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is underlain by chalk deposits, which contain artificial caves used for wine storage; some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark – both once formed a single island.
Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Chalk was one of the earliest rocks made up of microscopic particles to be studied under the microscope, when it was found to be composed entirely of coccoliths, their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer built up over millions of years and, through the weight of overlying sediments became consolidated into rock. Earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level; the chemical composition of chalk is calcium carbonate, with minor amounts of clay. It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, made of mineral chalk, since it crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be erased.
Blackboard chalk manufacture now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum. While gypsum-based blackboard chalk is the lowest cost to produce, thus used in the developing world, calcium-based chalk can be made where the crumbling particles are larger and thus produce less dust, is marketed as "dustless chalk". Colored chalks, pastel chalks, sidewalk chalk, used to draw on sidewalks and driveways, are made of gypsum. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In southeast England, deneholes are a notable example of ancient chalk pits; such bell pits may mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but the most famous is the extensive complex at Grimes Graves in Norfolk. Woodworking joints may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface.
Chalk transferring to cover the complete surface indicates a good fit. Builder's putty mainly contains chalk as a filler in linseed oil. Chalk may be used for its properties as a base. In agriculture, chalk is used for raising pH in soils with high acidity; the most common forms are CaCO3 and CaO. Small doses of chalk can be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for polishing. For example, toothpaste contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a controlled grain size, for fine polishing of metals. Chalk can be used as fingerprint powder. Several traditional uses of chalk have been replaced by other substances, although the word "chalk" is still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth by tailors, it is now made of talc. Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court.
If a ball hits the line, a cloud of chalk or p
White Cliffs of Dover
The White Cliffs of Dover, part of the North Downs formation, is the name given to the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of 350 feet, owes its striking appearance to its composition of chalk accented by streaks of black flint; the cliffs, on both sides of the town of Dover in Kent, stretch for eight miles. A section of coastline encompassing the cliffs was purchased by the National Trust in 2016; the cliffs are part of the Dover to Kingsdown Cliffs Site of Special Scientific Interest and Special Area of Conservation. The cliffs are part of the coastline of Kent in England between 51°06′N 1°14′E and 51°12′N 1°24′E, at the point where Great Britain is closest to continental Europe. On a clear day they are visible from the French coast; the chalk cliffs of the Alabaster Coast of Normandy in France are part of the same geological system. The White Cliffs are at one end of the Kent Downs designated Area of Outstanding Natural Beauty.
In 1999 a sustainable National Trust visitor centre was built in the area. The Gateway building, designed by van Heyningen and Haward Architects, houses a restaurant, an information centre on the work of the National Trust, details of local archaeology and landscape.cheese spread About 70 million years ago Great Britain and much of Europe was submerged under a great sea. The sea bottom was covered with white mud formed from fragments of coccoliths, the skeletons of tiny algae that floated in the surface waters and sank to the bottom during the Cretaceous period and, together with the remains of bottom-living creatures, formed muddy sediments, it is thought that the sediments were deposited slowly half a millimetre a year, equivalent to about 180 coccoliths piled one on top of another. Up to 1,600 feet 500 metres of sediments were deposited in some areas; the weight of overlying sediments caused the deposits to become consolidated into chalk. Subsequent earth movements related to the formation of the Alps raised the sea-floor deposits above sea level.
Until the end of the last glacial period, the British Isles were part of continental Europe, linked by the unbroken Weald-Artois Anticline, a ridge that acted as a natural dam to hold back a large freshwater pro-glacial lake, now submerged under the North Sea. The land masses remained connected until between 450,000 and 180,000 years ago when at least two catastrophic glacial lake outburst floods breached the anticline and destroyed the ridge that connected Britain to Europe. A land connection across the southern North Sea existed intermittently at times when periods of glaciation resulted in lower sea levels. At the end of the last glacial period, around 10,000 years ago, rising sea levels severed the last land connection; the cliffs' chalk face shows horizontal bands of dark-coloured flint, composed of the remains of sea sponges and siliceous planktonic micro-organisms which hardened into the microscopic quartz crystals. Quartz silica filled cavities left by dead marine creatures which are found as flint fossils the internal moulds of Micraster echinoids.
Several different ocean floor species such as brachiopods, bivalves and sponges be can found in the chalk deposits, as can sharks' teeth. In some areas, layers of a soft, grey chalk known as a hardground complex can be seen. Hardgrounds are thought to reflect disruptions in the steady accumulation of sediment when sedimentation ceased and/or the loose surface sediments were stripped away by currents or slumping, exposing the older hardened chalk sediment. A single hardground may have been exhumed 16 or more times before the sediments were compacted and hardened to form chalk; the cliff face continues to weather at an average rate of 1 centimetre per year, although large pieces will fall. In 2001, a large chunk of the cliff edge, as large as a football pitch, fell into the Channel. Another large section collapsed on 15 March 2012; the chalk grassland environment above the cliffs provides an excellent environment for many species of wild flowers and birds, has been designated a Special Area of Conservation and a Site of Special Scientific Interest.
Rangers and volunteers work to clear invasive plants. A grazing programme involving Exmoor ponies has been established to help to clear faster-growing invasive plants, allowing smaller, less robust native plants to survive; the ponies are managed by the National Trust, Natural England, County Wildlife Trusts to maintain vegetation on nature reserves. The cliffs are the first landing point for many migratory birds flying inland from across the English Channel. After a 120-year absence, in 2009 it was reported. Similar in appearance but smaller, the jackdaw is abundant; the rarest of the birds that live along the cliffs is the peregrine falcon. In recent decline and endangered, the skylark makes its home on the cliffs; the cliffs are home to fulmars, which resemble gulls, to colonies of black-legged kittiwake, a species of gull. Bluebird, as mentioned in the classic World War II song " The White Cliffs of Dover" is an old country name for swallows and house martins, which make an annual migration to continental Europe, many of them crossing the English Channel at least twice a year.
Among the wildflowers are several varieties of orchids, the rarest of, the early spider orchid, which has yellow-green to brownish green petals and looks like the body of a large spider. The oxtongue broomrape is an unusual plant, it has yellow, white, or blue snapdragon-like flowers and about 90 per cent of the UK's population is found on the cliffs. V
Sediment is a occurring material, broken down by processes of weathering and erosion, is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may become sandstone and siltstone. Sediments are most transported by water, but wind and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian deposition. Glacial moraine deposits and till are ice-transported sediments. Sediment can be classified based on its grain composition. Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder". Composition of sediment can be measured in terms of: parent rock lithology mineral composition chemical make-up.
This leads to an ambiguity in which clay can be used as a composition. Sediment is transported based on the strength of the flow that carries it and its own size, volume and shape. Stronger flows will increase the lift and drag on the particle, causing it to rise, while larger or denser particles will be more to fall through the flow. Rivers and streams carry sediment in their flows; this sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle, the settling velocity of the particle. These relationships are shown in the following table for the Rouse number, a ratio of sediment fall velocity to upwards velocity. Rouse = Settling velocity Upwards velocity from lift and drag = w s κ u ∗ where w s is the fall velocity κ is the von Kármán constant u ∗ is the shear velocity If the upwards velocity is equal to the settling velocity, sediment will be transported downstream as suspended load. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move, it will move along the bed as bed load by rolling and saltating.
If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as wash load. As there are a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions. Sediment motion can create self-organized structures such as ripples, dunes, or antidunes on the river or stream bed; these bedforms are preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment. Overland flow can transport them downslope; the erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions. If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion. If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion". If the flow and the substrate permit channelization, gullies may form.
The major fluvial environments for deposition of sediments include: Deltas Point bars Alluvial fans Braided rivers Oxbow lakes Levees Waterfalls Wind results in the transportation of fine sediment and the formation of sand dune fields and soils from airborne dust. Glaciers carry a wide range of sediment sizes, deposit it in moraines; the overall balance between sediment in transport and sediment being deposited on the bed is given by the Exner equation. This expression states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment that falls out of the flow; this equation is important in that changes in the power of the flow change the ability of the flow to carry sediment, this is reflected in the patterns of erosion and deposition observed throughout a stream. This can be localized, due to small obstacles. Erosion and deposition can be regional. Deposition can occur due to dam emplacement that causes the river to pool and deposit its entire load, or due to base level rise.
Seas and lakes accumulate sediment over time. The sediment can consist of terrigenous material, which originates on land, but may be deposited in either terrestrial, marine, or lacustrine environments, or of sediments originating in the body of water. Terrigenous material is supplied by nearby rivers and streams or reworked marine sediment. In the mid-ocean, the exoskeletons of dead organisms are responsible for sediment accumulation. Deposited sediments are the source of sedimentary rocks, which can contain fossils of
Testate amoebae are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test that encloses the cell, with an aperture from which the pseudopodia emerge, that provides the amoeba with shelter from predators and environmental conditions. The test of some species is produced by the amoeba and may be organic, siliceous or calcareous depending on the species, whereas in other cases the test is made up of particles of sediment collected by the amoeba which are agglutinated together by secretions from within the cell. A few taxa can build either type, depending on the circumstances and availability of foreign material; the assemblage referred to as "testate amoebae" is composed of several, unrelated groups of organisms. However, some features they all share that have been used to group them together include the presence of a test and pseudopodia that do not anastomose. Testate amoebae can be found in most freshwater environments, including lakes, cenotes, as well as mires and soils.
The strong and resistant nature of the tests allows them to be preserved long after the amoeba has died. These characteristics, along with the sensitivity that some species display to changes in environmental conditions, has sparked their use as bioindicators and paleoclimate proxies in recent years. Testate amoebae are a polyphyletic assemblage; the main testate amoebae groups are the lobose Tubulinea, which include Arcellinida and Phryganellina, the filose Euglyphida, although there are smaller groups that include other testate amoebae. The following table includes a few examples of testate amoebae genera, reflects their position within the classification by Adl et al. where five supergroups were proposed to classify all eukaryotes. This classification purposefully avoids the use of Linnaean higher category names. While it has been noted that the names that Adl et al. provide for the clades may result confusing or uninformative regarding the relative degree of phenotypic distinctiveness amongst groups when used in isolation, this system avoids creating superfluous ranks where unnecessary and provides stable group names that can be retained when a group is moved to a different lineage, as is the case with protists, as their classification remains in constant review.
Traditionally, those species that form large networks of anastomosing pseudopodia, despite some of them having tests, are not counted amongst testate amoebae. The Thecamoebida, with the genus Thecamoeba, despite their name, do not have tests. Euglyphid testate amoebae are related to the Foraminifera. Microworld - World of ameboid organisms - A database of both testate and naked amoebae with over 6,700 microphotographs and videos and over 1,700 species descriptions, as well as dichotomous and visual keys for identification. Medioli, F. S.. B.. G.. The thecamoebian bibliography. Palaeontologia Electronica, 3: 1-161. Medioli, F. S.. B.. E.. The thecamoebian bibliography: 2nd edition. Palaeontologia Electronica, 61: 1-107
Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current. The individual organisms constituting plankton are called plankters, they provide a crucial source of food to many large aquatic organisms, such as fish and whales. These organisms include bacteria, algae and drifting or floating animals that inhabit—for example—the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification. Though many planktonic species are microscopic in size, plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish. Technically the term does not include organisms on the surface of the water, which are called pleuston—or those that swim in the water, which are called nekton; the name plankton is derived from the Greek adjective πλαγκτός, meaning errant, by extension, wanderer or drifter, was coined by Victor Hensen in 1887.
While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day, their horizontal position is determined by the surrounding water movement, plankton flow with ocean currents. This is in contrast to nekton organisms, such as fish and marine mammals, which can swim against the ambient flow and control their position in the environment. Within the plankton, holoplankton spend their entire life cycle as plankton. By contrast, meroplankton are only planktic for part of their lives, graduate to either a nektic or benthic existence. Examples of meroplankton include the larvae of sea urchins, crustaceans, marine worms, most fish; the amount and distribution of plankton depends on available nutrients, the state of water and a large amount of other plankton. The study of plankton is termed planktology and a planktonic individual is referred to as a plankter; the adjective planktonic is used in both the scientific and popular literature, is a accepted term.
However, from the standpoint of prescriptive grammar, the less-commonly used planktic is more the correct adjective. When deriving English words from their Greek or Latin roots, the gender-specific ending is dropped, using only the root of the word in the derivation. Plankton are divided into broad functional groups: Phytoplankton, autotrophic prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, cyanobacteria and coccolithophores. Zooplankton, small protozoans or metazoans that feed on other plankton; some of the eggs and larvae of larger nektonic animals, such as fish and annelids, are included here. Bacterioplankton and archaea, which play an important role in remineralising organic material down the water column. Mycoplankton and fungus-like organisms, like bacterioplankton, are significant in remineralisation and nutrient cycling; this scheme divides the plankton community into broad producer and recycler groups.
However, determining the trophic level of many plankton is not always straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species perform both roles. In this mixed trophic strategy — known as mixotrophy — organisms act as both producers and consumers, either at the same time or switching between modes of nutrition in response to ambient conditions. For instance, relying on photosynthesis for growth when nutrients and light are abundant, but switching to predation when growing conditions are poor. Recognition of the importance of mixotrophy as an ecological strategy is increasing, as well as the wider role this may play in marine biogeochemistry. Plankton are often described in terms of size; the following divisions are used: However, some of these terms may be used with different boundaries on the larger end. The existence and importance of nano- and smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity.
The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is much more important than its mass or inertia. Plankton inhabit oceans, lakes, ponds. Local abundance varies horizontally and seasonally; the primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy, confining primary production to surface waters, to geographical regions and seasons having abundant light. A secondary variable is nutrient availability. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience low primary production because they offer limited nutrients such as nitrate and silicate; this results from large-scale ocean water column stratification. In such regions, primary production occurs at greater depth, although at a reduced level. Despite significant macronutrient concentrations, some ocean regions are unproductive; the micronutrient iron is deficient in these reg
An amoeba called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape by extending and retracting pseudopods. Amoebas do not form a single taxonomic group. Amoeboid cells occur not only among the protozoa, but in fungi and animals. Microbiologists use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement. In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Amoeboid organisms are no longer classified together in one group; the best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, the multicellular "social amoeba" or slime mould Dictyostelium discoideum.
Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba have bulbous pseudopods, rounded at the ends and tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have thread-like pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like structures; some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia supported from within by bundles of microtubules. Free-living amoebae may be "testate", or "naked"; the shells of testate amoebae may be composed of various substances, including calcium, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms. To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.
This organelle is necessary because freshwater has a lower concentration of solutes than the amoeba's own internal fluids. Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and burst. Marine amoebae do not possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water; the food sources of amoebae vary. Some amoebae are live by consuming bacteria and other protists; some eat dead organic material. Amoebae ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, there is no fixed place on the cell at which phagocytosis occurs; some amoebae feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
The size of amoeboid cells and species is variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter, within the size range of many bacteria. At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter. Most of the free-living freshwater amoebae found in pond water and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye; some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, engulf them by phagocytosis. Amoeboid stages occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds classified in the class Myxogastria, the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage.
The amoeboid cells of the former combine to form a giant multinucleate organism, while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism. Other organisms may present amoeboid cells during certain life-cycle stages, e.g. the gametes of some green algae and pennate diatoms, the spores of some Mesomycetozoea, the sporoplasm stage of Myxozoa and of Ascetosporea. The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus". Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus; the term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid. In 1822, the genus Amiba was erected by the Frenc
The red algae, or Rhodophyta, are one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species with taxonomic revisions ongoing; the majority of species are found in the Florideophyceae, consist of multicellular, marine algae, including many notable seaweeds. 5% of the red algae occur in freshwater environments with greater concentrations found in the warmer area. There are no terrestrial species, assumed to be traced back to an evolutionary bottleneck where the last common ancestor lost about 25% of its core genes and much of its evolutionary plasticity; the red algae form a distinct group characterized by having eukaryotic cells without flagella and centrioles, chloroplasts that lack external endoplasmic reticulum and contain unstacked thylakoids, use phycobiliproteins as accessory pigments, which give them their red color. Red algae store sugars as floridean starch, a type of starch that consists of branched amylopectin without amylose, as food reserves outside their plastids.
Most red algae are multicellular, macroscopic and reproduce sexually. The red algal life history is an alternation of generations that may have three generations rather than two. Chloroplasts evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukarytoic phagotroph; this event resulted in the origin of the red and green algae, the glaucophytes, which make up the oldest evolutionary lineages of photosynthetic eukaryotes. A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such as Cryptophyta, Stramenopiles, Centrohelids and Telonemi; the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse and laver are a traditional part of European and Asian cuisines and are used to make other products such as agar and other food additives. Unicellular members of the Cyanidiophyceae are thermoacidophiles and are found in sulphuric hot springs and other acidic environments.
The remaining taxa are found in freshwater environments. Most rhodophytes are marine with a worldwide distribution, are found at greater depths compared to other seaweeds because of dominance in certain pigments within their chloroplasts; some marine species are found on sandy shores, while most others can be found attached to rocky substrata. Freshwater species account for 5% of red algal diversity, but they have a worldwide distribution in various habitats. A few freshwater species are found in black waters with sandy bottoms and fewer are found in more lentic waters. Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae and animals. In addition, some marine species have adopted a parasitic lifestyle and may be found on or more distantly related red algal hosts. In the system of Adl et al. 2005, the red algae are classified in the Archaeplastida, along with the glaucophytes and green algae plus land plants.
The authors use a hierarchical arrangement. No subdivisions are given. However, other studies have suggested; as of January 2011, the situation appears unresolved. Below are other published taxonomies of the red algae using molecular and traditional alpha taxonomic data. If one defines the kingdom Plantae to mean the Archaeplastida, the red algae will be part of that kingdom If Plantae are defined more narrowly, to be the Viridiplantae the red algae might be considered their own kingdom, or part of the kingdom Protista. A major research initiative to reconstruct the Red Algal Tree of Life using phylogenetic and genomic approaches is funded by the National Science Foundation as part of the Assembling the Tree of Life Program; some sources place all red algae into the class "Rhodophyceae". A subphylum - Proteorhodophytina - has been proposed to encompass the existing classes Compsopogonophyceae, Porphyridiophyceae and Stylonematophyceae; this proposal was made on the basis of the analysis of the plastid genomes.
Over 7,000 species are described for the red algae, but the taxonomy is in constant flux with new species described each year. The vast majority of these are marine with about 200; some examples of species and genera of red algae are: Cyanidioschyzon merolae, a primitive red alga Atractophora hypnoides Gelidiella calcicola Lemanea, a freshwater genus Palmaria palmata, dulse Schmitzia