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Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
community. They form a group of about 200 species, and belong either to the kingdom
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to a newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae). Coccolithophores are almost exclusively
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...
, are photosynthetic, and exist in large numbers throughout the sunlight zone of the
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
. Coccolithophores are the most productive
calcifying organism Marine biogenic calcification is the process by which marine organisms such as oysters and clams form calcium carbonate. Seawater is full of dissolved compounds, ions and nutrients that organisms can use for energy and, in the case of calcificati ...
s on the planet, covering themselves with a calcium carbonate shell called a ''
coccosphere Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the king ...
''. However, the reasons they
calcify Hard tissue, refers to "normal" calcified tissue, is the tissue which is mineralized and has a firm intercellular matrix. The hard tissues of humans are bone, tooth enamel, dentin, and cementum. The term is in contrast to soft tissue. Bone Bone ...
remains elusive. One key function may be that the coccosphere offers protection against
microzooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along b ...
predation, which is one of the main causes of phytoplankton death in the ocean. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Coccolithophores are ecologically important, and biogeochemically they play significant roles in the marine biological pump and the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
They are of particular interest to those studying global climate change because, as ocean acidity increases, their coccoliths may become even more important as a carbon sink. Management strategies are being employed to prevent
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
-related coccolithophore blooms, as these blooms lead to a decrease in nutrient flow to lower levels of the ocean. The most abundant species of coccolithophore, '' Emiliania huxleyi'', belongs to the order Isochrysidales and family
Noëlaerhabdaceae Noelaerhabdaceae is a family of coccolithophorids. Some genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification ...
. It is found in temperate, subtropical, and tropical oceans. This makes ''E. huxleyi'' an important part of the planktonic base of a large proportion of
marine food webs Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. Thi ...
. It is also the fastest growing coccolithophore in laboratory cultures. It is studied for the extensive blooms it forms in nutrient depleted waters after the reformation of the summer thermocline. and for its production of molecules known as alkenones that are commonly used by earth scientists as a means to estimate past sea surface temperatures..


Overview

Coccolithophores (or coccolithophorids, from the adjective) form a group of about 200 phytoplankton species. They belong either to the kingdom
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae).. Coccolithophores are distinguished by special calcium carbonate plates (or scales) of uncertain function called '' coccoliths'', which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths (e.g. in genus '' Prymnesium''), so not every member of Prymnesiophyceae is a coccolithophore. Coccolithophores are single-celled phytoplankton that produce small calcium carbonate (CaCO3) scales ( coccoliths) which cover the cell surface in the form of a spherical coating, called a
coccosphere Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the king ...
. They have been an integral part of marine plankton communities since the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
. Today, coccolithophores contribute ~1–10% to primary production in the surface ocean and ~50% to pelagic CaCO3 sediments. Their calcareous shell increases the sinking velocity of photosynthetically fixed into the deep ocean by ballasting organic matter. At the same time, the
biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...
precipitation of calcium carbonate during coccolith formation reduces the total alkalinity of seawater and releases . Thus, coccolithophores play an important role in the marine carbon cycle by influencing the efficiency of the biological carbon pump and the oceanic uptake of atmospheric . As of 2021, it is not known why coccolithophores calcify and how their ability to produce coccoliths is associated with their ecological success. The most plausible benefit of having a coccosphere seems to be a protection against predators or viruses. Viral infection is an important cause of phytoplankton death in the oceans, and it has recently been shown that calcification can influence the interaction between a coccolithophore and its virus. The major predators of marine phytoplankton are
microzooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along b ...
like
ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...
s and dinoflagellates. These are estimated to consume about two-thirds of the primary production in the ocean and microzooplankton can exert a strong grazing pressure on coccolithophore populations. Although calcification does not prevent predation, it has been argued that the coccosphere reduces the grazing efficiency by making it more difficult for the predator to utilise the organic content of coccolithophores. Heterotrophic
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
s are able to selectively choose prey on the basis of its size or shape and through chemical signals and may thus favor other prey that is available and not protected by coccoliths.


Structure

Coccolithophores are spherical cells about 5–100 micrometres across, enclosed by calcareous plates called coccoliths, which are about 2–25 micrometres across. Each cell contains two brown chloroplasts which surround the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
. Enclosed in each coccosphere is a single cell with membrane bound
organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
. Two large chloroplasts with brown pigment are located on either side of the cell and surround the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
, mitochondria,
golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...
, endoplasmic reticulum, and other organelles. Each cell also has two flagellar structures, which are involved not only in motility, but also in mitosis and formation of the
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is co ...
.. In some species, a functional or vestigial haptonema is also present. This structure, which is unique to haptophytes, coils and uncoils in response to environmental stimuli. Although poorly understood, it has been proposed to be involved in prey capture.


Ecology


Life history strategy

The life cycle of coccolithophores is characterized by an alternation of diploid and haploid phases. They alternate from the haploid to diploid phase through
syngamy Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
and from diploid to haploid through
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
. In contrast with most organisms with alternating life cycles, asexual reproduction by mitosis is possible in both phases of the life cycle. Both abiotic and
biotic factors An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
may affect the frequency with which each phase occurs. Coccolithophores
reproduce asexually Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the ...
through binary fission. In this process the coccoliths from the parent cell are divided between the two daughter cells. There have been suggestions stating the possible presence of a sexual reproduction process due to the diploid stages of the coccolithophores, but this process has never been observed. K or r- selected strategies of coccolithophores depend on their life cycle stage. When coccolithophores are diploid, they are r-selected. In this phase they tolerate a wider range of nutrient compositions. When they are haploid they are K- selected and are often more competitive in stable low nutrient environments. Most coccolithophores are K strategist and are usually found on nutrient-poor surface waters. They are poor competitors when compared to other phytoplankton and thrive in habitats where other phytoplankton would not survive. These two stages in the life cycle of coccolithophores occur seasonally, where more nutrition is available in warmer seasons and less is available in cooler seasons. This type of life cycle is known as a complex heteromorphic life cycle.


Global distribution

Coccolithophores occur throughout the world's oceans. Their distribution varies vertically by stratified layers in the ocean and geographically by different temporal zones.. While most modern coccolithophores can be located in their associated stratified oligotrophic conditions, the most abundant areas of coccolithophores where there is the highest species diversity are located in subtropical zones with a temperate climate. While water temperature and the amount of light intensity entering the water's surface are the more influential factors in determining where species are located, the ocean currents also can determine the location where certain species of coccolithophores are found. Although motility and colony formation vary according to the life cycle of different coccolithophore species, there is often alternation between a motile, haploid phase, and a non-motile diploid phase. In both phases, the organism's dispersal is largely due to ocean currents and circulation patterns. Within the Pacific Ocean, approximately 90 species have been identified with six separate zones relating to different Pacific currents that contain unique groupings of different species of coccolithophores. The highest diversity of coccolithophores in the Pacific Ocean was in an area of the ocean considered the Central North Zone which is an area between 30 oN and 5 oN, composed of the North Equatorial Current and the Equatorial Countercurrent. These two currents move in opposite directions, east and west, allowing for a strong mixing of waters and allowing a large variety of species to populate the area. In the Atlantic Ocean, the most abundant species are '' E. huxleyi'' and ''Florisphaera profunda'' with smaller concentrations of the species ''Umbellosphaera'' ''irregularis'', ''Umbellosphaera tenuis'' and different species of ''Gephyrocapsa''. Deep-dwelling coccolithophore species abundance is greatly affected by nutricline and thermocline depths. These coccolithophores increase in abundance when the nutricline and thermocline are deep and decrease when they are shallow. The complete distribution of coccolithophores is currently not known and some regions, such as the Indian Ocean, are not as well studied as other locations in the Pacific and Atlantic Oceans. It is also very hard to explain distributions due to multiple constantly changing factors involving the ocean's properties, such as coastal and equatorial upwelling, frontal systems, benthic environments, unique oceanic topography, and pockets of isolated high or low water temperatures. The upper photic zone is low in nutrient concentration, high in light intensity and penetration, and usually higher in temperature. The lower photic zone is high in nutrient concentration, low in light intensity and penetration and relatively cool. The middle photic zone is an area that contains the same values in between that of the lower and upper photic zones.


Great Calcite Belt

The Great Calcite Belt of the Southern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores, despite the region being known for its diatom predominance. The overlap of two major phytoplankton groups, coccolithophores and diatoms, in the dynamic frontal systems characteristic of this region provides an ideal setting to study environmental influences on the distribution of different species within these taxonomic groups. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
The Great Calcite Belt, defined as an elevated
particulate inorganic carbon Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is somet ...
(PIC) feature occurring alongside seasonally elevated chlorophyll a in austral spring and summer in the Southern Ocean, plays an important role in climate fluctuations, accounting for over 60% of the Southern Ocean area (30–60° S). The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO2) alongside the North Atlantic and North Pacific oceans.


Effect of global climate change on distribution

Recent studies show that climate change has direct and indirect impacts on Coccolithophore distribution and productivity. They will inevitably be affected by the increasing temperatures and thermal stratification of the top layer of the ocean, since these are prime controls on their ecology, although it is not clear whether global warming would result in net increase or decrease of coccolithophores. As they are calcifying organisms, it has been suggested that ocean acidification due to increasing carbon dioxide could severely affect coccolithophores. Recent increases have seen a sharp increase in the population of coccolithophores.


Role in the food web

Coccolithophores are one of the more abundant primary producers in the ocean. As such, they are a large contributor to the primary productivity of the tropical and subtropical oceans, however, exactly how much has yet to have been recorded.


Dependence on nutrients

The ratio between the concentrations 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 ...
,
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
and silicate in particular areas of the ocean dictates competitive dominance within phytoplankton communities. Each ratio essentially tips the odds in favor of either diatoms or other groups of phytoplankton, such as coccolithophores. A low silicate to nitrogen and phosphorus ratio allows coccolithophores to outcompete other phytoplankton species; however, when silicate to phosphorus to nitrogen ratios are high coccolithophores are outcompeted by diatoms. The increase in agricultural processes lead to
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
of waters and thus, coccolithophore blooms in these high nitrogen and phosphorus, low silicate environments.


Impact on water column productivity

The calcite in calcium carbonate allows coccoliths to scatter more light than they absorb. This has two important consequences: 1) Surface waters become brighter, meaning they have a higher albedo, and 2) there is induced photoinhibition, meaning photosythetic production is diminished due to an excess of light. In case 1), a high concentration of coccoliths leads to a simultaneous increase in surface water temperature and decrease in the temperature of deeper waters. This results in more stratification in the water column and a decrease in the vertical mixing of nutrients. However, a recent study estimated that the overall effect of coccolithophores on the increased in radiative forcing of the ocean is less than that from anthropogenic factors. Therefore, the overall result of large blooms of coccolithophores is a decrease in water column productivity, rather than a contribution to global warming.


Predator-prey interactions

Their predators include the common predators of all phytoplankton including small fish, zooplankton, and shellfish larvae. Viruses specific to this species have been isolated from several locations worldwide and appear to play a major role in spring bloom dynamics.


=Toxicity

= No environmental evidence of coccolithophore toxicity has been reported, but they belong to the class Prymnesiophyceae which contain orders with toxic species. Toxic species have been found in the genera ''Prymnesium'' Massart and ''Chrysochromulina'' Lackey. Members of the genus ''Prymnesium'' have been found to produce haemolytic compounds, the agent responsible for toxicity. Some of these toxic species are responsible for large fish kills and can be accumulated in organisms such as shellfish; transferring it through the food chain. In laboratory tests for toxicity members of the oceanic coccolithophore genera ''Emiliania, Gephyrocapsa, Calcidiscus'' and ''Coccolithus'' were shown to be non-toxic as were species of the coastal genus ''Hymenomonas'', however several species of ''Pleurochrysis'' and ''Jomonlithus'', both coastal genera were toxic to ''Artemia''.


Community interactions

Coccolithophorids are predominantly found as single, free-floating haploid or diploid cells.


Competition

Most phytoplankton need sunlight and nutrients from the ocean to survive, so they thrive in areas with large inputs of nutrient rich water upwelling from the lower levels of the ocean. Most coccolithophores require sunlight only for energy production, and have a higher ratio of nitrate uptake over ammonium uptake (nitrogen is required for growth and can be used directly from nitrate but not ammonium). Because of this they thrive in still, nutrient-poor environments where other phytoplankton are starving. Trade-offs associated with these faster growth rates include a smaller cell radius and lower cell volume than other types of phytoplankton.


Viral infection and coevolution

Giant DNA-containing viruses are known to lytically infect coccolithophores, particularly ''E. huxleyi''. These viruses, known as E. huxleyi viruses (EhVs), appear to infect the coccosphere coated diploid phase of the life cycle almost exclusively. It has been proposed that as the haploid organism is not infected and therefore not affected by the virus, the co-evolutionary " arms race" between coccolithophores and these viruses does not follow the classic Red Queen evolutionary framework, but instead a "Cheshire Cat" ecological dynamic. More recent work has suggested that viral synthesis of sphingolipids and induction of programmed cell death provides a more direct link to study a Red Queen-like coevolutionary arms race at least between the coccolithoviruses and diploid organism.


Evolution and diversity

Coccolithophores are members of the clade Haptophyta, which is a sister clade to Centrohelida, which are both in Haptista. The oldest known coccolithophores are known from the Late Triassic, around the Norian- Rhaetian boundary. Diversity steadily increased over the course of the Mesozoic, reaching its apex during the Late Cretaceous. However, there was a sharp drop during the Cretaceous-Paleogene extinction event, when more than 90% of coccolithophore species became extinct. Coccoliths reached another, lower apex of diversity during the Paleocene-Eocene thermal maximum, but have subsequently declined since the Oligocene due to decreasing global temperatures, with species that produced large and heavily calcified coccoliths most heavily affected.


Coccolithophore shells

* Exoskeleton: coccospheres and coccoliths Each coccolithophore encloses itself in a protective shell of coccoliths, calcified scales which make up its
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...
or coccosphere. The coccoliths are created inside the coccolithophore cell and while some species maintain a single layer throughout life only producing new coccoliths as the cell grows, others continually produce and shed coccoliths.


Composition

The primary constituent of coccoliths is calcium carbonate, or
chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
. Calcium carbonate is transparent, so the organisms' photosynthetic activity is not compromised by encapsulation in a coccosphere.


Formation

Coccoliths are produced by a
biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...
process known as coccolithogenesis. Generally, calcification of coccoliths occurs in the presence of light, and these scales are produced much more during the exponential phase of growth than the stationary phase. Although not yet entirely understood, the biomineralization process is tightly regulated by calcium signaling. Calcite formation begins in the golgi complex where protein templates nucleate the formation of CaCO3 crystals and complex acidic polysaccharides control the shape and growth of these crystals. As each scale is produced, it is exported in a Golgi-derived vesicle and added to the inner surface of the coccosphere. This means that the most recently produced coccoliths may lie beneath older coccoliths. Depending upon the phytoplankton's stage in the life cycle, two different types of coccoliths may be formed. Holococcoliths are produced only in the haploid phase, lack radial symmetry, and are composed of anywhere from hundreds to thousands of similar minute (ca 0.1 μm) rhombic calcite crystals. These crystals are thought to form at least partially outside the cell. Heterococcoliths occur only in the diploid phase, have radial symmetry, and are composed of relatively few complex crystal units (fewer than 100). Although they are rare, combination coccospheres, which contain both holococcoliths and heterococcoliths, have been observed in the plankton recording coccolithophore life cycle transitions. Finally, the coccospheres of some species are highly modified with various appendages made of specialized coccoliths.


Function

While the exact function of the coccosphere is unclear, many potential functions have been proposed. Most obviously coccoliths may protect the phytoplankton from predators. It also appears that it helps them to create a more stable pH. During photosynthesis carbon dioxide is removed from the water, making it more basic. Also calcification removes carbon dioxide, but chemistry behind it leads to the opposite pH reaction; it makes the water more acidic. The combination of photosynthesis and calcification therefore even out each other regarding pH changes. In addition, these exoskeletons may confer an advantage in energy production, as coccolithogenesis seems highly coupled with photosynthesis. Organic precipitation of calcium carbonate from bicarbonate solution produces free carbon dioxide directly within the cellular body of the alga, this additional source of gas is then available to the Coccolithophore for photosynthesis. It has been suggested that they may provide a cell-wall like barrier to isolate intracellular chemistry from the marine environment. More specific, defensive properties of coccoliths may include protection from osmotic changes, chemical or mechanical shock, and short-wavelength light. It has also been proposed that the added weight of multiple layers of coccoliths allows the organism to sink to lower, more nutrient rich layers of the water and conversely, that coccoliths add buoyancy, stopping the cell from sinking to dangerous depths. Coccolith appendages have also been proposed to serve several functions, such as inhibiting grazing by zooplankton.


Uses

Coccoliths are the main component of the Chalk, a Late Cretaceous rock formation which outcrops widely in southern England and forms the White Cliffs of Dover, and of other similar rocks in many other parts of the world. At the present day sedimented coccoliths are a major component of the calcareous oozes that cover up to 35% of the ocean floor and is kilometres thick in places. Because of their abundance and wide geographic ranges, the coccoliths which make up the layers of this ooze and the chalky sediment formed as it is compacted serve as valuable microfossils.
Calcification Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature M ...
, the biological production of calcium carbonate (CaCO3), is a key process in the marine carbon cycle. Coccolithophores are the major planktonic group responsible for pelagic CaCO3 production. The diagram on the right shows the energetic costs of coccolithophore calcification: : (A) Transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca2+ and HCO3 (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca2+ through the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
to the CV is the dominant cost associated with calcification. : (B) Metabolic processes include the synthesis of CAPs (gray rectangles) by the Golgi complex (white rectangles) that regulate the nucleation and geometry of CaCO3 crystals. The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO3 crystals. : (C) Mechanical and structural processes account for the secretion of the completed coccoliths that are transported from their original position adjacent to the nucleus to the cell periphery, where they are transferred to the surface of the cell. The costs associated with these processes are likely to be comparable to organic-scale exocytosis in noncalcifying haptophyte algae. The diagram on the left shows the benefits of coccolithophore calcification. (A) Accelerated photosynthesis includes CCM (1) and enhanced light uptake via scattering of scarce photons for deep-dwelling species (2). (B) Protection from photodamage includes sunshade protection from ultraviolet (UV) light and photosynthetic active radiation (PAR) (1) and energy dissipation under high-light conditions (2). (C) Armor protection includes protection against viral/bacterial infections (1) and grazing by selective (2) and nonselective (3) grazers. The degree by which calcification can adapt to ocean acidification is presently unknown. Cell physiological examinations found the essential H+ efflux (stemming from the use of HCO3− for intra-cellular calcification) to become more costly with ongoing ocean acidification as the electrochemical H+ inside-out gradient is reduced and passive proton outflow impeded. Adapted cells would have to activate
proton channel A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction: : n one side of a biological membrane/sub> + energy n the other side of the membr ...
s more frequently, adjust their membrane potential, and/or lower their internal pH. Reduced intra-cellular pH would severely affect the entire cellular machinery and require other processes (e.g.
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 in ...
) to co-adapt in order to keep H+ efflux alive. The obligatory H+ efflux associated with calcification may therefore pose a fundamental constraint on adaptation which may potentially explain why "calcification crisis" were possible during long-lasting (thousands of years) CO2 perturbation events even though evolutionary adaption to changing
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
chemistry conditions is possible within one year. Unraveling these fundamental constraints and the limits of adaptation should be a focus in future coccolithophore studies because knowing them is the key information required to understand to what extent the calcification response to carbonate chemistry perturbations can be compensated by evolution. Silicate- or cellulose-armored functional groups such as diatoms and dinoflagellates do not need to sustain the calcification-related H+ efflux. Thus, they probably do not need to adapt in order to keep costs for the production of structural elements low. On the contrary, dinoflagellates (except for calcifying species; with generally inefficient CO2-fixing RuBisCO enzymes may even profit from chemical changes since photosynthetic carbon fixation as their source of structural elements in the form of cellulose should be facilitated by the ocean acidification-associated CO2 fertilization. Under the assumption that any form of shell/exoskeleton protects phytoplankton against predation non-calcareous armors may be the preferable solution to realize protection in a future ocean. The diagram on the right is a representation of how the comparative energetic effort for armor construction in diatoms, dinoflagellates and coccolithophores appear to operate. The frustule (diatom shell) seems to be the most inexpensive armor under all circumstances because diatoms typically outcompete all other groups when silicate is available. The coccosphere is relatively inexpensive under sufficient O2 high CO3 and low +because the substrate is saturating and protons are easily released into seawater. In contrast, the construction of thecal elements, which are organic (
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
) plates that constitute the dinoflagellate shell, should rather be favored at high H+ concentrations because these usually coincide with high O2 Under these conditions dinoflagellates could down-regulate the energy-consuming operation of carbon concentrating mechanisms to fuel the production of organic source material for their shell. Therefore, a shift in carbonate chemistry conditions toward high O2may promote their competitiveness relative to coccolithophores. However, such a hypothetical gain in competitiveness due to altered carbonate chemistry conditions would not automatically lead to dinoflagellate dominance because a huge number of factors other than carbonate chemistry have an influence on species composition as well. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
*


Defence against predation

Currently, the evidence supporting or refuting a protective function of the coccosphere against predation is limited. Some researchers found that overall microzooplankton predation rates were reduced during blooms of the coccolithophore '' Emiliania huxleyi'', while others found high microzooplankton grazing rates on natural coccolithophore communities. In 2020, researchers found that ''in situ'' ingestion rates of microzooplankton on ''E. huxleyi'' did not differ significantly from those on similar sized non-calcifying phytoplankton. In laboratory experiments the heterotrophic dinoflagellate '' Oxyrrhis marina'' preferred calcified over non-calcified cells of ''E. huxleyi'', which was hypothesised to be due to size selective feeding behaviour, since calcified cells are larger than non-calcified ''E. huxleyi''. In 2015, Harvey et al. investigated predation by the dinoflagellate ''O. marina'' on different genotypes of non-calcifying ''E. huxleyi'' as well as calcified strains that differed in the degree of calcification. They found that the ingestion rate of ''O. marina'' was dependent on the genotype of ''E. huxleyi'' that was offered, rather than on their degree of calcification. In the same study, however, the authors found that predators which preyed on non-calcifying genotypes grew faster than those fed with calcified cells. In 2018, Strom et al. compared predation rates of the dinoflagellate '' Amphidinium longum'' on calcified relative to naked ''E. huxleyi'' prey and found no evidence that the coccosphere prevents ingestion by the grazer. Instead, ingestion rates were dependent on the offered genotype of E. huxleyi. Altogether, these two studies suggest that the genotype has a strong influence on ingestion by the microzooplankton species, but if and how calcification protects coccolithophores from microzooplankton predation could not be fully clarified.


Importance in global climate change


Impact on the carbon cycle

Coccolithophores have both long and short term effects on the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
. The production of coccoliths requires the uptake of dissolved inorganic carbon and calcium. Calcium carbonate and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
are produced from calcium and
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemi ...
by the following chemical reaction: :Ca2+ + 2HCO3 CaCO3 + CO2 + H2O. Because coccolithophores are photosynthetic organisms, they are able to use some of the CO2 released in the calcification reaction for
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 in ...
. However, the production of calcium carbonate drives surface alkalinity down, and in conditions of low alkalinity the CO2 is instead released back into the atmosphere. As a result of this, researchers have postulated that large blooms of coccolithophores may contribute to global warming in the short term. A more widely accepted idea, however, is that over the long term coccolithophores contribute to an overall decrease in atmospheric CO2 concentrations. During calcification two carbon atoms are taken up and one of them becomes trapped as calcium carbonate. This calcium carbonate sinks to the bottom of the ocean in the form of coccoliths and becomes part of sediment; thus, coccolithophores provide a sink for emitted carbon, mediating the effects of greenhouse gas emissions.


Evolutionary responses to ocean acidification

Research also suggests that ocean acidification due to increasing concentrations of CO2 in the atmosphere may affect the calcification machinery of coccolithophores. This may not only affect immediate events such as increases in population or coccolith production, but also may induce
evolutionary adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
of coccolithophore species over longer periods of time. For example, coccolithophores use H+ ion channels in to constantly pump H+ ions out of the cell during coccolith production. This allows them to avoid
acidosis Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma. The term ''acidemia'' describe ...
, as coccolith production would otherwise produce a toxic excess of H+ ions. When the function of these ion channels is disrupted, the coccolithophores stop the calcification process to avoid acidosis, thus forming a feedback loop. Low ocean alkalinity, impairs ion channel function and therefore places evolutionary selective pressure on coccolithophores and makes them (and other ocean calcifiers) vulnerable to ocean acidification. In 2008, field evidence indicating an increase in calcification of newly formed ocean sediments containing coccolithophores bolstered the first ever experimental data showing that an increase in ocean CO2 concentration results in an increase in calcification of these organisms. Decreasing coccolith mass is related to both the increasing concentrations of CO2 and decreasing concentrations of CO32– in the world's oceans. This lower calcification is assumed to put coccolithophores at ecological disadvantage. Some species like ''Calcidiscus'' ''leptoporus'', however, are not affected in this way, while the most abundant coccolithophore species, ''E. huxleyi'' might be (study results are mixed). Also, highly calcified coccolithophorids have been found in conditions of low CaCO3 saturation contrary to predictions. Understanding the effects of increasing ocean acidification on coccolithophore species is absolutely essential to predicting the future chemical composition of the ocean, particularly its carbonate chemistry. Viable conservation and management measures will come from future research in this area. Groups like the European-based CALMARO are monitoring the responses of coccolithophore populations to varying pH's and working to determine environmentally sound measures of control. File:Gephyrocapsa oceanica.jpg, ''
Gephyrocapsa oceanica ''Gephyrocapsa oceanica'' is a species of coccolithophorid. It is the type species of the genus ''Gephyrocapsa''. The species is an important Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch t ...
''
Scale bar = 1.0 μm File:Diversity of coccolithophores (cropped) (Rhabdosphaera clavigera).jpg, '' Rhabdosphaera clavigera'' File:Diversity of coccolithophores (cropped).(Discosphaera tubifera).jpg, ''
Discosphaera tubifera ''Discosphaera tubifer'' (original name: ''Discosphaera tubifera'') is a marine, unicellular species of coccolithophore in the genus Discosphaera. It exhibits a very delicate structure and arrangement of coccoliths Coccoliths are individual ...
''


Impact on microfossil record

Coccolith fossils are prominent and valuable
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...
microfossils. They are the largest global source of biogenic calcium carbonate, and significantly contribute to the global
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
. They are the main constituent of chalk deposits such as the white cliffs of Dover. Of particular interest are fossils dating back to the Palaeocene-Eocene Thermal Maximum 55 million years ago. This period is thought to correspond most directly to the current levels of CO2 in the ocean. Finally, field evidence of coccolithophore fossils in rock were used to show that the deep-sea fossil record bears a rock record bias similar to the one that is widely accepted to affect the land-based fossil record.


Impact on the oceans

The coccolithophorids help in regulating the temperature of the oceans. They thrive in warm seas and release dimethyl sulfide (DMS) into the air whose nuclei help to produce thicker clouds to block the sun. When the oceans cool, the number of coccolithophorids decrease and the amount of clouds also decrease. When there are fewer clouds blocking the sun, the temperature also rises. This, therefore, maintains the balance and equilibrium of nature.


See also

* CLAW hypothesis * Dimethyl sulfide * Dimethylsulfoniopropionate * ''Emiliania huxleyi'' virus 86 * ''
Pleurochrysis carterae ''Pleurochrysis carterae'' is a marine species of unicellular coccolithophorid algae that has the ability to calcify subcellularly. They produce calcified scales, known as coccoliths, which are deposited on the surface of the cell resulting in t ...
''


References


External links

Sources of detailed information
Nannotax3
– illustrated guide to the taxonomy of coccolithophores and other nannofossils.
INA — International Nannoplankton Association''Emiliania huxleyi'' Home Page
Introductions to coccolithophores

* ttp://www.paleoportal.org/index.php?globalnav=fossil_gallery§ionnav=taxon&taxon_id=104 The Paleontology Portal: Calcareous Nanoplanktonbr>RadioLab
– podcast on coccolithophores {{Taxonbar, from=Q1647990 Haptophytes Microfossils Extant Late Triassic first appearances Planktology Sedimentology de:Haptophyta