| Coccolithophore Paraphyletic grouping of algae | |
|---|---|
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| Coccolithus pelagicus | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Haptista |
| Division: | Haptophyta |
| Class: | Prymnesiophyceae |
| Groups included | |
Coccolithophores, orcoccolithophorids, aresingle-celled organisms which are part of thephytoplankton, theautotrophic (self-feeding) component of theplankton community. They form a group of about 200 species, and belong either to the kingdomProtista, according toRobert Whittaker'sfive-kingdom system, or cladeHacrobia, according to a newer biological classification system. Within the Hacrobia, the coccolithophores are in thephylum ordivisionHaptophyta, classPrymnesiophyceae (orCoccolithophyceae). Coccolithophores are almost exclusivelymarine, arephotosynthetic andmixotrophic, and exist in large numbers throughout thesunlight zone of theocean.
Coccolithophores are the most productivecalcifying organisms on the planet, covering themselves with acalcium carbonate shell called acoccosphere. It is formed when a cell arrange a series of interlockingcoccoliths to completely cover its surface.[1] Coccoliths (minute calcium carbonate platelets) are generated within the cell, suggesting that the cell diameter cannot be exceeded by the maximum coccolith size.[2] In addition, coccolith size and the orientation of calcite crystals forming coccoliths can vary among species. Although the interlocking structure of coccoliths offers strong mechanical protection for cell, the specific topology of the coccoliths (the way they interconnect within the coccosphere) and the mechanisms governing their arrangement remain unclear.[3] One key function may be that the coccosphere offers protection againstmicrozooplankton predation, which is one of the main causes of phytoplankton death in the ocean.[4]
Coccolithophores are ecologically important, and biogeochemically they play significant roles in the marinebiological pump and thecarbon cycle.[5][4] Depending on habitat, they can produce up to 40 percent of the localmarine primary production.[6] They are of particular interest to those studying globalclimate change because, asocean acidity increases, their coccoliths may become even more important as acarbon sink.[7] Management strategies are being employed to preventeutrophication-related coccolithophore blooms, as these blooms lead to a decrease in nutrient flow to lower levels of the ocean.[8]
The most abundant species of coccolithophore,Emiliania huxleyi, belongs to the orderIsochrysidales and familyNoelaerhabdaceae.[9] It is found intemperate,subtropical, andtropical oceans.[10] This makesE. huxleyi an important part of theplanktonic base of a large proportion ofmarine food webs. It is also the fastest growing coccolithophore in laboratory cultures.[11] It is studied for the extensiveblooms it forms in nutrient depleted waters after the reformation of the summerthermocline.[12][13] and for its production of molecules known asalkenones that are commonly used byearth scientists as a means to estimate pastsea surface temperatures.[14]
Coccolithophores (or coccolithophorids, from the adjective[15]) form a group of about 200 phytoplankton species.[16] They belong either to the kingdomProtista, according toRobert Whittaker'sFive kingdom classification, or cladeHacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophores are in thephylum ordivisionHaptophyta, classPrymnesiophyceae (orCoccolithophyceae).[9] Coccolithophores are distinguished by specialcalcium carbonate plates (or scales) of uncertain function calledcoccoliths, which are also importantmicrofossils. However, there are Prymnesiophyceae species lacking coccoliths (e.g. in genusPrymnesium), so not every member of Prymnesiophyceae is a coccolithophore.[17]
Coccolithophores are single-celledphytoplankton that produce smallcalcium carbonate (CaCO3) scales (coccoliths) which cover the cell surface in the form of a spherical coating, called acoccosphere. Many species are alsomixotrophs, and are able to photosynthesise as well as ingest prey.[18]
Coccolithophores have been an integral part ofmarine plankton communities since theJurassic.[19][20] Today, coccolithophores contribute ~1–10% to inorganic carbon fixation (calcification) to total carbon fixation (calcification plus photosynthesis) in the surface ocean[21] and ~50% to pelagic CaCO3 sediments.[22] Their calcareous shell increases the sinking velocity of photosynthetically fixed CO2 into the deep ocean byballasting organic matter.[23][24] At the same time, thebiogenic precipitation of calcium carbonate during coccolith formation reduces the total alkalinity of seawater and releases CO2.[25][26] Thus, coccolithophores play an important role in themarine carbon cycle by influencing the efficiency of thebiological carbon pump and the oceanic uptake of atmospheric CO2.[4]
As of 2021, it is not known why coccolithophores calcify and how their ability to produce coccoliths is associated with their ecological success.[27][28][29][30][31] The most plausible benefit of having a coccosphere seems to be a protection against predators or viruses.[32][30] Viral infection is an important cause of phytoplankton death in the oceans,[33] and it has recently been shown that calcification can influence the interaction between a coccolithophore and its virus.[34][35] The major predators of marine phytoplankton aremicrozooplankton likeciliates anddinoflagellates. These are estimated to consume about two-thirds of the primary production in the ocean[36] and microzooplankton can exert a strong grazing pressure on coccolithophore populations.[37] 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.[38]Heterotrophicprotists are able to selectively choose prey on the basis of its size or shape and through chemical signals[39][40] and may thus favor other prey that is available and not protected by coccoliths.[4]
Coccolithophores are spherical cells about 5–100 micrometres across, enclosed by calcareous plates calledcoccoliths, which are about 2–25 micrometres across. Each cell contains two brownchloroplasts which surround thenucleus.[43]
Enclosed in each coccosphere is a single cell withmembrane boundorganelles. Two largechloroplasts with brownpigment are located on either side of the cell and surround thenucleus,mitochondria,golgi apparatus,endoplasmic reticulum, and other organelles. Each cell also has twoflagellar structures, which are involved not only in motility, but also inmitosis and formation of thecytoskeleton.[44] In some species, a functional orvestigialhaptonema is also present.[45] This structure, which is unique tohaptophytes, coils and uncoils in response to environmental stimuli. Although poorly understood, it has been proposed to be involved in prey capture.[44]
The complex life cycle of coccolithophores is known as ahaplodiplontic life cycle, and is characterized by an alternation of both asexual and sexual phases. The asexual phase is known as thehaploid phase, while the sexual phase is known as thediploid phase. During the haploid phase, coccolithophores produce haploid cells throughmitosis. These haploid cells can then divide further through mitosis or undergo sexual reproduction with other haploid cells. The resulting diploid cell goes throughmeiosis to produce haploid cells again, starting the cycle over. With coccolithophores, asexual reproduction by mitosis is possible in both phases of the life cycle, which is a contrast with most other organisms that have alternating life cycles.[46] Bothabiotic andbiotic factors may affect the frequency with which each phase occurs.[47]
Coccolithophoresreproduce asexually throughbinary 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.[48]
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.[48] 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.[49] 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.[48]
Coccolithophores occur throughout the world's oceans. Their distribution varies vertically by stratified layers in the ocean and geographically by different temporal zones.[50] While most modern coccolithophores can be located in their associated stratifiedoligotrophic conditions, the most abundant areas of coccolithophores where there is the highest species diversity are located in subtropical zones with a temperate climate.[51] 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.[52]
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 oceancurrents and circulation patterns.[53]
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.[54] 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 30oN and 5oN, 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.[54]
In the Atlantic Ocean, the most abundant species areE. huxleyi andFlorisphaera profunda with smaller concentrations of the speciesUmbellosphaerairregularis,Umbellosphaera tenuis and different species ofGephyrocapsa.[54] Deep-dwelling coccolithophorespecies abundance is greatly affected bynutricline andthermocline depths. These coccolithophores increase in abundance when the nutricline and thermocline are deep and decrease when they are shallow.[55]
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 equatorialupwelling, frontal systems,benthic environments, unique oceanic topography, and pockets of isolated high or low water temperatures.[57]
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.[51]
TheGreat Calcite Belt of theSouthern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores, despite the region being known for itsdiatom 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 environmentalinfluences on the distribution of different species within these taxonomic groups.[60]
The Great Calcite Belt, defined as an elevatedparticulate inorganic carbon (PIC) feature occurring alongside seasonally elevatedchlorophyll a inaustral spring and summer in the Southern Ocean,[61] plays an important role in climate fluctuations,[62][63] accounting for over 60% of the Southern Ocean area (30–60° S).[64] The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO2) alongside the North Atlantic and North Pacific oceans.[65]
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 thatocean acidification due to increasing carbon dioxide could severely affect coccolithophores.[55] Recent CO2 increases have seen a sharp increase in the population of coccolithophores.[66]
Coccolithophores are one of the more abundant primary producers in the ocean. As such, they are a large contributor to theprimary productivity of the tropical and subtropical oceans, however, exactly how much has yet to have been recorded.[70]
The ratio between the concentrations ofnitrogen,phosphorus andsilicate in particular areas of the ocean dictatescompetitive dominance within phytoplankton communities. Each ratio essentially tips the odds in favor of eitherdiatoms 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 toeutrophication of waters and thus, coccolithophore blooms in these high nitrogen and phosphorus, low silicate environments.[8]
Thecalcite 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 higheralbedo, and 2) there is inducedphotoinhibition, 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 morestratification in the water column and a decrease in the vertical mixing of nutrients. However, a 2012 study estimated that the overall effect of coccolithophores on the increase inradiative forcing of the ocean is less than that from anthropogenic factors.[71] Therefore, the overall result of large blooms of coccolithophores is a decrease in water column productivity, rather than a contribution to global warming.
Their predators include the common predators of all phytoplankton including small fish, zooplankton, and shellfish larvae.[49][72] Viruses specific to this species have been isolated from several locations worldwide and appear to play a major role in spring bloom dynamics.
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 generaPrymnesium Massart andChrysochromulina Lackey. Members of the genusPrymnesium 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 generaEmiliania, Gephyrocapsa, Calcidiscus andCoccolithus were shown to be non-toxic as were species of the coastal genusHymenomonas, however several species ofPleurochrysis andJomonlithus, both coastal genera were toxic toArtemia.[72]
Coccolithophorids are predominantly found as single, free-floating haploid or diploid cells.[50]
Mostphytoplankton 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.[73]Trade-offs associated with these faster growth rates include a smaller cell radius and lower cell volume than other types of phytoplankton.
GiantDNA-containing viruses are known tolytically infect coccolithophores, particularlyE. 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 classicRed Queen evolutionary framework, but instead a "Cheshire Cat" ecological dynamic.[74] More recent work has suggested that viral synthesis ofsphingolipids and induction ofprogrammed cell death provides a more direct link to study a Red Queen-likecoevolutionary arms race at least between the coccolithoviruses and diploid organism.[47]
Coccolithophores are members of the cladeHaptophyta, which is a sister clade toCentrohelida, which are both inHaptista.[75] The oldest known coccolithophores are known from the Late Triassic, around theNorian-Rhaetian boundary.[76] Diversity steadily increased over the course of the Mesozoic, reaching its apex during theLate Cretaceous. However, there was a sharp drop during theCretaceous-Paleogene extinction event, when more than 90% of coccolithophore species became extinct. Coccoliths reached another, lower apex of diversity during thePaleocene-Eocene thermal maximum, but have subsequently declined since theOligocene due to decreasing global temperatures, with species that produced large and heavily calcified coccoliths most heavily affected.[30]
| Part of a series related to |
| Biomineralization |
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Teeth, scales, tusks etc |
Each coccolithophore encloses itself in a protective shell ofcoccoliths, calcified scales which make up itsexoskeleton orcoccosphere.[77] 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.
The primary constituent of coccoliths is calcium carbonate, orchalk. Calcium carbonate is transparent, so the organisms' photosynthetic activity is not compromised by encapsulation in a coccosphere.[49]
Coccoliths are produced by abiomineralization process known as coccolithogenesis.[43] 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.[78] Although not yet entirely understood, the biomineralization process is tightly regulated bycalcium signaling.Calcite formation begins in thegolgi complex where protein templates nucleate the formation of CaCO3 crystals and complex acidicpolysaccharides control the shape and growth of these crystals.[53] As each scale is produced, it is exported in a Golgi-derivedvesicle and added to the inner surface of the coccosphere. This means that the most recently produced coccoliths may lie beneath older coccoliths.[46]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) rhombiccalcite 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.[57]
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 stablepH. 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.[79] 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.[80] More specific, defensive properties of coccoliths may include protection from osmotic changes, chemical or mechanical shock, and short-wavelength light.[45] 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.[81] Coccolith appendages have also been proposed to serve several functions, such as inhibiting grazing by zooplankton.[57]
Coccoliths are the main component ofthe Chalk, a Late Cretaceous rock formation which outcrops widely in southern England and forms theWhite Cliffs of Dover, and of other similar rocks in many other parts of the world.[13] At the present day sedimented coccoliths are a major component of thecalcareous oozes that cover up to 35% of the ocean floor and is kilometres thick in places.[53] 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 valuablemicrofossils.
Calcification, the biological production ofcalcium carbonate (CaCO3), is a key process in themarine carbon cycle. Coccolithophores are the major planktonic group responsible for pelagic CaCO3 production.[82][83] The diagram on the right shows the energetic costs of coccolithophore calcification:
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.[30]
The degree by which calcification can adapt toocean acidification is presently unknown. Cell physiological examinations found the essentialH+ 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.[84] Adapted cells would have to activateproton channels more frequently, adjust theirmembrane potential, and/or lower their internalpH.[85] Reduced intra-cellular pH would severely affect the entire cellular machinery and require other processes (e.g.photosynthesis) to co-adapt in order to keep H+ efflux alive.[86][87] 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[88][89] even though evolutionary adaption to changingcarbonate chemistry conditions is possible within one year.[88][89] 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.[90]
Silicate- or cellulose-armored functional groups such asdiatoms anddinoflagellates 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;[91] with generally inefficient CO2-fixingRuBisCO enzymes[92] may even profit from chemical changes since photosyntheticcarbon fixation as their source of structural elements in the form of cellulose should be facilitated by the ocean acidification-associated CO2 fertilization.[93][94] Under the assumption that any form of shell/exoskeleton protects phytoplankton against predation[32] non-calcareous armors may be the preferable solution to realize protection in a future ocean.[90]
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. Thefrustule (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 [CO2], high [HCO3−], and low [H+] because the substrate is saturating and protons are easily released into seawater.[84] In contrast, the construction ofthecal elements, which are organic (cellulose) plates that constitute the dinoflagellate shell, should rather be favored at high H+ concentrations because these usually coincide with high [CO2]. 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 [CO2] may 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 onspecies composition as well.[90][95]
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 coccolithophoreEmiliania huxleyi,[96][97] while others found high microzooplankton grazing rates on natural coccolithophore communities.[98] In 2020, researchers found thatin situ ingestion rates of microzooplankton onE. huxleyi did not differ significantly from those on similar sized non-calcifying phytoplankton.[99] In laboratory experiments the heterotrophic dinoflagellateOxyrrhis marina preferred calcified over non-calcified cells ofE. huxleyi, which was hypothesised to be due to size selective feeding behaviour, since calcified cells are larger than non-calcifiedE. huxleyi.[100] In 2015, Harvey et al. investigated predation by the dinoflagellateO. marina on different genotypes of non-calcifyingE. huxleyi as well as calcified strains that differed in the degree of calcification.[101] They found that the ingestion rate ofO. marina was dependent on the genotype ofE. 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-calcifyinggenotypes grew faster than those fed with calcified cells.[101] In 2018, Strom et al. compared predation rates of the dinoflagellateAmphidinium longum on calcified relative to nakedE. huxleyi prey and found no evidence that the coccosphere prevents ingestion by the grazer.[102] Instead, ingestion rates were dependent on the offered genotype of E. huxleyi.[102] 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.[4]
| Part of a series on |
| Plankton |
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Coccolithophores have both long and short term effects on thecarbon cycle. The production of coccoliths requires the uptake ofdissolved inorganic carbon and calcium.Calcium carbonate andcarbon dioxide are produced from calcium andbicarbonate by the following chemical reaction:[103]
Because coccolithophores are photosynthetic organisms, they are able to use some of the CO2 released in the calcification reaction forphotosynthesis.[104]
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.[105]As a result of this, researchers have postulated that large blooms of coccolithophores may contribute to global warming in the short term.[106] 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.[106]
Research also suggests thatocean 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 induceevolutionary adaptation 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 avoidacidosis, 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 afeedback loop.[107] Low oceanalkalinity, impairs ion channel function and therefore places evolutionary selective pressure on coccolithophores and makes them (and other ocean calcifiers) vulnerable to ocean acidification.[108] 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 ofCO2−3 in the world's oceans. This lower calcification is assumed to put coccolithophores at ecological disadvantage. Some species likeCalcidiscusleptoporus, however, are not affected in this way, while the most abundant coccolithophore species,E. huxleyi might be (study results are mixed).[107][109] Also, highly calcified coccolithophorids have been found in conditions of low CaCO3 saturation contrary to predictions.[7] 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-basedCALMARO[110] are monitoring the responses of coccolithophore populations to varying pH's and working to determine environmentally sound measures of control.
_(Rhabdosphaera_clavigera).jpg)
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Coccolith fossils are prominent and valuablecalcareousmicrofossils. They are the largest global source of biogenic calcium carbonate, and significantly contribute to the globalcarbon cycle.[111] They are the main constituent of chalk deposits such as thewhite cliffs of Dover.
Of particular interest are fossils dating back to thePalaeocene-Eocene Thermal Maximum 55 million years ago. This period is thought to correspond most directly to the current levels of CO2 in the ocean.[112] Finally, field evidence of coccolithophore fossils in rock were used to show that the deep-sea fossil record bears arock record bias similar to the one that is widely accepted to affect the land-basedfossil record.[113]
The coccolithophorids help in regulating the temperature of the oceans. They thrive in warm seas and releasedimethyl sulfide (DMS) into the air whosenuclei help to produce thicker clouds to block the sun.[114] 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.[115][116]
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Introductions to coccolithophores
