Coccoliths are individual plates or scales of calcium carbonate formed by

coccolithophore 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 kingdom ...

s (single-celled

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. P ...

such as ''

Emiliania huxleyi ''Emiliania huxleyi'' is a species of coccolithophore found in almost all ocean ecosystems from the equator to sub-polar regions, and from nutrient rich upwelling zones to nutrient poor oligotrophic waters. It is one of thousands of different ...

'') and cover the cell surface arranged in the form of a spherical shell, called a '' coccosphere''.

Overview

Coccolithophores are spherical cells about 5–100 micrometres across, enclosed by calcareous plates called

coccoliths Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere''. ...

, which are about 2–25 micrometres across. Coccolithophores are an important group of about 200 marine phytoplankton species which cover themselves with a calcium carbonate shell called a "coccosphere". They are ecologically and biogeochemically important but the reason why 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 predation, which is one of the main causes of phytoplankton death in the ocean. CC-BY icon

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File:Cross section of a coccolithophore with coccolith layer.png, Partial cross section of a coccolithophore with coccolith layer File:Diagram of a coccolithophore cell and its shield of coccoliths.png, Coccolithophore cell surrounded by its shield of coccoliths. The coccolith-bearing cell is called the coccosphere. Coccolithophores have been an integral part of marine plankton communities since the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), 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 J ...

. Today, coccolithophores contribute ~1–10% to

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

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 Track ballast forms the trackbed upon which railroad ties (sleepers) are laid. It is packed between, below, and around the ties. It is used to bear the load from the railroad ties, to facilitate drainage of water, and also to keep down vegetat ...

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 The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...

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 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

dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

s. 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 A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

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.

Formation and composition

Coccoliths are formed within the cell in

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features ...

s derived from the

golgi body 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 in ...

. When the coccolith is complete these vesicles fuse with the cell wall and the coccolith is exocytosed and incorporated in the coccosphere. The coccoliths are either dispersed following death and breakup of the coccosphere, or are shed continually by some species. They sink through the water column to form an important part of the deep-sea

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

s (depending on the water depth).

Thomas Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...

was the first person to observe these forms in modern marine sediments and he gave them the name 'coccoliths' in a report published in 1858. Coccoliths are composed of calcium carbonate as the mineral

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scra ...

and are the main constituent of

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. Cha ...

deposits such as the

white cliffs of Dover The White Cliffs of Dover is the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, deposi ...

(deposited in

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

times), in which they were first described by Henry Clifton Sorby in 1861. File:Collapsed coccosphere of Pleurochrysis carterae.jpg, Collapsed coccosphere of ''

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 th ...

Types

There are two main types of coccoliths, heterococcoliths and holococcoliths. Heterococcoliths are formed of a radial array of elaborately shaped crystal units. Holococcoliths are formed of minute (~0.1 micrometre) calcite rhombohedra, arranged in continuous arrays. The two coccolith types were originally thought to be produced by different families of coccolithophores. Now, however, it is known through a mix of observations on field samples and laboratory cultures, that the two coccolith types are produced by the same species but at different life cycle phases. Heterococcoliths are produced in the diploid life-cycle phase and holococcoliths in the haploid phase. Both in field samples and laboratory cultures, there is the possibility of observing a cell covered by a combination of heterococcoliths and holococcoliths. This indicates the transition from the diploid to the haploid phase of the species. Such combination of coccoliths has been observed in field samples, with many of them coming from the Mediterranean. Cocolithtypes

Shape

Coccoliths are also classified depending on shape. Common shapes include: * Calyptrolith – basket-shaped with openings near the base * Caneolith – disc- or bowl-shaped * Ceratolith – horseshoe or wishbone shaped * Cribrilith – disc-shaped, with numerous perforations in the central area * Cyrtolith – convex disc shaped, may with a projecting central process * Discolith – ellipsoidal with a raised rim, in some cases the high rim forms a vase or cup-like structure * Helicolith – a placolith with a spiral margin * Lopadolith – basket or cup-shaped with a high rim, opening distally * Pentalith – pentagonal shape composed of five four-sided crystals * Placolith – rim composed of two plates stacked on top of one another * Prismatolith – polygonal, may have perforations * Rhabdolith – a single plate with a club-shaped central process * Scapholith – rhombohedral, with parallel lines in center File:Coccolith structures of representative Noelaerhabdaceae.jpg, Coccolith structures of representative Noelaerhabdaceae. CC-BY icon

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Each morphospecies is associated with a SEM image in the next diagram File:Images of representative Noelaerhabdaceae and other coccolithophores.jpg, SEM images correspond to coccolith drawings in the previous diagram
(A) ''

Gephyrocapsa ''Gephyrocapsa'' is a genus of haptophyte The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for ''Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used ins ...

ericsonii'' RCC4032 (B) ''Gephyrocapsa muellerae'' (C) '' Gephyrocapsa oceanica'' (D) '' Reticulofenestra parvular'' RCC4033; (E) Reticulofenestra parvular RCC4034; (F) Reticulofenestra parvular RCC4035; (G) Reticulofenestra parvular RCC4036; (H) ''

Emiliania huxleyi ''Emiliania huxleyi'' is a species of coccolithophore found in almost all ocean ecosystems from the e