Coccoliths are individual plates or scales of

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

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 Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

such as ''

Emiliania huxleyi ''Gephyrocapsa huxleyi'', also called ''Emiliania huxleyi'', is the most abundant species of coccolithophore in modern oceans found in almost all ecosystems from the equator to sub-polar regions, and from nutrient rich upwelling zones to nutr ...

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

Overview

Coccoliths, which are about 2-25 micrometres across, enclose coccolithophores, which are spherical cells about 5-100 micrometres across. Coccolithophores are an important group of about 200 marine phytoplankton species which cover themselves with a

shell called a "coccosphere". They are ecologically and biogeochemically important but the reason why they calcify 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. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
File:Cross section of a coccolithophore with coccolith layer.png, Partial cross section of a coccolithophore with coccolith layer, alt=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 143.1 Mya. ...

. 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 ...

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 The oceanic carbon cycle (or marine carbon cycle) is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the Seabed, seafloor. The carbon cycle is a result of ma ...

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 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 flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

s and

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

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 ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

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 vesicles derived from the golgi body. 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 solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s (depending on the water depth). Thomas Huxley 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 minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

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. Ch ...

deposits such as the

white cliffs of Dover The White Cliffs of Dover are 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, depo ...

(deposited in

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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''

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 In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a special case of a parallelepiped in which all six faces are congruent rhombus, rhombi. It can be used to define the rhombohedral lattice system, a Ho ...

, 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 Noelaerhabdaceae is a family of coccolithophore, coccolithophorids. Some genus, genera include: ''Emiliania (coccolithophore), Emiliania'' W.W.Hay & H.P.Mohler, ''Gephyrocapsa'' Kamptner, and ''Reticulofenestra'' W.W.Hay, Mohler & M.Wade. Genus ...

. Modified material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License

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 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) ''

'' morphotype R; (I) ''Emiliania huxleyi'' morphotype A; (J) ''Emiliania huxleyi'' morphotype B.

Function

Although coccoliths are remarkably elaborate structures whose formation is a complex product of cellular processes, their function is unclear. Hypotheses include defence against grazing by zooplankton or infection by

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almo ...

; maintenance of

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

; release of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

for

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

; to filter out harmful UV light; or in deep-dwelling species, to concentrate light for photosynthesis.

Fossil record

Because coccoliths are formed of low-Mg calcite, the most stable form of calcium carbonate, they are readily fossilised. They are found in sediments together with similar microfossils of uncertain affinities (nanoliths) from the

Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. T ...

to recent. They are widely used as biostratigraphic markers and as paleoclimatic proxies. Stable oxygen and carbon isotope data from coccoliths is used to reconstruct estimates of oceanic CO₂ concentrations in the geologic past. Coccoliths and related fossils are referred to as '' calcareous nanofossils'' or ''calcareous nannoplankton (nanoplankton)''.

References

External links

The EHUX website
- site dedicated to ''Emiliania huxleyi'', containing essays on blooms, coccolith function, etc.
International Nannoplankton Association site
- includes an illustrated guide to coccolith terminology and several image galleries.
Nannotax
- illustrated guide to the taxonomy of coccolithophores and other nannofossils.
Cocco Express - Coccolithophorids Expressed Sequence Tags (EST) & Microarray Database
{{Authority control Calcium compounds