Coccolith

Coccolith
False-colour scanning electron micrograph of Gephyrocapsa oceanica, showing the coccoliths.

Coccoliths are individual plates of calcium carbonate formed by coccolithophores (single-celled algae such as Emiliania huxleyi) which are arranged around them in a coccosphere.

Contents

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 sediments (depending on the water depth). Thomas Huxley was the first person to observe coccoliths in modern marine sediments and he gave them the name coccoliths. Coccoliths are composed of calcium carbonate as the mineral calcite and are the main constituent of chalk deposits such as the white cliffs of Dover.

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 (ca 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.

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 or viruses; maintenance of buoyancy; release of carbon dioxide for photosynthesis; 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 (nannoliths) from the Triassic to recent. Coccoliths and related fossils are referred to as calcareous nannofossils or calcareous nannoplankton.

External links


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Coccolith — Coc co*lith, n. [Gr. ? a grain, seed + lith.] (Biol.) One of a kind of minute, calcareous bodies, probably vegetable, often abundant in deep sea mud. [1913 Webster] …   The Collaborative International Dictionary of English

  • coccolith — [käk′ə lith΄] n. [< ModL coccus (see COCCUS) + LITH] a minute calcareous plate covering the body of certain golden algae (division Chromophycota) …   English World dictionary

  • Coccolith — REM Aufnahme eines Coccolithen von Discoaster surculus aus dem Paläogen Coccolithen sind mikroskopische (maximal 0,01 mm große) scheibenförmige Plättchen aus Kalk, die die Schale von Kalkalgen der Ordnung Coccolithophorida bilden. Das aus… …   Deutsch Wikipedia

  • coccolith — coccolithic, adj. /kok euh lith /, n. a microscopic calcareous disk or ring making up part of the covering of certain marine plankton and forming much of the content of chalk rocks. [1865 70; < NL Coccolithus orig. a genus name; see COCCUS, O ,… …   Universalium

  • coccolith — noun A microscopic skeletal plate of calcite on the surface of certain marine phytoplankton; it forms chalk and limestone when fossilized …   Wiktionary

  • coccolith — tiny calcareous plate whose fossils form chalk and limestone deposits Stones and Rocks …   Phrontistery dictionary

  • coccolith — [ kɒkəlɪθ] noun Biology a minute rounded calcareous platelet, numbers of which form the spherical shells of coccolithophores. Origin C19: from Gk kokkos grain or berry + lithos stone …   English new terms dictionary

  • coccolith — coc·co·lith …   English syllables

  • coccolith — coc•co•lith [[t]ˈkɒk əˌlɪθ[/t]] n. gel mcr a microscopic calcareous disk or ring making up part of the covering of certain marine plankton and forming much of the content of chalk rocks • Etymology: 1865–70; < NL Coccolithus orig. a genus… …   From formal English to slang

  • coccolith — …   Useful english dictionary

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”