Coccolith

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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.

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 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 and are the main constituent of chalk deposits such as the white cliffs of Dover (deposited in Cretaceous times), in which they were first described by Henry Clifton Sorby in 1861.

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.

Shape

Helicoliths of Helicosphaera carteri
Coccosphere of Emiliania huxleyi consisting of overlapping placoliths

Coccoliths are also classified depending on shape. Common shapes include:[1][2]

  • 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

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.

References

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External links

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