Coccoliths are individual plates of calcium carbonate formed by coccolithophores 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. 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 ofcalcium carbonate as the mineral calcite and are the main constituent of chalk deposits such as the white cliffs of Dover, 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 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.
Shape
Coccoliths are also classified depending on shape. Common shapes include:
Calyptrolith – basket-shaped with openings near the base
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 from the Upper Triassic to recent. They are widely used as biostratigraphic markers and as paleoclimatic proxies. Coccoliths and related fossils are referred to as calcareous nanofossils or calcareous nannoplankton .
