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.
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.
- 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
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.
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 Upper Triassic to recent. Coccoliths and related fossils are referred to as calcareous nannofossils or calcareous nannoplankton.
|Wikimedia Commons has media related to Coccolith.|
- 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
- Possible functions of Coccoliths