Everything about Coccoliths totally explained
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 compete these vesicles fuse with the cell wall and the coccolith is excytosed 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's 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're readily fossilised. We find them in the sediment 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.
Further Information
Get more info on 'Coccoliths'.
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