Coccolith 382890 221825475 2008-06-26T07:43:56Z SpBot 7016748 robot Adding: [[pl:Kokolit]] '''Coccoliths''' are individual [[plate|plates]] of [[calcium carbonate]] formed by [[coccolithophore]]s (single-celled [[Alga|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 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 [[sediment]]s (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. 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''. == External links == * [http://bioinfo.csusm.edu Cocco Express - Coccolithophorids Expressed Sequence Tags (EST) & Microarray Database] *[http://www.soes.soton.ac.uk/staff/tt/eh/coccoliths.html The EHUX website] - A site dedicated to ''Emiliania huxleyi'', containing essays on blooms, coccolith function, etc. *[http://www-ocean.tamu.edu/Quarterdeck/QD5.2/shatto-slowey2.html Possible functions of Coccoliths] *[http://www.nhm.ac.uk/hosted_sites/ina/ International Nannoplankton Association site] - includes an illustrated guide to coccolith terminology and several image galleries. *[http://nannotax.org/ Nannotax] - illustrated guide to Neogene coccolithophores and other nannofossils. [[Category:Calcium compounds]] [[de:Coccolith]] [[fr:Coccolithe]] [[nl:Coccoliet]] [[pl:Kokolit]] [[ru:Кокколиты]]