Description of an Emiliania huxleyi cell

Jeremy Young
Palaeontology Dept.
The Natural History Museum
London, SW7 5BD
Great Britain.

[click on the picture to see it full size]

Schematic of Ehux cell: ER = endoplasmic reticulatum; (diagram from Peter Westbroek)

Organelles: this drawing shows a cross-section through a single Emiliania huxleyi. It is based mainly on observations made during transmission electron microscope studies of ultrathin (/<1 micron) sections of cells. As the figure clearly shows Ehux is unicellular, i.e. formed of a single cell, but this cell contains a number of internal organelles. Dominating the cell is the chloroplast - the photosynthetic body. This contains photosynthetic pigments and works in essentially the same way as chloroplasts in land plants, although the pigments are slightly diferent so the colour is yellow or golden brown colour rather than green. In the centre of the cell, above the chloroplast is the nucleus. This contains the genetic material, DNA and is the site for replication of DNA during cell division and production of RNA. The mitochondrion is a distinct organelle, present in all eukaryotic cells, and is the source of ATP (adenosine triphosphate) the major energy transferring molecule of biological systems.

Membranes: the nuclear membrane connects with an internal network of membranes, the endoplasmic reticulum [ER] and through this to the cell membrane. All these are formed by the Golgi Body. By contrast the chloroplast and mitochodrion have discrete embranes which separate them from the rest of the cell, and they contain separate DNA. These indicate that the chloroplasts and mitochondrion represent once discrete singe-celled organisms which were incorporated via endosymbiosis into the host protist. These endosymbioses are now thought, at least in the case of coccolithophorids, to have occurred only once, in the distant evolutionary past of the group's ancestors.

Coccolith formation: the most distinctive feature of the cell is the covering of coccoliths. The coccoliths are formed inside the cell via a highly organised process, coccolithogenesis. As shown, only one coccolith at a time is formed in Ehux and they develop within a coccolith vesicle close to the nuclear membrane. Above the coccolith vesicle is the Reticular Body which controls the coccolith formation. This is quite separate from the Golgi Body but is closely related to it, and microvesicles can be observed between the two organelles. In other coccolithophorids the coccolith vesicles form inside the golgi body without a distinct reticular body being present. When the coccolith is fully formed the coccolith vesicle migrates toward the edge of the cell, the vesicle mebranes fuse with the cell membrane and the external parts of the membrane are resorbed so that the coccolith is extruded to the outside of the cell.

Further Reading: for more information, see (Green & Leadbeater, 1994; Green et al, 1989; Jordan et al, 1995; Pienaar, 1994; and refs in Westbroek, 1984).


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