Emiliania huxleyi Coccoliths


Jeremy Young
Palaeontology Dept.
The Natural History Museum
London, SW7 5BD
Great Britain.
Email: j.young@nhm.ac.uk


This page describes Ehux coccoliths, i.e. the characteristic calcareous plates the cell produces. People often loosely refer to Ehux as a coccolith, which is strictly speaking incorrect. This is not drastically important but the correct terminology is:
Coccolithophore - the organism, including the cell and external coccoliths.
Coccosphere - the more or less spherical layer of coccoliths surrounding the cell.
Coccolith - the individual calcareous plates.

Ehux produces coccoliths that are approx. 2.5 x 10^-6 metres in diameter, and which weigh approx. 1.8 x 10^-12 g each. Of this weight, the average contents of carbon, oxygen and calcium atoms in the coccolith CaCO3 have been measured to be 0.28, 0.87 and 0.67 x 10^-12 g per coccolith respectively (Fagerbakke et al, 1994).

For comparison, the wavelength of visible light is in the range 0.4-0.7 x 10^-6 metres, and a pinhead is about 2000 x 10^-6 metres in diameter.

[click on the small pictures on this page to view them at full size]



Three SEMs of E. huxleyi coccoliths: the first two of the under-side of a coccolith (the side lying closest to the cell surface), the third from 'above' (pictures from Jeremy Young).

The coccoliths are produced inside the cell (see also cell page) then extruded to the outside. The coccoliths themselves are not living structures but dead mineral structures, analogous to human bone or fingernails, or the shells of oysters and other molluscs. Such biominerals are intriguing structures and study of their formation, or biomineralization, is a challenging interdisciplinary science which seeks both to understand natural structures and processes and to learn from nature how complex mineral structures can be formed. Coccoliths are one of the more remarkable types of biominerals owing to the very precise control by the organic system of every aspect of inorganic mineral growth. For this reason and because coccolithophores are easily studied in the laboratory coccolith formation, particularly in Ehux has been intensively studied.



Two side-views of coccoliths. Star Trek? (Pictures from Jeremy Young).

Coccolith shape and arrangement on the cell: Ehux coccoliths have a very characteristic shape (placolith morphology for terminological tyros) which has been variously likened to rivets, collar studs, and cable-reels. There is a central tube from which two flanges or shields extend. The shields are curved to match the curvature of the cell wall so there is a clear top and bottom to the coccolith. The concave side of the coccolith is nearer to the cell and is termed the proximal or lower surface, as opposed to the upper or distal surface. The area inside the tube is open on the distal side but filled on the proximal side by a grill. On the coccosphere the shields of the individual coccoliths overlap and interlock forming a robust structure.

This seems to be a self-organising effect - we believe individual coccoliths are pushed out by the cell the right way-up, but otherwise more or less at random into the coccosphere. Extra-cellular polysaccharide almost certainly helps keep the coccoliths attached to the cell and random shuffling between them results in the interlocking. This irregular self-organising structure to the coccosphere means that the number of coccoliths per cell does not need to be regulated closely. Typically there is a single layer of about 10 coccoliths around the cell, but some cells accumulate multi-layered coccospheres with hundreds of coccoliths. Conversely in cultures of Ehux it is quite common to see cells with incomplete coccospheres, or with no coccoliths at all.

Coccolith structure and growth: coccolith formation occurs inside the cell and has been studied both by looking at sections through cells with coccoliths at various stages of growth and by looking at isolated incompletely formed coccoliths. Growth starts with nucleation of an elliptical ring of simple calcite crystals - a proto-coccolith ring. This proto-coccolith ring is situated at what will become the base of the tube. Growth then progresses simultaneously in various well-constrained directions: outwards to form the proximal shield; inwards to form the central grill; upwards and then outwards to form an outer layer of the tube and the distal shield; obliquely upwards to form the inner layer of the tube. This growth occurs entirely by enlargement of the crystals of the proto-coccolith ring, with no formation of new crystals. Also each of the developing crystal-units produces elements in each of the growth directions, thus developing a very complex final form - particularly when contrasted to the rhombohedral and prismatic crystals produced during inorganic calcite growth.

One result of the complex shape is that the crystals interlock with each other, particularly due to the difference in obliquity of the inner and out layers of the tube. This interlocking gives the structure considerable strength - Ehux coccoliths normally stay intact even during quite robust laboratory preparation techniques. In addition the complex crystal-unit structure provides the best morphological criteria for determining the evolutionary relationships of different coccoliths. Coccoliths formed by species closely related to Emiliania huxleyi have the same basic structure of four elements formed from a single crystal-unit. Ehux is characterised by the delicate hammer-like form of the distal shield elements, with openings between each element. By contrast most related species have solid distal shields and may have other distinguishing features, for example in Gephyrocapsa species there is bridge across the central area.

Coccolithogenesis: Alison Taylor has produced a wonderful time-lapse video showing another coccolithophore (Coccolithus pelagicus) building its coccoliths inside the cell and then extruding them out onto the cell surface. The coccoliths are rather large relative to the cell size; if scaled up to human size it would be like a person giving birth to a car wheel or a dustbin lid.

For more detailed and specialised information on the geometry and formation of the coccoliths, click here.

References


Ehux home page