More About Coccoliths ...


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


Some more advanced discussion of coccolith shape and formation.

Chirality: one of the many remarkable features of the coccolith structure is that they show pervasive and consistent chirality, or handedness. Although there is a general radial symmetry to the structure in detail almost every aspect shows also rotational symmetry. For instance the distal shield elements are deflected clockwise when seen in distal view) and the inner tube elements are deflected anti-clockwise. Again the proximal shield elements are terminated by a pair of faces of the which the one on the clockwise side is always longer. These chiral features are not always perfectly developed but whenever they occur they always show the same handedness - this has been tested by checking numerous specimens and examples of reverse chirality have never been found. The chirality extends back to the earliest growth stages and appears to be a product of the nucleation stage.

Number of crystal units: whilst the structure of the crystal units is extremely regular the actual number of crystal units is variable. Typically an Ehux coccolith has about 30 crystal units - this can easily be determined by counting the number of hammer head elements in the distal shield. Small specimens, however, may have as few as 20 crystal units and large specimens more than 40. As with the arrangement of coccoliths in the coccosphere this appears to be a self-organising type of structure. There is no precise coccolith blueprint, instead the crystal units nuclei are laid down at more or less uniform spacing around the proto-coccolith ring so if the ring is larger there are more crystal units.

Crystallographic orientation: calcite is a mineral with a precise chemical composition (CaCO3) and atomic structure with its own internal symmetry. Inorganic calcite crystals have shapes which reflect the internal symmetry, typically rhombohedra or hexagonal prisms elongated along the dominant c-axis of the atomic structure. In coccoliths the shape of the individual crystal units is strongly modified but the c-axis orientation is precisely related to their shape, specifically the c-axis is parallel to the elongation of the hammer-like distal shield elements. We can conclude from this that the initial nucleation stage of coccolith formation is characterised by precise control of crystallographic orientation, as well as the spacing and location of nuclei.

V&R model of nucleation: Ehux coccoliths despite their complexity are actually somewhat simpler structures than many others in that the entire structure is formed of only type of crystal-unit. In most other coccoliths the proto-coccolith ring consists of two crystal unit types one with approximately radially oriented calcite crystals (R-units), as in Ehux and the other with approximately vertically directed c-axes (V-units). These two crystal unit types can be traced back to the proto-coccolith ring where the two alternate, V-R-V-R-V-R. Whilst only R-units are seen in fully grown Ehux coccoliths study of proto-coccolith rings shows that two types of crystal units occur in the earliest growth stages; larger crystal nuclei which develop into the R-unit crystals of the complete Ehux coccoliths and smaller crystal nuclei which do not grow. We surmise that the smaller nuclei are relict V-units which are superfluous to the final coccolith structure but which are still nucleated since the nucleation stage is based on a conserved mechanism which cannot readily be changed during evolution.

Biochemical control: ultimately biomineralization must be the product of biochemical systems and we believe that with coccoliths we are close to being able to determine and characterise them. This is a tantalising goal since if we can understand how coccoliths, one of the most precisely regulated biomineral structures known are formed then we will be a significant step closer to understanding biominerization in general and to being able to create biomineral like structures. It seems likely that two separate system are involved.

1. Nucleation regulation: the evidence described above indicates that the nucleation of calcite is very precisely controlled. A straightforward hypothesis for explaining this is that an organic macromolecular template exists with an array of binding sites which mirror the structure of a plane through the calcite structure. On such a template Ca2+ and CO32- ions will fit into the binding sites automatically producing a proto-calcite crystal. Subsequent crystal growth can then occur inorganically. The alternation of V and R nuclei seen in coccoliths has lead us to speculate (Young et al. 1992) that the macromolecular template is folded so that a single template can produce the alternating nuclei with alternating vertical and radial c-axes.

This template is entirely hypothetical but if it exists we would expect it to be a protein directly controlled by RNA. Biochemistry research groups in Japan and Europe are attempting to isolate this coccolith precursor template molecule.

2. Growth regulation: an extremely complex acidic polysaccharide has been identified by the Leiden research group (de Jong et al. 1984). This has been shown to be closely associated with coccolith growth and to have a high affinity for calcite. One likely hypothesis is that this molecule binds to calcite and blocks further mineral growth on surfaces to which it binds. This effect in combination with vesicle growth in particular directions under cytoskeletal control may be sufficient to regulate crystal growth.

Genetic control: the approach to understanding coccolith proto described above has been to work down from observed coccolith structure toward the biochemical systems controlling growth regulation. An alternative approach is to attempt to identify and characterise the genes responsible for regulating biominerization. This should be possible since in culture strains often lose the ability to calcify, i.e. the genetic systems which promote calcification are suppressed, so by comparing the RNA produced by calcifying and non-calcifying Ehux strains the genes regulating calcification should be identifiable. This type of enquiry is being followed by a Dutch research group lead by Prof. Westbroek.

Our hope is that in this area as others the common focus on a single species, Ehux, will enable us to address fundamental problems using a wide range of different techniques and so develop detailed understanding of one system as a model for wide application.

References


Ehux home page