Chemistry Reference
In-Depth Information
of nutrient elements in proteins associated with biomineralization reduces their
availability for synthesis of DNA and physiologically essential proteins and so is
maladaptive in a nutrient-limited autotroph. Hence it is likely that proteins associated
with coccolith formation have enzymatic functions, such as controlling production of
polysaccharides, rather than having a direct role in regulating crystal growth, thus
minimizing the total amount of protein used for biomineralization.
Morphological observations
An unusual aspect of coccoliths as biomineral structures is that they are routinely
studied at the level of the biomineral fabric. In most other mineralized structures, for
instance bivalves or vertebrates, or even foraminifera, the routine level of study is gross
morphology and the biomineral fabric is only investigated in special higher resolution
studies. With coccoliths, by contrast, the component crystals are directly visible and form
the basis of classification and identification. This obviously applies to routine observations
by scanning electron microscopy (SEM), but also less obviously to observation by light
microscopy (LM). In normal illumination only the basic form of coccoliths can be seen;
because coccoliths are formed of calcite they can be better studied in cross-polarized light.
Coccoliths are typically about one micron thick, and the very high birefringence of calcite
means that they show first-order polarization colors. The radial fabric of the coccoliths
causes them to show bright extinction crosses (e.g., Young 1992). This is the routine means
of study of fossil coccoliths (Bown and Young 1998), although it is less commonly used by
biologists studying plankton. As a result of these light microscopy observations, it has long
been appreciated that coccoliths consist in part of crystals with sub-radial orientations,
forming bright pseudo-extinction crosses in polarized light but also in part of crystals with
sub-vertical calcite
c-
axis orientations that appear dark in cross-polarized light in plan view
(e.g., Kamptner 1954; Prins 1969; Romein 1979).
A key advance in knowledge of coccolith formation came from combining these
light microscope observations with study of coccoliths at different growth stages, either
from culture samples or low-diversity natural assemblages (Young and Bown 1991;
Young et al. 1992). Representative structures are shown in Figures 3-6 and are described
briefly below, then discussed in terms of nucleation and growth regulation. Terminology:
Biscutum, Coccolithus
and
Emiliania
all have
placolith
type coccoliths; these have a
basic shape similar to that of a cable reel consisting of a central
tube
which connects a
lower
proximal shield
and upper
distal shield
and encloses the
central area.
The placolith
morphology allows the coccoliths to interlock closely on the coccosphere, forming a
robust structure. This morphology has evolved repeatedly, but many other coccolith
morphologies also occur. The structures shown have been worked out primarily from
SEM and LM observations. Further details of the structures are given by Young et al.
(1992, in press), Young (1992), and Henriksen et al. (in press b). SEM observations of
multiple specimens, including broken ones and specimens at different growth stages,
allow the shapes and interconnections of coccolith elements to be worked out. LM
observations are the primary source of information on broad crystallographic orientation
of the crystal units, although this has been supplemented by use of selected area electron
diffraction (SAED), (e.g.
,
Davis 1995; Marsh 1999), atomic force microscopy (Henriksen
et al. in press a,b) and crystal face morphology (e.g., Black 1963; Young et al. in press).
Biscutum
provides a straightforward example of a V/R structure. The distal (upper)
shield and outer tube cycle is formed of V-units, which appear dark in cross-polarized light.
The proximal shield, inner tube and central area elements all interconnect and are formed
from R-units, with sub-radial calcite
c-
axes. The two unit types can be seen to alternate
with each other in a ring on the proximal face of the coccolith and this is interpreted to be
the proto-coccolith ring locus, i.e., the location where nucleation occurred.
