Chemistry Reference
In-Depth Information
consequences for pH and aragonite saturation state is discussed in detail in the section on
Calcification Mechanism (below).
Organic matrix models
Despite the remarkable similarities between the spherulitic structures found in rocks
and those characteristic of coral skeleton, the role of organic material in coral
skeletogenesis—either as an organic matrix framework or a seed for nucleation—remains
a topic of debate, central to which is the fact that in almost all instances of biological
mineralization, the mineral is associated with organic material (Watabe 1981). Indeed,
some consider the presence of an organic matrix to be a prerequisite step for the
formation and growth of most biominerals. While few would argue against some level of
involvement of organic material in some part of the coral calcification process, it is both
the level of control exerted over skeletogenesis and the type of control (promotional
versus inhibitory) that is central to the debate. The classic organic matrix not only
facilitates nucleation but also controls crystal mineralogy, orientation and growth. In
these systems, oriented nucleation is considered to arise from specific molecular
mechanisms that lower the activation energy of nucleation along a particular
crystallographic direction (Mann 2001). Formation of mollusk shell nacre is a good
example. In this case, the nuclei are crystallographically aligned with regard to the
underlying organic matrix sheet. As a result, the plate-like aragonite crystals grow with
their c-axes perpendicular to the organic surface. However, while the composition (proteins
rich in aspartic and glutamic acids, acidic and sulfated polysaccharides—Crenshaw 1990;
Figure 11 (on facing page).
Changes in crystal morphology, calcification rate, skeletal extension and
Sr/Ca ratio of a
Porites
coral between night (a,b) and day (c,d) are explicable in terms of the light-
sensitive action of the Ca
2+
-ATPase pump. The skeletal surface depicted in (a) and (c) are three
“fingers” shown in Figure 3. In (a) the Ca
2+
-ATPase pump is turned off at night. As a result, pH within
the calcifying space (CS) is ~8 (Al-Horani et al. 2003) and the aragonite saturation state is low (<10)
(see Figs. 13 and 14). Low calcification and crystal growth rates cause low density aggregates of
granular shaped, submicron aragonite crystals (CG) to precipitate on the old skeletal surface (SS). The
main pathway for entry of strontium and calcium into the CS at night is in seawater transported via
pericellular pathways (PC) and vacuoles (V) that form by invagination (IV) of the basal membrane
(BM). The contents of the vacuoles are exocytosed (EX) through the apical membrane (AM) and into
the CS (Clode and Marshall 2002). The Sr/Ca ratio of the calcifying fluid = seawater Sr/Ca (~8.6
mmol/mol)(de Villiers et al. 1994) and the Sr/Ca ratio of the precipitating crystals is the same as
inorganic aragonite (~9
10 mmol/mol)(Kinsman and Holland 1969, Enmar et al. 2002). In (b)
nighttime skeletal growth occurs mainly at the tips of the fingers (Barnes and Crossland 1980, Vago et
al. 1997). The calicoblastic ectoderm is tight (TE) against the skeletal surface (SS) except at the tips
where the tissue lifts away from the skeleton forming a small pocket (PO). The granular crystals are
precipitated in bundles to form a new center of calcification (COC). Growth of aragonite fibers is
inhibited at point of contact between tissue and skeleton forming a daily growth band (GB). In (c), The
Ca
2+
-ATPase pump is turned on in daylight. pH within the CS increases to ~9 (Al-Horani et al. 2003)
and the aragonite saturation state increases (>100) (see Figs. 13 and 14). High calcification and crystal
growth rates causes high densities of spherulites to grow from the granular surfaces of the new COC.
Epitaxial crystal growth also continues along the entire skeletal surface lengthening the existing
fasciculi until adjacent bundles meet and growth stops. The main pathway for entry of calcium into the
CS is via the Ca
2+
-ATPase pump, which may concentrate Ca
2+
ions within the vacuoles (Marshall and
Wright 1993) and/or transport Ca
2+
ions directly across the apical membrane (AM). The relative
transport of Sr
2+
ions is low and the Sr/Ca ratio in the CS decreases relative to seawater (~7.9
mmol/mol). Assuming a constant K
d
of 1.1, the Sr/Ca ratio of crystals precipitating from this fluid will
drop to ~8.6 mmol/mol. In (d) daytime skeletal growth occurs mainly at the sides of the fingers which
thicken and eventually consolidate to form a solid spine (Barnes 1970). Increased osmotic pressure in
the CS pushed the ectoderm up off the skeletal surface creating a space (PO) into which the aragonite
fibers grow. Growth continues until nighttime when the osmotic pressure within the CS decreases
causing the tissue to sink back down onto the skeletal surface. MG = mesoglea, CE = calicoblastric
epithelium, CS = calcifying space, CG = new crystal growth, SS = old skeletal surface.
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