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crystal morphologies are found in other biogenic mineralized systems, including egg
shells, and generally considered characteristic of precipitates from highly supersaturated
solutions that form very rapidly (Lowenstam and Weiner 1989). The assertion by Adkins
et al. (2003) that the spherulitic aragonite “bouquets” in the skeleton of a deep sea coral
are characteristic of slow growth is therefore somewhat puzzling.
Despite the resemblance between crystals in corals and crystals in inorganic
minerals, Bryan and Hill (1941) were hesitant to explain the fasciculate organization of
the aragonite fibers as a purely physical system, a reluctance rooted in the observation
that coral skeletons are highly intricate and complex structures at both microscopic and
macroscopic scales. Thus, Bryan and Hill proposed the existence of an organic “gel”
enveloping each fiber and penetrating deeply within the fibers to guide crystal growth.
Several years later, a model proposed by Barnes (1970) showed that the morphology of
the aragonite fibers and their organization into bundles are explicable entirely in terms of
factors controlling abiotic crystal growth. Fundamental to this model is the idea that
calcification occurs most rapidly in micron-sized spaces formed where the calicoblastic
ectoderm lifts away from the skeletal surface. Barnes proposed that, given this limited
space in which to grow, fast-growing crystals precipitated from a supersaturated solution
will compete with each other. Crystals that happen to be oriented perpendicular to the
calicoblastic ectoderm will extend most rapidly and occlude those growing horizontally
or at low angles. The tendency for these crystals to diverge from the optimum axis of
growth gives rise to three-dimensional fans (Fig. 10a). In this way, the fine aragonite
needles grow as fan systems all over the basal plate, large fans outcompeting small ones
for space until stable fan systems develop. These are the sclerodermites that grow
upwards together to form the trabeculae.
Further compelling evidence for the predominance of physicochemical factors in
the growth of aragonite fibers in coral skeletons is the correlation between fiber
morphology and coral growth rate. Constantz (1986) observed very distinct and
consistent differences in aragonite fiber morphology amongst the scleractinian taxa. In
general, the narrowest fibers (~0.1
m) are characteristic of the fastest growing genera,
the Acroporidae. By contrast, the slow growing genera including the Favids have the
widest fibers. Variations within the range of naturally-occurring spherulite
morphologies in rocks and minerals can also be related to growth rate as the crystals of
the spherulite become progressively finer and more tightly bunched together as growth
rate increases (Lofgren 1974). Slower-growing fibers are larger and more widely
spaced. Interestingly, the secondary aragonite crystals that grow within pore spaces of
skeleton evacuated by coral tissue have the coarse, open morphology characteristic of
slower-growing spherulites in rocks (Fig. 10b). Thus, the range of aragonite fiber
morphologies found amongst the scleractinian taxa could be explained by basic theories
of crystal growth in inorganic systems without the need for mediation by an organic
macromolecular framework or matrix. These ideas form the basis for the
physicochemical model of coral calcification.
Combining observations of diurnal changes in coral calcification, skeletal extension,
crystal morphology and Sr/Ca ratio of the skeleton, we propose a model in which these
aspects of coral mineralization can be explained in terms of the light-sensitive action of
the Ca
2+
-ATPase pump. The model is summarized in Figure 11(a-d) and shows how
changes in the chemistry and pH of the calcifying fluid between night and day (Fig.
11a,c), and associated changes in the relationship between tissue ectoderm and skeletal
surface (Fig. 11b,d) result in the observed cycle of calcification and extension rate,
crystal morphology and chemistry. The dual role of the Ca
2+
-ATPase enzyme in
transporting cations into the calcifying space while removing protons, and the
µ
