Chemistry Reference
In-Depth Information
Their evidence indicates that each group of nuclear packets making up the calcification
center is added to the growing tip of the skeleton within 24 hours. However, whether
entire packets of crystalline aggregates are precipitated at once or whether individual
amorphous granules are exocytosed into the subskeletal space, transported within their
protein sheaths to the site of calcification and added one by one to a growing nuclear
packet remains an open question. It is tempting to consider that the high concentrations of
organic material detected in centers of calcification (Cuif et al. 2003) may be remnants of
the organic wrappings of thousands of tiny granular crystals (see discussion of granular
crystals in foraminifera in Erez 2003).
In considering the possibility of an amorphous CaCO
3
precursor of the granular seed
nuclei, identification of calcite in centers of calcification of
Mussa angulosa
by Constantz
and Meike (1989) may have relevance because phase transformation would favor the
lowest energy state, calcite, over aragonite. While the latter study did not replicate the
finding of Houck et al. (1975) who reported up to 46% calcite in two
Porites
skeletons, it is
not inconsistent with the report of Vandermeulen and Watabe (1973) of trace amounts of
calcite in the larval plate of
Pocillopora damicornis
. However, the problem with having
calcite granules at centers of calcification is the unlikelihood of them being seed nuclei for
the growth of aragonite fibers. Furthermore, the combined results of recent independent
investigations of the mineralogy and chemistry of centers of calcification in a range of
Scleractinian species do not support this proposition. Cuif and Dauphin (1998), using
Raman spectroscopy, found no evidence of calcite in either the calcification centers or the
fibers of any of fifteen Scleractinian species, including
Mussa angulosa
(Fig. 6). They did
find that calcification centers are preferentially invaded by endolithic algae which might
explain observations of calcite in centers of calcification of
M. angulosa
. Bacterial
membrane encrustation might have been the source of calcite in the larval plate, i.e.,
bacteria located on the substrate before larval fixation and metamorphosis (Jean-Pierre
Cuif, personal communication 2003).
Sr/Ca geochemistry
Geochemical measurements also indicate that crystals in calcification centers are
aragonitic (Cohen et al. 2001, 2004). Strontium, with an ionic radius 28% larger than that
of Ca
2+
prefers the open crystal structure of the orthorhombic aragonite to the hexagonal
structure of calcite. The experimentally determined exchange co-efficient for Sr/Ca (K
d
)
in aragonite determined by Kinsman and Holland (1969) is >1 while that for calcite is
~0.08 (Lorens 1981; Tesoriero and Pankow 1996; Huang and Fairchild 2001). Therefore,
the Sr/Ca ratio of a CaCO
3
crystal is a good indication of its mineralogy. Crystals within
calcification centers of
Diploria labyrinthiformis
measured selectively by SIMS ion
microprobe have Sr/Ca ratios as high as 9.7 mmol/mol Sr/Ca (Cohen et al. 2004) (Fig. 7).
Given an average seawater Sr/Ca value of 8.56 mmol/mol (de Villiers et al. 1994), the
exchange coefficient for Sr/Ca in crystals within centers of calcification (K
d
= 1.10) is
close to that for Sr/Ca in aragonite precipitated experimentally at 25ºC (K
d
= 1.13)
(Kinsman and Holland 1969). These data support independent evidence for the aragonitic
mineralogy of the calcification centers.
The Sr/Ca ratio of crystals in the calcification centers also provide information about
the calcification process. The similarity amongst Sr/Ca ratios in inorganic aragonite
crystals precipitated from seawater (Sr/Ca = 9.7 mmol/mol at 25ºC; Kinsman and
Holland 1969), aragonite precipitated abiotically within skeletal pore spaces evacuated by
coral tissue (Sr/Ca = ~10 mmol/mol; Enmar et al. 2000) and the Sr/Ca ratio of crystals
within the calcification centers indicates that crystals at centers of calcification are
precipitated from a solution with a Sr/Ca ratio close to that of seawater.
