re: Biomineralization

From
Date
>From: Mike Simmons <gmi267@abdn.ac.uk>

>
>My major question here is what governs if an organism secretes a
>calcitic or aragonitic shell?
>m.d.simmons@abdn.ac.uk

Having gone through this sort of study with coccoliths (references below) I
may be able to offer a pointer or two. As far as the aragonite vs calcite
question goes a key perspective is that there is a wide spectrum in
biomineralization from very precise biological control of mineralisation to
rather vague biologically induced mineralization - as is well discussed in
the extensive literature on biomineralization. The more precise the control
the less possibility there is of mineralogical substitution. One
interesting test of degree of biological control in calcareous systems is
whether the orientation of calcite a-axes are regulated as well as the
calcite c-axis. Control of calcite a-axis orientation implies very precise
regulation of nucleation, probably by a precursor template and in these
circumstances calcite-aragonite substitution is unlikely - or so at least
we have argued with coccoliths. This may seem a trifle arcane but it is an
avenue you can easily follow via SEM observations on well-preserved to
slightly overgrown forams (look for rhombohedral faces and observe whether
they are randomly oriented or not).

YOUNG J.R., DIDYMUS J.M., & MANN S., 1991: On the reported presence of
vaterite and aragonite in coccoliths of Emiliania huxleyi. Botanica Marina,
34/6, 589-591.
	YOUNG J.R., DIDYMUS J.M., BOWN P.R., PRINS B., MANN S., 1992: Crystal
assembly and phylogenetic evolution in heterococcoliths. Nature, 356,
516-518.
	YOUNG J.R. 1992: The description and analysis of coccolith structure. In,
B. Hamrsmid & YOUNG J.R. (eds.) Proceedings of the 4th INA Conference,
Prague 1992. Knihovnicka ZPN, 14a, 35-71.
	DIDYMUS J.M., YOUNG J.R., & MANN S. 1994: Construction and morphogenesis
of the chiral ultrastructure of coccoliths of the marine alga Emiliania
huxleyi. Procs. Royal Society, London, B258, 237-245.
	DAVIS S.A.,  YOUNG J.R. & MANN S. (1995) Electron  microscopy  studies  of
shield  elements  of  Emiliania  huxleyi  coccoliths. Botanica Marina. 38,
493-497.
	


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