The massive basal skeleton of a few remnant living hypercalcified
sponges rediscovered since the 1960s are valuable representatives of
ancient calcium carbonate biomineralization mechanisms in basal Metazoa.
A multi-scale mineralogical characterization of the easily accessible
Mediterranean living hypercalcified sponge belonging to Calcarea.
Petrobiona massiliana (Vacelet and Levi, 1958), was conducted. Oriented
observations in light and electron microscopy of mature and growing
areas of the Mg-calcite basal skeleton were combined in order to
describe all structural levels from the submicronic to the macroscopic
scale. The smallest units produced are ca. 50-100 nm grains that are in
a mushy amorphous state before their crystallization. Selected area
electron diffraction (SAED) further demonstrated that submicronic grains
are assembled into crystallographically coherent clusters or fibers, the
latter are even laterally associated into single-crystal bundles. A
model of crystallization propagation through amorphous submicronic
granular units is proposed to explain the formation of coherent
micron-scale structural units. Finally, XRD and EELS analyses
highlighted, respectively, inter-individual variation of skeletal Mg
contents and heterogeneous spatial distribution of Ca ions in skeletal
fibers. All mineralogical features presented here cannot be explained by
classical inorganic crystallization principles in super-saturated
solutions, but rather underlined a highly biologically regulated
formation of the basal skeleton. This study extending recent
observations on corals, mollusk and echinoderms confirms that occurrence
of submicronic granular units and a possible transient amorphous
precursor phase in calcium carbonate skeletons is a common
biomineralization strategy already selected by basal metazoans. (C) 2011
Elsevier Inc. All rights reserved.