title: Calcareous sponge biomineralization: ultrastructural and compositional heterogeneity of spicules in Leuconia johnstoni.
abstract: In contrast to siliceous sponge spicules, the biomineralization in calcareous sponges is poorly understood. In particular, the existence of a differentiated central core in calcareous spicules is still controversial. Here we combine high-spatial resolution analyses, including NanoSIMS, Raman, SXM, AFM, SEM and TEM to investigate the composition, mineralogy and ultrastructure of the giant tetractines of Leuconia johnstoniCarter, 1871 (Baeriidae, Calcaronea) and the organization of surrounding cells. A compositionally distinct core is present in these spicule types. The core measures 3.5-10 μm in diameter and is significantly depleted in Mg and lightly enriched in S compared with the adjacent outer layer in the spicule. Measured Mg/Ca ratios in the core range from 70 to 90 mmol/mol compared to 125-130 mmol/mol in the adjacent calcite envelope. However, this heterogeneous distribution of Mg and S is not reflected in the mineralogy and the microstructure. Raman spectroscopy demonstrates a purely calcitic mineralogy. SEM examination of slightly etched spicules indicates an ultrastructure organized hierarchically in a concentric pattern, with layers less than 250 nm in width inside layers averaging 535 ± 260 nm. No change in structural pattern corresponds to the Mg/Ca variation observed. AFM and TEM observations show a nanogranular organization of the spicules with a network of intraspicular organic material intercalated between nanograins 60-130 nm in diameter. Observations of sclerocyte cells in the process of spiculogenesis suggest that the compositionally distinct core is produced by a sub-apical sclerocyte "founder cell" that controls axial growth, while the envelope is secreted by lateral sclerocytes "thickener cells", which control radial growth.
authors.affiliation: Muséum National d'Histoire Naturelle, Laboratoire de Minéralogie et Cosmochimie du Muséum, UMR 7202, Case Postale 52, 61 rue Buffon, 75005 Paris, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Kopp
authors.forename: Anders
authors.initials: A
authors.lastname: Meibom
authors.forename: Olivier
authors.initials: O
authors.lastname: Beyssac
authors.forename: Jarosław
authors.initials: J
authors.lastname: Stolarski
authors.forename: Shakib
authors.initials: S
authors.lastname: Djediat
authors.forename: Jakub
authors.initials: J
authors.lastname: Szlachetko
authors.forename: Isabelle
authors.initials: I
authors.lastname: Domart-Coulon
mesh_terms: D000825:Animal Structures / Q000737:chemistry* / Q000648:ultrastructure*; D000818:Animals; D002113:Calcification, Physiologic / Q000502:physiology*; D002118:Calcium / Q000032:analysis; D005602:France; D008274:Magnesium / Q000032:analysis; D018625:Microscopy, Atomic Force; D008854:Microscopy, Electron; D011161:Porifera / Q000033:anatomy & histology* / Q000502:physiology; D013059:Spectrum Analysis, Raman; D013455:Sulfur / Q000032:analysis
chemical_list: D013455:Sulfur; D008274:Magnesium; D002118:Calcium