Mesostructure from hydration gradients in demosponge biosilica

Abstract

Organisms of the phylum Porifera, that is, sponges, utilize enzymatic hydrolysis to concatenate bioavailable inorganic silicon to produce lightweight, strong, and often flexible skeletal elements called spicules. In their optical transparency, these remarkable biomaterials resemble fused silica, despite having been formed under ambient marine biological conditions. Although previous studies have elucidated the chemical mechanisms of spicule formation and revealed the extensive hydration of these glasses, their precise composition and local and medium-range structures had not been determined. We have employed a combination of compositional analysis, (1) H and (29) Si solid-state nuclear magnetic resonance spectroscopy, and synchrotron X-ray total scattering to characterize spicule-derived silica produced by the demosponge Tethya aurantia. These studies indicate that the materials are highly hydrated, but in an inhomogeneous manner. The spicule-derived silica is, on average, perfectly dense for the given extent of hydration and regions of fully condensed and unstrained SiO networks persist throughout each monolithic spicule. To accommodate chemical strain and defects, the extensive hydration is concentrated in distinct regions that give rise to mesostructural features. The chemistry responsible for producing spicule silica resembles hydrolytic sol-gel processing, which offers exceptional control over the precise local atomic arrangement of materials. However, the specific processing involved in forming the sponge spicule silica further results in regions of fully condensed silica coexisting with regions of incomplete condensation. This mesostructure suggests a mechanism for atomistic defect tolerance and strain relief that may account for the unusual mechanical properties of the biogenic spicules.

Keywords: NMR spectroscopy; biomineralization; pair distribution function; silica; spicules.