Silica-protein composite layers of the giant basal spicules from Monorhaphis: Basis for their mechanical stability

Abstract

The hexactinellid sponge Monorhaphis chuni possesses with its giant basal spicules the largest biosilica structure on Earth. The approximately 8.5-mm-thick spicules are composed of up to 800 lamellae. By application of high-resolution electron microscopy (HR-SEM), it is shown that within the siliceous lamellae a proteinaceous scaffold exists which is composed of one protein of a size of 27 kDa. Analyses with Fourier transform infrared (FT-IR) emission and energy-dispersive X-ray (EDX) spectroscopy support this localization of the protein. No evidence for the presence of protein on the surfaces of the lamellae could be obtained. Heating the giant basal spicule to 600 °C destroys and eliminates the protein scaffold. At a temperature of 1600 °C, the lamellae fuse to solid glass via a nonstructured, foamed-up molten transition state. Elevation of the temperature to 2700 °C results in the formation of silica drops (Euplectella aspergillum). After the elimination of the protein scaffold from the silica lamellae, the spicules lose their mechanical characteristics of the original hydrated silica/protein composite to be flexible and simultaneously stiff and tough. The data presented here are expected to contribute to technologies suited to fabricate novel organic/inorganic (silica) hybrid fibers.