From biosilica of sponges (Demospongiae and hexactinellida) to fabricated biomedical materials

Abstract

Only 13 years after realizing, during a repair of a telegraph cable pulled out from the deep sea, that the bottom of the ocean is plentifully populated with a highly diverse fauna and flora, the Challenger expedition (1873-1876) treasured up a rich collection of vitreous sponges (Hexactinellida). They have been described by Schulze and represent the phylogenetically oldest class of siliceous sponges (phylum Porifera); they are eye-catching because of their distinct body plan, which relies on a filigree skeleton. It is constructed by an array of morphologically determined elements, the spicules. During the German Deep Sea Expedition “Valdivia” (1898-1899), Schulze could describe the largest siliceous hexactinellid sponge on Earth, the up to 3 m high Monorhaphis chuni, which likewise forms the largest biosilica structure, the giant basal spicule. Using such spicules as a model, basic knowledge on the morphology, formation, and development of the skeletal elements could be acquired. They are formed by a proteinaceous scaffold (composed of a 27-kDa protein), which mediates the formation of siliceous lamellae that encase the protein. The 27-kDa protein represents an enzyme that forms polysilicate from silicic acid monomers. The silica matrix is composed of almost pure silicon and oxygen, providing it with unusual optophysical properties that are superior to those of man-made waveguides. Experiments suggest that the spicules function in vivo as a nonocular photoreception system. In addition, the spicules are provided with exceptional mechanical properties, combining mechanical stability with strength and stiffness. These basic insights, obtained from the spicule formation in sponges, will surely contribute to a further applied utilization and exploration of silica in biomaterial/biomedical science.