Composition and Mechanical Properties of a Protein/Silica Hybrid Material Forming the Micron‐Thick Axial Filament in the Spicules of Marine Sponges

Synthetic hybrid polymer/silica composites draw increasfilament, has been suggested. Nowadays, it is well ing attention of researchers due to their potential applications in catalysis, drug delivery, and light-emitting devices. Remarkable examples of akin hybrid composites are also provided by nature, where amorphous silica is frequently used by the organisms as stiff skeletal building blocks or for light wave-guiding and focusing elements; all being formed in the course of the protein-directed biomineralization. One of the most studied classes of silica-producing organisms are the glass sponges, whose spicules exhibit complex hierarchical arrangement of stiff (silica) and compliant (proteins) structural components. In this paper, we focus on the inter-relationship between the composition and mechanical properties of hybrid protein/ silica material, forming the axial filament of the giant anchor spicule of the marine sponge Monorhaphis chuni (M. chuni). M. chuni is a deep sea glass sponge found at ocean depths down to 2 km. It belongs to the class of Hexactinellida and is among the earliest multicellular animals found as fossils. Themost fascinating feature of theM. chuni is the giant basal spicule around which the animal is assembled. This spicule is used for anchoring the animal to the bottom of the ocean and can reach up to 3m in length and 8mm in diameter. The cross-section of the investigated spicule with marked location of the axial filament is shown in Figure 1a. The filament itself is clearly visible in the AFM topography image in Figure 1b. Despite the fact that the filament is only 2mm in diameter, it plays a primary role in spicule formation and development. The filament provides the central vertical axis of the entire spicule, the latter being built of relatively thick silica core ( 150mm in diameter) and nearly concentric cylindrical biosilica layers (2–10mm wide, not visible here) separated by 35 nm thick organic layers. In the studies of different silica sponges, the presence of proteins within the