The Largest Bio-Silica Structure on Earth: The Giant Basal Spicule from the Deep-Sea Glass Sponge Monorhaphis chuni
Figure 5
Tauactin spicules with open tips. (A-1–A-3) All spicules in Hexactinellida display a square opening of the axial canal (ac); SEM analysis. The quadrangular axial filament (af) is connected with the outer surface of the spicules and permits longitudinal growth; it also determines the direction of spicule formation. Whereas in most spicules, the opening does not contain any material (A-2, A-3), the axial canal of some spicules contains an axial filament (af; A-1). (B-1–B-5) Scheme of the longitudinal growth of the spicules. (B-1) In the initial stage, the spicule with its silica layers (dark green) has within its axial canal the axial filament (red), which reaches almost to the tip of the spicule. (B-2–B-4) During the growth of the spicule, silicatein-like material mediates the deposition of the polymeric silica (green dots and patches), which is deposited as a new layer on top of the previous silica layer (light green). (B-4-B-5). With progress of the axial growth of the spicules, the organic material becomes internalized into the spicule and contributes to the elongation of the axial filament. (C) Proposed formation of spicules in the hexactinellid M. chuni by appositional lamellar growth. The center of the spicule comprises an axial canal filled with an axial filament (af, red); the protein composition includes also the silicatein(-related) protein. Around the axial filament, the first lamella has been formed (1). The formation of the next silica lamella is thought to be mediated by silicatein(-related) proteins (red ellipsoid dots) arranged on both the surface of the first lamella and on a proteinaceous tube/cage stabilized in its outer layer by lectin molecules (yellow dots). The final orientation of the tube is provided by the collagen mat. Within the cage, a solid silica lamella is formed through an association of the silica clusters (left to right). During this growth process of the spicules, a thickening of the spicules takes place by the formation of new silica lamellae (2-3). The organic material of the cage undergoes proteolytic disintegration, as indicated in layer 2. The concentric arrangement of the silicatein(-related) proteins/lectin associates is proposed to be stabilized by collagen (gray fibers).
