Electron microscope analyses of the bio-silica basal spicule from the Monorhaphis chuni sponge

We report on a structural analysis of several basal spicules of the deep-sea silica sponge Monorhaphis chuni by electron microscope techniques supported by a precise focused ion beam (FIB) target preparation. To get a deeper understanding of the spicules length growth, we concentrated our investigation onto the apical segments of two selected spicules with apparently different growth states and studied in detail permanent and temporary growth structures in the central compact silica axial cylinder (AC) as well as the structure of the organic axial filament (AF) in its center. The new findings concern the following morphology features: (i) at the tip we could identify thin silica layers, which overgrow as a tongue-like feature the front face of the AC and completely fuse during the subsequent growth state. This basically differs from the radial growth of the surrounding lamellar zone of the spicules made of alternating silica lamellae and organic interlayers. (ii) A newly detected disturbed cylindrical zone in the central region of the AC (diameter about 30 μm) contains vertical and horizontal cavities, channels and agglomerates, which can be interpreted as permanent leftover of a formerly open axial channel, later filled by silica. (iii) The AF consists of a three-dimensional crystal-like arrangement of organic molecules and amorphous silica surrounding these molecules. Similar to an inorganic crystal, this encased protein crystal is typified by crystallographic directions, lattice planes and surface steps. The 〈001〉 growth direction is especially favored, thereby scaffolding the axial cylinders growth and consequently the spicules' morphology.