Biosilicification of loricate choanoflagellate: organic composition of the nanotubular siliceous costal strips of Stephanoeca diplocostata.

Loricate choanoflagellates (unicellular, eukaryotic flagellates; phylum Choanozoa) synthesize a basket-like siliceous lorica reinforced by costal strips (diameter of approximately 100 nm and length of 3 μm). In the present study, the composition of these siliceous costal strips is described, using Stephanoeca diplocostata as a model. Analyses by energy-dispersive X-ray spectroscopy (EDX), coupled with transmission electron microscopy (TEM), indicate that the costal strips comprise inorganic and organic components. The organic, proteinaceous scaffold contained one major polypeptide of mass 14 kDa that reacted with wheat germ agglutinin. Polyclonal antibodies were raised that allowed mapping of the proteinaceous scaffold, the (glyco)proteins, within the costal strips. Subsequent in vitro studies revealed that the organic scaffold of the costal strips stimulates polycondensation of ortho-silicic acid in a concentration- and pH-dependent way. Taken together, the data gathered indicate that the siliceous costal strips are formed around a proteinaceous scaffold that supports and maintains biosilicification. A scheme is given that outlines that the organic template guides both the axial and the lateral growth of the strips.