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Silica-carbonate biomorphs are inorganic self-organized structures that mimic the morphology of living organisms. In this study, we present the effect that proteins involved in the in vivo biomineralization of silica and calcium carbonate have on the formation of silica-carbonate biomorphs. We tested four different sources of protein: (1) struthiocalcin-1, (2) the catalytic domain of silicatein-α of Tethya aurantia, (3) a protein extract obtained from the spicules of a vitreous sponge ( Protosuberitis sp.), and (4) a protein extract obtained from the spines of the sea urchin Echinometra lucunter. In addition to the well-established role that pH plays in biomorph formation, all the proteins tested controlled the morphology of these aggregates. Biomorphs obtained in the presence of the catalytic domain of silicatein-α were similar in shape to those observed in the control though considerably smaller in size. Struthiocalcin-1 affected the availability of carbonate ions and completely inhibited the formation of biomorphs resulting only in worm-like aggregates. Finally, novel biomorphs with shapes such as twisting rods, sunflowers, and mitotic cells were obtained in the presence of protein extracts from the marine sponge spicules and sea urchin spines. [ABSTRACT FROM AUTHOR]