Sponge biosilica formation involves syneresis following polycondensation in vivo

Syneresis is a process observed during the maturation/aging of silica gels obtained by sol-gel synthesis that results in shrinkage and expulsion of water due to a rearrangement and increase in the number of bridging siloxane bonds. Here we describe how the process of biosilica deposition during spicule ("biosilica" skeleton of the siliceous sponges) formation involves a phase of syneresis that occurs after the enzyme-mediated polycondensation reaction. Primmorphs from the demosponge Suberites domuncula were used to study syneresis and the inhibition of this mechanism. We showed by scanning electron microscopy that spicules added to primmorphs that have been incubated with manganese sulfate fuse together through the deposition of silica spheres and bridges. Energy-dispersive X-ray mapping of the newly formed deposits showed high silicon and oxygen content. These biosilica deposits contain a comparably higher percentage of water than mature/aged spicules. Quantitative real-time polymerase chain reaction analyses revealed that the addition of silicate to primmorph cultures resulted in a marked upregulation of the expression of the aquaporin gene and of the genes encoding the silica anabolic enzyme silicatein-α and the silica catabolic enzyme silicase. On the other hand, addition of manganese sulfate, either alone or together with silicate, caused a strong reduction in the level of aquaporin transcripts, although this metal ion did not essentially affect the silicate-induced increase in silicatein-α and silicase gene expression. We conclude that the secondary silica deposits formed on spicules under physiological conditions in the presence of silicate fuse together and subsequently undergo syneresis, which is facilitated by the removal of water through aquaporin channels. In growing spicules, these processes of biosilica formation and syneresis in the lamellar monolithic structures precede the final step of "biosintering" during which the massive biosilica rods of the spicules are formed.