The earliest skeletal metazoans, the siliceous sponges, can be dated back to about 800 million years ago. The ‘big bang’ in sponges that enabled them to introduce an evolutionary metazoan novelty, the formation of a hard, inorganic skeleton, was the acquisition of a master gene, that encoding silicatein, for biosilica deposition. This evolutionary novelty proved to be extremely successful for the siliceous sponges. The fact that they did not replace their inorganic polymer with a calcium-based mineral might imply that, besides its primary purpose as a framework for the body, the skeleton may serve additional roles (such as acting as a light waveguide network, substituting for a nervous system, which is lacking in sponges).
The molecular basis of the sponge silicification process is largely understood. The silicateins are highly diversified in the two classes of siliceous sponges, the demosponges, and the hexactinellids. Like any enzyme, the silicateins have to follow the laws of thermodynamics, meaning that their reactions proceed along a chemical gradient, driven by negative Gibbs free energy. They increase the reaction velocity substantially by lowering the activation energy barrier. Besides genes for silicateins, the genes for the silicatein-associated proteins (silintaphins) have also been identified. A further recently discovered reaction during silicatein-catalyzed spicule formation is the aging process of the biosilica gel network, during which biosilica shrinks and hardens. The availability of the genes controlling sponge spicule formation allows the sustainable exploitation for new biotechnological applications of bio-silica in the fabrication of novel materials.
The three minireviews in this series summarize recent findings in our understanding of inorganic polycondensation reactions and of silicatein-driven biosilica formation, a process that is controlled by silicatein and silicatein-associated proteins. Finally, we outline new avenues for sustainable application of the silicateins in the synthesis of nanobiomaterials.
The first minireview, ‘An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances’, focuses on the condensation of silica from orthosilicic acid solutions. It also describes the mechanisms and conditions that determine the morphology of the final condensed siliceous materials. The methodologies available to monitor all stages of this process, from solution species to solid state, are reviewed. In addition, there is a brief discussion of more recent advances of in silico methods.
In the second minireview, ‘Silicateins, silicatein interactors, and cellular interplay in sponge skeletogenesis: Formation of the glass fiber-like spicules’, the regulatory network controlling and enabling the synthesis of biosilica and the subsequent aging/hardening processes of the enzymatic product are highlighted. Silicatein is unique not only because it is the first enzyme known that can synthesize an inorganic polymer from inorganic precursors, but also because it provides a structural basis for understanding how the intricate morphology of the spicules is shaped.
The third minireview, entitled ‘Bioinspired synthesis of multifunctional inorganic and bio-organic hybrid materials’, presents new biologically inspired materials, based on silicatein templating and catalytic abilities. This review highlights some examples of silicatein-inspired, low-temperature-fabricated materials, and some biologically inspired methods.
[ Werner E. G. Müller is a university professor at the University Medical Center of the Johannes Gutenberg University of Mainz (Germany) and a holder of the ERC Advanced Investigator Grant. His major achievements are on the molecular evolution of sponges, molecular biomineralization and its bionic applications, and deep sea biominerals. In addition, he has contributed to the sustainable exploitation of secondary metabolites for application in biomedicine. ]
[ Xiaohong Wang is a professor at the National Research Center for Geoanalysis (Chinese Academy of Geological Sciences). She was a winner of the Excellent Young Scientists Fund of the Ministry of Land and Resources, China. At present, she is working at the University Medical Center Mainz (Germany), where, together with W. E. G. Müller, she coordinates the Joint German–Chinese Laboratory on ‘Nano-Bio-Composites’, with the aim of developing biologically inspired medical materials. ]