Silica formation in diatoms: the function of long-chain polyamines and silaffins

The formation of inorganic minerals under the control of an organism (biomineralization) is a widespread phenomenon in nature. Silica is the second most abundant biomineral being exceeded only by biogenic CaCO3. 1 Many landplants (e.g. rice, cereals, cucumber) deposit silica in significant amounts to reinforce their tissues and as a systemic response to pathogen attack. Furthermore, there is evidence that silica is required in animals including mammals for proper development of cartilage and bone. Silica biomineralization on earth, however, is dominated by simple aquatic life forms including unicellular organisms like diatoms, radiolaria and synurophytes as well as multicellular sponges. These organisms produce silica-based exoand endo-skeletons that account for the majority of their body mass and, most notably in diatoms, exhibit intricate cell wall patterns in the nanoto micro-meter range (biosilica nanopatterns) (Fig. 1). Since the biosilica nanopatterns are precisely reproduced in a species-specific manner in each generation, a genetic control of this biomineralization process is obvious which has been regarded as a paradigm for controlled production of nanostructured silica. Therefore, understanding the mechanism of silica nanofabrication by diatoms may inspire synthetic routes to produce novel silica-based materials under mild reaction conditions. Diatom biosilica is mainly composed of amorphous, hydrated SiO2 (silica) containing a small proportion of organic macromolecules, which have long been speculated to control silica deposition and nanopatterning. Only recently have proteins and other organic molecules associated with diatom biosilica been purified to homogeneity and extensively characterized. The current knowledge about the structure and function of these molecules will be summarized in this review.