Silicification: The Processes by Which Organisms Capture and Mineralize Silica

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Abstract
Silicification is widespread in the biological world and occurs in bacteria, single-celled protists, plants, invertebrates and vertebrates. Minerals formed in the biological environment often show unusual physical properties (e.g., strength, degree of hydration) and often have structures that exhibit order on many length scales. The minerals are formed from an environment that is undersaturated with respect to silicon and under conditions of around neutral pH and low temperature ca. 4–40°C. Formation of the mineral may occur intra- or extra-cellularly and specific biochemical locations for mineral deposition that include lipids, proteins and carbohydrates are known. The significance of the cellular machinery cannot be over emphasized and it is with advances in experimental techniques (cell biology and materials characterization) and advances in understanding (including the ability to design laboratory experiments to mimic the biological environment) that much progress has been made in the field in recent years. In most cases the formation of the mineral phase is linked to cellular processes and if we understand this process the knowledge so gained could be used to good effect in designing new materials for biomedical, optical and other applications. The study of living organisms could result in wealth generation/creation. It should be noted that although significant advances have been made in the last ten years, new questions have arisen and there are many areas requiring exploration. This contribution will place emphasis on the systems for which most is known, namely sponges and diatoms, however, it should be borne in mind that many other organisms from single-celled species such as choanoflagellates (Mann et al. 1982) and radiolaria through to higher plants and molluscs such as the limpet (Mann et al. 1986) make use of silica and form species-specific structures that show structural organization on several length scales. Examples of some of the silicified structures observed …