Most biominerals appear to be composites of organic material and mineral. Whether biosilica is such a composite is unresolved because of a lack of evidence for such organic components. We present evidence that organic material exists within diatom biosilica and can be extracted using HF/NH4F solutions from frustules isolated from Cyclotella meneghiniana Kütz and diatomaceous earth. To eliminate organic casing on the silicified frustules as a source of organic materials, the casing was removed by oxidation of frustules with NaOCl before extraction. The removal of the casing was confirmed in that oxidized frustules no longer displayed the ability to be stained with ruthenium red and fluorescamine. Frustules examined with EDXA showed an emission peak from sulfur before treatment but no peak following treatment, indicating that oxidation removed organic sulfur. The organic material obtained from extracts of fresh frustules contained both soluble and insoluble components. Only soluble material was evident in extracts from diatomaceous earth. The soluble material appears to contain glycoproteins with relatively high levels of serine and glycine. The soluble proteins from fresh frustules also appear to be phosphorylated. Indirect evidence is presented that suggests the soluble proteins may contain regions of primary structure enriched in anionic amino acids. The soluble extracts differ from general cell contents when the two fractions are compared, suggesting that frustules contain specialized organic material. The identification of silica‐specific organic material suggests that mineralization in diatoms may be in part matrix‐mediated.

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The organic matrix from sea urchin tests was extracted using 2% acetic acid. This material contained 8145% protein, 14-19% carbohydrate, and < 1% phosphate. The whole matrix was separated into aqueous soluble and insoluble fractions to determine their individual characteristics and functional activities. Electrophoresis of the soluble matrix (SM) resulted in a single band of approximately 170kD. The SM was capable of inhibiting in vitro CaC03 precipitation (at nanomolar concentrations) in solutions supersaturated with Ca2+ and C032-. It was also capable of inhibiting CaC03 spicule formation in sea urchin larvae at 10 pg SM per ml of larvae as measured by 14C incorporation. The insoluble matrix (IM) fraction had a higher protein content than the SM component and a lower PO4 content.That the SM from urchin tests is capable of inhibiting both in vitro crystallization and organismal calcification lends credence to its probable role as a mineralization regulator. What role it plays in vivo can only be surmised at this point, and whether or not this role is the same when it acts in conjunction with other components of matrix is unknown.

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