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A frustule is the intricately patterned, silica-containing cell wall of a diatom. Its two principal overlapping halves fit rather like a lid and a box, with connecting bands. Pores and ridges differ among diatom groups, making the mineral structure useful in microscopic identification.

How can a biological cell make a patterned mineral structure under mild conditions? Isolated diatom peptides called silaffins caused silica nanospheres to form rapidly when added to a solution of silicic acid. This demonstrated that cellular organic components can directly promote mineral formation.^[ref] The experiment accounts for one chemical capacity; a complete frustule also requires control over where that capacity acts and how the resulting material is organized.

N. Kröger investigated how diatoms build intricately patterned silica structures. A 1999 study isolated positively charged peptides, named silaffins, from diatom biosilica. Adding these molecules to a solution of silicic acid generated networks of silica nanospheres outside the living cell.^[ref]

Reconstituting this step gave evidence that organic components can directly influence mineral formation. It did not require an intact diatom to make every sphere, while also not claiming that the isolated peptides alone reconstruct the whole shell. The experiment connects biomineralization to the properties of particular biological molecules.

A silaffin is a modified peptide associated with the silica-containing cell wall of a diatom. Silaffins help explain how an organism can promote mineral formation under conditions compatible with a living biological cell.

In a 1999 experiment, purified silaffins extracted from diatom walls rapidly generated networks of silica nanospheres when added to a silicic-acid solution. Chemical analysis identified modified lysine residues, and the modifications affected silica-precipitating activity.^[ref] The result demonstrates direct promotion of mineral deposition by an organic component. It does not alone explain the full patterned geometry of a frustule: producing mineral and organizing it into an elaborate wall are distinct experimental questions.