Pleiotropic Role of Recombinant Silaffin‐Like Cationic Polypeptide P5S3: Peptide‐Induced Silicic Acid Stabilization, Silica Formation and Inhibition of Silica Dissolution

Silica‐mineralizing organisms such as diatoms manage several aspects of silica chemistry when polymerizing monomeric silicic acid into amorphous silica. Silicic acid is undersaturated in the diatoms’ habitats and mechanisms of enrichment and prevention of uncontrolled mineralization are not well understood. Diatom‐biosilica is associated with organic compounds, including polycationic, post‐translationally modified peptides termed silaffins, which induce the condensation of silicic acid under supersaturated conditions. Here, we report the pleiotropic action of the designed silaffin‐like peptide P5S3, which (i) stabilizes 4–8x silicic acid (in supersaturated conditions of 8.3 mm), (ii) decelerates silica hydrolysis in non‐saturated conditions (1 mm) and (iii) enhances silica precipitation at 15–30x supersaturation (30 mm), forming a composite precipitate. Abundant cationic amino acids in P5S3 can interact with silica, though blocking experiments with selected anions also indicate a role of non‐electrostatic (e. g. amide‐silica) interactions. This shows how silaffin‐like peptides could potentially contribute to both keeping silica soluble, and forming peptide‐silica precipitates. The silaffin‐like cationic polypeptide P5S3 can play a triple role in stabilizing molybdate‐reactive silica, in enhancing colloidal silica precipitation and in decelerating silica dissolution. We attribute these functions to electrostatic and non‐electrostatic interactions of P5S3 with silica, and show that the peptide‐effect depends on the respective concentrations of P5S3 and silicic acid.