The silicified bipartite cell walls of diatoms (Bacillariophyceae) are produced in intracellular compartments by precipitation from supersaturated Si(OH)(4) and are then externalized. Fossil evidence of silicification is for marine, probably neritic, centric diatoms from approx. 120 Mya. Regardless of the initial selective significance of silicification, and of other current roles of silicification, the increased density resulting from silicification increases the sinking rate of cells; this can be partly or wholly offset by regulation of the protoplast solute content. Acclimatory and regulatory changes in silicification (relatively slow), and intracellular solute composition (relatively rapid), and intracellular solute composition (relatively rapid) of marine diatoms alter cell density over periods of hours to days. Density changes via changes in resource supply and, probably, parasitism, would move cells into optimal resource supply conditions, and remove parasitized, infective cells from surface populations of uninfected cells. Regulation of sinking rate could have been the first function of external or internal silica if the earliest silicified diatoms were planktonic.
Raven, J. A., & Waite, A. (2004). The evolution of silicification in diatoms: inescapable sinking and sinking as escape? New Phytologist, 162(1), 45-61. https://doi.org/10.1111/j.1469-8137.2004.01022.x