Silica

Silica is only really required by diatoms (excepting the rarely cultured silicoflagellates) and is therefore often left out of media which are selective for other organisms. Diatoms only need trace quantities of silica because they are extremely efficient at scouring silica from the environment. Some diatoms may grow for many generations in seawater media which has no added silica and some may continue to divide for several weeks, albeit with poor frustule development.  Media such as GSe where the only added silica, apart from the seawater, comes from soil extract may readily support the growth of  some diatom species.  Enrichment cultures for isolating microalgae devoid of added silicate may still become rich in diatoms therefore if selection is for non-diatom species a diatom inhibitor is advantageous. Germanium dioxide (GeO₂) is toxic to diatoms because it disrupts silica deposition and the addition of low concentrations (5 mg/L) of GeO₂ to a culture medium can inhibit diatom growth.

The silica stock may precipitate if made up in glass bottles so teflon bottles can be used instead. Precipitation is a greater problem when the stock is first added to seawater especially if autoclaved and in glass containers. However it will slowly disassociate and be available for uptake. Polycarbonate carboys for larger volumes or teflon bottles (1 L) may be used to limit the precipitation if it is problematic, noting that teflon is especially expensive.