text: In aquatic environments (rivers, lakes and ocean), dissolved Si is utilized by diatoms, dictyochales, radiolarians and sponges to produce solid bSiO structures. The biomineralized silica has an amorphous structure and therefore its properties may vary among the different organisms. Biomineralization by diatoms induces the largest Si flux within the ocean, and thus it has a crucial role in the global Si cycle. During Si uptake by diatoms, there is an isotopic discrimination against the heavy isotope, forming Si-depleted biogenic silica minerals. As a result, the remaining dissolved Si in the surrounding water is Si-enriched. Since diatoms rely on sunlight for photosynthesis, they inhabit in surface waters, and thus the surface water of the ocean are typically Si-enriched. Although there is less available data on the isotopic fractionation during biomineralization by radiolarians, it has been suggested that radiolarians also discriminate against the heavy isotope (Si), and that the magnitude of isotopic fractionation is of a similar range as biomineralization by diatoms. Sponges also show an isotopic preference for Si over Si, but the magnitude of their isotopic fractionation is often larger (For quantitative comparation, see Figure 2).