In nature, there is a wide array of biominerals, ranging from iron oxide to strontium sulfate, with calcareous biominerals being particularly notable. However, the most taxonomically widespread biomineral is silica (SiO2·nH2O), being present in all eukaryotic supergroups. Notwithstanding, the degree of silicification can vary even between closely related taxa, from being found in composite structures with other biominerals (e.g., limpet teeth; to forming minor structures (e.g., ciliate granules; or being a major structural constituent of the organism. The most extreme degree of silicification is evident in the diatoms, where almost all species have an obligate requirement for silicon to complete cell wall formation and cell division. Biogeochemically and ecologically, diatoms are the most important silicifiers in modern marine ecosystems, with radiolarians (polycystine and phaeodarian rhizarians), silicoflagellates (dictyochophyte and chrysophyte stramenopiles), and sponges with prominent roles as well. In contrast, the major silicifiers in terrestrial ecosystems are the land plants (embryophytes), with other silicifying groups (e.g., testate amoebae) having a minor role.