Earth and Life by Andrew H. Knoll

Author:Andrew H. Knoll
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2026-01-10T00:00:00+00:00

The other main constituent of biomineralized skeletons is silica (SiO2). Some sponges fashion spicules of amorphous silica, but, otherwise, silica is at best a minor feature of animal biomineralization. In protists, on the other hand, the formation of siliceous scales and tests is common—indeed, more common in terms of phylogenetic distribution than calcium carbonate minerals. Radiolaria and diatoms are particularly important in the marine silica cycle (figure 12.2). Radiolaria, among the most diverse of marine protozoans, form complex skeletons of amorphous silica, fashioning shapes that range from starbursts to micro–Eiffel Towers. These structures serve to orient cells within the water column, facilitating feeding, and they may also help to defend against protistan predators. Diatoms are among the world’s major primary producers, accounting, as noted in chapter 4, for an estimated 20 percent of all photosynthesis on Earth. Found in marine environments, fresh water, and moist soils, diatoms fashion complex skeletal jewel boxes of amorphous silica that encompass the cell, providing protection, ballast in planktonic cells, and perhaps a defense against invading viruses.

Also, many plants secrete small wafers of amorphous silica called phytoliths within surface cells. It has been proposed that phytoliths constitute an energy-efficient way of strengthening plant surfaces, and the abundance of phytoliths in grasses and sedges further suggests that they provide a defense against herbivores. Consistent with this view, the fossil record shows that as grasslands spread across the Great Plains of North America 20–5 million years ago, the teeth of regional herbivores evolved ever-higher crowns, needed where phytolith-armored grasses wear away enamel.

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