Many protists have protective shells or tests,[2] usually made from silica (glass) or calcium carbonate (chalk).

Their numbers are such that their shells play a huge part in the formation of ocean sediments and in the global cycling of elements and nutrients.

Protists such as diatoms and radiolaria have intricate, glass-like shells made of silica that are hard and protective, and serve as a barrier to prevent water loss.

Cellular life likely originated as single-celled prokaryotes (including modern bacteria and archaea) and later evolved into more complex eukaryotes.

Nonetheless, the term continues to be used informally to refer to those eukaryotes that cannot be classified as plants, fungi or animals.

[3][4] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered.

[14] Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes.

[15] The frustules of dead diatoms drift to the ocean floor where, over millions of years, they can build up as much as half a mile deep.

[22] In coastal zones, diatoms serve as the major phytoplanktonic organisms and greatly contribute to biogenic silica production.

[23] The Southern Ocean is referred to as having a "biogeochemical divide", since only minuscule amounts of silicon is transported out of this region.

[24] Diatom frustules have been accumulating for over 100 million years, leaving rich deposits of nano and microstructured silicon oxide in the form of diatomaceous earth around the globe.

However, in 2018 it was shown that absorption of ultraviolet light by nanostructured silica protects the DNA in the algal cells, and this may be an evolutionary cause for the formation of the glass cages.

[28][29] Radiolarians are unicellular predatory protists encased in elaborate globular shells (or "capsules"), usually made of silica and pierced with holes.

In the diagram, (A) represents accelerated photosynthesis including carbon concentrating mechanisms (CCM) and enhanced light uptake via scattering of scarce photons for deep-dwelling species.

The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO3 crystals.

[39] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.

Many choanoflagellates build complex basket-shaped "houses", called lorica, from several silica strips cemented together.

Fossils which can be studied by the naked eye or low-powered magnification, such as a hand lens, are referred to as macrofossils.

While every kingdom of life is represented in the microfossil record, the most abundant forms are protist skeletons or cysts from the Chrysophyta, Pyrrhophyta, Sarcodina, acritarchs and chitinozoans, together with pollen and spores from the vascular plants.