Protist shell

Diatoms form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year,[12] take in over 6.7 billion metric tons of silicon each year from the waters in which they live,[13] and contribute nearly half of the organic material found in the oceans.

Diatoms are enclosed in protective silica (glass) shells called frustules. The beautifully engineered and intricate structure of many of these frustules is such that they are often referred to as "jewels of the sea".[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.[16]

Diatoms uses silicon in the biogenic silica (BSIO₂) form,[17] which is taken up by the silicon transport protein to be predominantly used in constructing these protective cell wall structures.[18] Silicon enters the ocean in a dissolved form such as silicic acid or silicate.[19] Since diatoms are one of the main users of these forms of silicon, they contribute greatly to the concentration of silicon throughout the ocean. Silicon forms a nutrient-like profile in the ocean due to the diatom productivity in shallow depths, which means there is less concentration of silicon in the upper ocean and more concentration of silicon in the deep ocean.[19]

Diatom productivity in the upper ocean contribute to the amount of silicon exported to the lower ocean.[20] When diatom cells are lysed in the upper ocean, their nutrients like, iron, zinc, and silicon, are brought to the lower ocean through a process called marine snow. Marine snow involves the downward transfer of particulate organic matter by vertical mixing of dissolved organic matter.[21] Availability of silicon appears crucial for diatom productivity, and as long as silicic acid is available for diatoms to utilize, the diatoms contribute other important nutrient concentrations in the deep ocean.[22]

In coastal zones, diatoms serve as the major phytoplanktonic organisms and greatly contribute to biogenic silica production. In the open ocean, however, diatoms have a reduced role in global annual silica production. Diatoms in North Atlantic and North Pacific subtropical gyres contribute only about 6% of global annual marine silica production, while the Southern Ocean produces about one-third of the global marine biogenic silica.[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 shapes

          Drawings by Haeckel 1904 (click for details)

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  Diatoms are one of the most common types of phytoplankton
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  Their protective shells (frustles) are made of silicon
• 
• 
  They come in many shapes and sizes

Diatoms

Centric

Pennate

Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry

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  Silicified frustule of a pennate diatom with two overlapping halves
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  Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[25]
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  Fossil diatom
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  There are over 100,000 species of diatoms which account for 50% of the ocean's primary production

Different diatom frustule shapes and sizes

Structure of a centric diatom frustule [26]

Diatoms

Diatoms, major components of marine plankton, have silica skeletons called frustules. "The microscopic structures of diatoms help them manipulate light, leading to hopes they could be used in new technologies for light detection, computing or robotics.[27]

SEM images of pores in diatom frustules[28]

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. The evolutionary causes for the generation of nano and microstructured silica by photosynthetic algae are not yet clear. However, in 2018 it was shown that reflection 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]

Triparma laevis and a drawing of its silicate shell, scale bar = 1 μm.

Exploded drawing of the shell, D = dorsal plate, G = girdle plate, S = shield plate and V = ventral plate.

Triparma laevis belongs to the Bolidophyceae, a sister taxon to the diatoms.[30][31]

External video
Diatoms: Tiny factories you can see from space
How diatoms build their beautiful shells – Journey to the Microcosmos

Radiolarian shapes

          Drawings by Haeckel 1904 (click for details)

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.[32]

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  Like diatoms, radiolarians come in many shapes
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  Also like diatoms, radiolarian shells are usually made of silicate
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  However acantharian radiolarians have shells made from strontium sulfate crystals

An animation of the diversity of radiolarian shells  [33] (click to activate)

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  Cutaway schematic diagram of a spherical radiolarian shell
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  Cladococcus abietinus

Fossil radiolarian

X-ray microtomography of Triplococcus acanthicus

This is a microfossil from the Middle Ordovician with four nested spheres. The innermost sphere is highlighted red. Each segment is shown at the same scale.[34]

Turing and radiolarian morphology

Shell of a spherical radiolarian

Shell micrographs

Computer simulations of Turing patterns on a sphere
closely replicate some radiolarian shell patterns[35]

External video
Radiolarian geometry
Ernst Haeckel's radiolarian engravings

Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell called a coccosphere. Coccospheres are covered with ornate circular plates or scales called coccoliths. The coccoliths are made from calcium carbonate. The term coccolithophore derives from the Greek for a seed carrying stone, referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white.[36]

• 
• 
  Coccolithophores named after the BBC documentary series
  The Blue Planet
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  The coccolithophore Emiliania huxleyi

Coccolithophores

...have plates called coccoliths

...extinct fossil

Coccolithophores build calcite skeletons important to the marine carbon cycle[37]

There are benefits for protists that carry protective shells. The diagram on the left below shows some benefits coccolithophore get from carrying coccoliths. 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. (B) represents protection from photodamage including sunshade protection from ultraviolet light (UV) and photosynthetic active radiation (PAR) and energy dissipation under high-light conditions. (C) represents armour protection includes protection against viral/bacterial infections and grazing by selective and nonselective grazers.[38]

Benefits of having shells

Benefits in coccolithophore calcification [38] – see text above

Costs of having shells

Energetic costs in coccolithophore calcification [38]

There are also costs for protists that carry protective shells. The diagram on the right above shows some of the energetic costs coccolithophore incur from carrying coccoliths. In the diagram, the energetic costs are reported in percentage of total photosynthetic budget. (A) represents transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca₂+ and HCO₃− (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca₂+ through the cytoplasm to the coccolith vesicle (CV) is the dominant cost associated with calcification. (B) represents metabolic processes include the synthesis of coccolith-associated polysaccharides (CAPs – gray rectangles) by the Golgi complex (white rectangles) that regulate the nucleation and geometry of CaCO₃ crystals. The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO₃ crystals. (C) Mechanical and structural processes account for the secretion of the completed coccoliths that are transported from their original position adjacent to the nucleus to the cell periphery, where they are transferred to the surface of the cell.[38]

Foraminiferan shapes

          Drawings by Haeckel 1904 (click for details)

Like radiolarians, foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, may be single-chambered or multi-chambered; multi-chambered forams add more chambers as they grow. The most famous of these are made of calcite, but tests may also be made of aragonite, agglutinated sediment particles, chiton, or (rarely) of silica. Most forams are benthic, but about 40 living species are planktic.[39] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.[32] Some foraminifera lack tests altogether.[40]

Foraminiferans

Empty Foraminiferan test, showing multiple chambers and pores

...and in life, showing pseudopodia streaming from pores

Foraminiferans are important unicellular zooplankton protists, often with calcite tests

External video
foraminiferans
Foraminiferal networks and growth

Benthic foraminifera Favulina hexagona, together with nanofossils enclosed inside the shell hexagons [41]

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  section showing chambers of a spiral foram
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  Live Ammonia tepida streaming granular ectoplasm for catching food
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  Group of planktonic forams
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  Fossil nummulitid forams of various sizes from the Eocene
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  The Egyptian pyramids were constructed from limestone that contained nummulites.[42]