Although silicon is readily available in the form of
silicates, very few organisms use it directly.
Diatoms,
radiolaria, and
siliceous sponges use
biogenic silica as a structural material for their skeletons. In more advanced plants, the silica
phytoliths (opal phytoliths) are rigid microscopic bodies occurring in the cell; some plants, including
rice, need silica for their growth. Silica has been shown to improve plant cell wall strength and structural integrity in some plants.
Diatoms Diatoms form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 per cent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of
silicon each year from the waters in which they live, 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". Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes. 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. Diatoms uses silicon in the
biogenic silica (BSiO2) form, which is taken up by the silicon
transport protein to be predominantly used in constructing these protective cell wall structures. Silicon enters the ocean in a dissolved form such as
silicic acid or
silicate. 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. 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. 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. 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. The Southern Ocean is referred to as having a "biogeochemical divide", since only minuscule amounts of silicon is transported out of this region. File:Diatoms (248 05) Various diatoms.jpg|
Diatoms are one of the most common types of phytoplankton File:Diatom Helipelta metil.jpg|Their protective shells (frustles) are made of silicon File:Diatom - Triceratium favus.jpg File:Diatom2.jpg|They come in many shapes and sizes File:Diatom algae Amphora sp.jpg|Silicified frustule of a pennate diatom with two overlapping halves File:Fjouenne sbrmvr012w 20070924163039 small.jpg|
Guinardia delicatula, a diatom responsible for
algal blooms in the North Sea and the English Channel File:Ископаемая диатомовая водоросль.jpg|Fossil diatom File:Pinnularia major.jpg|There are over 100,000 species of
diatoms which account for 50% of the ocean's primary production 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 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.
Radiolarians Radiolarians are unicellular predatory
protists encased in elaborate globular shells (or "capsules"), 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. File:Mikrofoto.de-Radiolarien 6.jpg|Like diatoms, radiolarians come in many shapes File:Theocotylissa ficus Ehrenberg - Radiolarian (34638920262).jpg|Also like diatoms, radiolarian shells are usually made of silicate File:Acantharian radiolarian Xiphacantha (Haeckel).jpg|However
acantharian radiolarians have shells made from
strontium sulfate crystals File:Spherical radiolarian 2.jpg|Cutaway schematic diagram of a spherical radiolarian shell File:Cladococcus abietinus.jpg|
Cladococcus abietinus ==Calcium-based shells==