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Many protists have protective shells or tests, usually made from silica (glass) or calcium carbonate (chalk). are a diverse group of eukaryote organisms that are not plants, animals, or fungi. They are typically microscopic unicellular organisms that live in water or moist environments.
Protists shells are often tough, mineralised forms that resist degradation, and can survive the death of the protist as a microfossil. Although protists are typically very small, they are ubiquitous. Their numbers are such that their shells play a huge part in the formation of and in the global cycling of elements and nutrients.
The role of protist shells depends on the type of protist. Protists such as and radiolaria have intricate, glass-like shells made of silica that are hard and protective, and serve as a barrier to prevent water loss. The shells have small pores that allow for gas exchange and nutrient uptake. and foraminifera also have hard protective shells, but the shells are made of calcium carbonate. These shells can help with buoyancy, allowing the organisms to float in the water column and move around more easily.
In addition to protection and support, protist shells also serve scientists as a means of identification. By examining the characteristics of the shells, different species of protists can be identified and their ecology and evolution can be studied.
The term protist came into use historically to refer to a group of biologically similar organisms; however, modern research has shown it to be a Paraphyly group that does not contain all descendants of a common ancestor. As such it does not constitute a clade and is not currently in formal scientific use. Nonetheless, the term continues to be used informally to refer to those that cannot be classified as plants, fungi or animals.
Most protists are too small to be seen with the naked eye. They are highly diverse organisms currently organised into 18 phyla, but are not easy to classify. 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. As eukaryotes, protists possess within their cell at least one Cell nucleus, as well as such as mitochondria and Golgi bodies. Many protists are asexual but can reproduce rapidly through mitosis or by fragmentation; others (including foraminifera) may reproduce either sexually or asexually. (1982). 9780910249058, University of Tennessee, Dept. of Geological Sciences. ISBN 9780910249058
In contrast to the cells of bacteria and archaea, the cells of protists and other eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic, but there are exceptions, and some are neither single-celled nor microscopic, such as seaweed.
Diatoms are enclosed in protective silica (glass) shells called . The beautifully engineered and intricate structure of many of these frustules is such that they are often referred to as "jewels of the sea".Ireland, T., "Engineering with algae". Biologist, 63(5): 10. Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes.Wassilieff, Maggy (2006) "Plankton - Plant plankton", Te Ara - the Encyclopedia of New Zealand. Accessed: 2 November 2019. 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.
Diatom productivity in the upper ocean contribute to the amount of silicon exported to the lower ocean. When diatom cells are Lysis 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.
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.De Tommasi, E., Congestri, R., Dardano, P., De Luca, A.C., Managò, S., Rea, I. and De Stefano, M. (2018) "UV-shielding and wavelength conversion by centric diatom nanopatterned frustules". Scientific Reports, 8(1): 1–14. . Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
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.Monteiro, F.M., Bach, L.T., Brownlee, C., Bown, P., Rickaby, R.E., Poulton, A.J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S. and Gutowska, M.A. (2016) "Why marine phytoplankton calcify". Science Advances, 2(7): e1501822. . Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
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 (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.
Microfossils are a common feature of the geological record, from the Precambrian to the Holocene. They are most common in , but also occur in brackish water, fresh water and terrestrial sedimentary deposits. While every kingdom of prehistoric life is represented in the microfossil record, the most abundant forms are protist skeletons or Microbial cyst from the Chrysophyta, Pyrrhophyta, Sarcodina, and , together with pollen and spores from the .
In 2017, fossilized , or microfossils, were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an almost instantaneous emergence of life" (in a geological time-scale sense), after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.
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