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Class of microalgae, found in the oceans, waterways and soils of the world / From Wikipedia, the free encyclopedia

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A diatom (Neo-Latin diatoma)[lower-alpha 1] is any member of a large group comprising several genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year,[10][11] take in over 6.7 billion tonnes of silicon each year from the waters in which they live,[12] and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile (800 m) deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara, much of it from the Bodélé Depression, which was once made up of a system of fresh-water lakes.[13][14]

Quick facts: Diatoms Temporal range Jurassic–Present Pre...
Temporal range: Jurassic–Present
Light microscopy of a sampling of marine diatoms found living between crystals of annual sea ice in Antarctica, showing a multiplicity of sizes and shapes
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Stramenopiles
Phylum: Gyrista
Subphylum: Ochrophytina
Infraphylum: Diatomista
Superclass: Khakista
Class: Bacillariophyceae
Dangeard, 1933[1]

Diatoms are unicellular organisms: they occur either as solitary cells or in colonies, which can take the shape of ribbons, fans, zigzags, or stars. Individual cells range in size from 2 to 200 micrometers.[15] In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles approximately every 24 hours by asexual multiple fission; the maximum life span of individual cells is about six days.[16] Diatoms have two distinct shapes: a few (centric diatoms) are radially symmetric, while most (pennate diatoms) are broadly bilaterally symmetric.

The unique feature of diatoms are that they are surrounded by a cell wall made of silica (hydrated silicon dioxide), called a frustule.[17] These frustules produce structural coloration, prompting them to be described as "jewels of the sea" and "living opals".

Movement in diatoms primarily occurs passively as a result of both ocean currents and wind-induced water turbulence; however, male gametes of centric diatoms have flagella, permitting active movement to seek female gametes. Similar to plants, diatoms convert light energy to chemical energy by photosynthesis, but their chloroplasts were acquired in different ways.[18]

Unusually for autotrophic organisms, diatoms possess a urea cycle, a feature that they share with animals, although this cycle is used to different metabolic ends in diatoms. The family Rhopalodiaceae also possess a cyanobacterial endosymbiont called a spheroid body. This endosymbiont has lost its photosynthetic properties, but has kept its ability to perform nitrogen fixation, allowing the diatom to fix atmospheric nitrogen.[19] Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera Hemiaulus, Rhizosolenia and Chaetoceros.[20]

Dinotoms are diatoms that have become endosymbionts inside dinoflagellates. Research on the dinoflagellates Durinskia baltica and Glenodinium foliaceum have shown that the endosymbiont event happened so recently, evolutionarily speaking, that their organelles and genome are still intact with minimum to no gene loss. The main difference between these and free living diatoms is that they have lost their cell wall of silica, making them the only known shell-less diatoms.[21]

The study of diatoms is a branch of phycology. Diatoms are classified as eukaryotes, organisms with a nuclear envelope-bound cell nucleus, that separates them from the prokaryotes archaea and bacteria. Diatoms are a type of plankton called phytoplankton, the most common of the plankton types. Diatoms also grow attached to benthic substrates, floating debris, and on macrophytes. They comprise an integral component of the periphyton community.[22] Another classification divides plankton into eight types based on size: in this scheme, diatoms are classed as microalgae. Several systems for classifying the individual diatom species exist.

Fossil evidence suggests that diatoms originated during or before the early Jurassic period, which was about 150 to 200 million years ago. The oldest fossil evidence for diatoms is a specimen of extant genus Hemiaulus in Late Jurassic aged amber from Thailand.[23]

Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatomaceous earth (diatomite) is a collection of diatom shells found in the earth's crust. They are soft, silica-containing sedimentary rocks which are easily crumbled into a fine powder and typically have a particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, and as a dynamite stabilizer.

Dwindling diatoms and the mixed layer
Earth's oceans teem with microscopic plants called phytoplankton. But according to a 2015 NASA study, populations of diatoms, the largest type of phytoplankton algae, have declined more than 1 percent per year from 1998 to 2012. Phytoplankton are an essential base of the marine food web and absorb carbon dioxide dissolved in the ocean that originally came from the atmosphere. The tiny organisms occupy the uppermost layer of ocean water, called the mixed layer, where waves and currents continually churn, drawing up nutrients from a deeper layer of water below. Scientists say the phytoplankton declines observed over the 15-year study period are due to the mixed layer becoming shallower, which results in fewer nutrients reaching the diatoms. The reduction in population may reduce the amount of carbon dioxide drawn out of the atmosphere and transferred to the deep ocean for long-term storage.[24][25]
3D-animation of the diatom Corethron sp.
Displays overlays from four fluorescent channels
(a) Green: [DiOC6(3) fluorescence] - stains cellular membranes indicating the core cell bodies
(b) Cyan: [PLL-A546 fluorescence] - generic counterstain for visualising eukaryotic cell surfaces
(c) Blue: [Hoechst fluorescence] - stains DNA, identifies nuclei
(d) Red: [chlorophyll autofluorescence] - resolves chloroplasts[26]
The animation starts by overlaying all available fluorescent channels, and then clarifies the visualisation by switching channels on and off