These plates were then exposed and the stronger the light, the more the silver iodide was converted back to silver metal. The iodine was then washed away leaving a positive image. So-called Daguerreotypes are now highly prized. Nuclear reactors produce radioactive iodine, and this was released in large amounts following the nuclear accident at Chernobyl, Russia, in Atom number: 53; atomic weight: Iodine is a black, shiny, non-metallic solid and a member of Group 17 of the Periodic Table.
It occurs naturally as a single, stable isotope, iodine, which is not radioactive. Site powered by Webvision Cloud. Skip to main content Skip to navigation. No comments. In this issue: iodine - a little goes a long way. Source: Thinkstock. Iodine cycles from land via erosion of rock, dissolved in water to the ocean, where it is taken up by marine organisms and released in various organic and inorganic forms, some of which are volatized into the atmosphere, where they react with ozone O 3 and other atmospheric gases and seed the particulate matter of clouds.
Movement of clouds over land re-deposits iodine dissolved in rainwater largely as iodate, IO 3 — and iodide, I — , where some is taken up by soil bacteria and other terrestrial organisms and released back into the soil and into ground water Jones and Truesdale ; Fuge and Johnson ; Truesdale and Jones ; Baker et al.
The six chemical building blocks of early life are usually considered to be hydrogen H , carbon C , oxygen O , nitrogen N , phosphorus P , and sulphur S , which combine in various ways to make up the essential sugars, fatty acids, amino acids, and nucleotides that form the basic constituents of metabolism Smith and Morowitz ; Falkowski et al.
Many of these elements are also important to the Earth's atmosphere e. Commoner ; Kasting and Siefert Researchers have attempted to explain how and why these basic elements came together in the earliest life forms e. Bada ; Griffiths ; Jalasvuori and Bamford , and while there is disagreement over whether the ability to replicate via RNA or generate energy via metabolism came first, or whether both arose at the same time before or after the advent of cell walls , such disputes are immaterial to this discussion.
The first early cells LUCA, Last Universal Common Ancestor almost certainly had a simple metabolism compared to modern forms, with less than a full complement of essential amino acids and perhaps a few non-specific enzymes remembering that enzymes are simply biochemical catalysts that are molecularly larger and more specific than inorganic catalysts. Aromatic amino acids phenylalanine, tyrosine, and tryptophan appear to have arisen some time after LUCA arose, as did more complex and specific enzymes Ahmad and Jensen While the actual incorporation of iodine into essential biochemistry appears to have come after the rise of LUCA Barrington , I suggest its influence probably started with LUCA itself.
To understand why this may be so, I review below the role of iodine in modern organisms and the atmosphere. Although it is not always clear exactly what iodine is used for biochemically, plants are known to take up iodine from water and store it, often as MIT, DIT, or THs Jones and Truesdale ; Eales Decaying vegetation absorbs and fixes even more iodine about 10 times more , so that soils rich in organic matter, such as peat, are known to be particularly rich in stored iodine Fuge and Johnson The question arises: how and why does iodine enter cells?
Many unicellular marine phytotplanktonic species e. While some iodine apparently gets transferred into the cytoplasm during this process, much appears to be released as iodide and various organic iodinated compounds, including methyl iodide, CH 3 I Baker et al. For some single-celled organisms, including most anaerobic bacteria, fungi, viruses, and yeasts, I 2 is toxic to the cell membrane McDonnell and Russell Just because some organisms are killed by I 2 does not necessarily mean they do not use iodine, only that they must get their iodine in another form.
For example, the Escherischia coli bacteria that inhabit rat intestines for which I 2 is lethal are known to bind significant amounts of host THs to their outer cell membranes and to excrete iodine in the form of iodide DiStefano et al. Certain ferric iron and sulphate-reducing anaerobic bacteria can also reduce iodate to iodide Councell et al. The fact that virtually all species are able to convert iodate to iodide and that iodide is known to enter cells in many of these suggests that iodine has always entered cell cytoplasm and participated in essential biochemistry.
While more research is clearly needed to unravel the precise roles that iodine and iodinated molecules play in cellular metabolism, if iodine has always been essential to cells then iodotyrosines may have a longer evolutionary history than previously thought. Iodotyrosines are known to form spontaneously Nishinaga ; Cahnmann and Funakoshi , although more slowly and with a lower yield than when catalyzed by enzymes, as occurs in vertebrate thyroid glands Hulbert Iodotyrosines are highly reactive with other molecules Harshman , suggesting they may have been one of the first and most crucial molecules effecting cell—cell signaling in multicellular organisms.
Iodide atoms at the sea surface enter the atmosphere as volatized I 2 and iodocarbons, which react readily with atmospheric oxygen. Volatized I 2 and iodine compounds not only break down ozone Wang et al. The incorporation of iodine into rain recycles it back to land, where it becomes available for uptake by terrestrial plants and soil bacteria Jones and Truesdale ; Fuge and Johnson ; Truesdale and Jones ; Amachi et al.
The intimate relationship between iodine used by unicellular marine organisms and atmospheric cycling almost certainly has a long evolutionary history. I suggest that iodine must have played a crucial role in the development and evolution of our atmosphere primarily due to its reactive relationship with oxygen.
Three to 4 billion years ago Ga , the Earth's atmosphere was very low in oxygen Falkowski et al. Since the by-product of photosynthesis is oxygen, it is presumed that the sudden increase in atmospheric oxygen was due to biological activity.
Since modern cyanobacteria and photosynthetic marine bacteria release significant amounts of iodine that subsequently enter the atmosphere, it is reasonable to suggest that in evolutionary terms, iodine became an important constituent of the atmosphere at the same time as oxygen.
This simultaneous rise of atmospheric oxygen and iodine appears to have been a turning point for the history of life on Earth, because only after iodine and oxygen became major constituents of the atmosphere did eukaryotes and multicellular organisms arise. I suggest that oxygen alone would not have done the trick. Iodine and oxygen became critical to multicellular life and the evolution of vertebrates e. Falkowski et al. As stated previously, tyrosine and other aromatic amino acids apparently arose some time after LUCA.
Phosphorylated tyrosines, which are critical cell-signaling molecules for all living metazoans and their closest protist relatives e. The genes for these enzymes are known to be evolutionarily ancient King ; Manning et al. Tyrosine in vertebrates is an essential precursor to melanin, catecholamines including neurohormones such as dopamine, epinephrine, and norepinephrine and of course, THs and all iodotyrosines.
Tyrosine has been described as uniquely reactive Harshman , especially with iodine. Tyrosine has thus been available and critical to cellular function for a very long time, although it was not present initially. So when and how did tyrosine enter the picture?
In all living organisms, tyrosine is synthesized from other compounds via an enzyme-mediated process e. I suggest, therefore, that early in its evolutionary history, tyrosine was initially generated by the catalytic actions of iodine in the cytoplasm of LUCA, as outlined below.
Oxidation reactions with iodate at surface of LUCA cells would have produced iodide, some of which probably entered the cytoplasm as I 2 through permeable cell walls Deamer ; Melkikh and Seleznev I 2 in the cytoplasm, acting as a catalyst, would have made new kinds of compounds available to the cell. One of those compounds may have been the amino-acid tyrosine. I suggest that some descendants of LUCA might have had the ability to generate tyrosine from an available sugar, with I 2 acting in its capacity as a metal-like catalyst, before the specialized enzymes now necessary for biosynthesis of this molecule had evolved Ahmad and Jensen ; Griffiths Catalysis of this sort has previously been proposed utilizing a little-known metal, montmorillonite to explain the origins of RNA Ferris Once tyrosine was present, the metabolic steps to photosynthesis could evolve and once tyrosine as a component of photosynthesis had become necessary for survival, non-specific enzymes could evolve into specialized ones capable of synthesizing tyrosine faster and more efficiently.
This kind of selective process has been used to explain the evolution of other specialized essential enzymes Gehring ; Badger et al. As tyrosine is especially reactive with iodine, some iodotyrosines probably formed spontaneously in these primitive cells. Iodotyrosines and iodoproteins are rather reactive themselves because of their iodine component; they bind to other molecules to make more complex compounds and can insert themselves into lipid membranes Harshman Iodotyrosines also catalyze other reactions and scavenge free radicals Oziol et al.
Such reactivity would have made iodotyrosines useful biochemically well before they were used as cell—cell signaling molecules in multicellular organisms. The first multicellular organisms appear to have needed two critical innovations: a way for cells to come together and a way for them to communicate.
It has been suggested, for example, that multicellularity might have arisen as a consequence of incomplete cell division, a development that would have created connections between cells, such as seen in some colonies of modern volvocine algae. Multicellular organisms might also have arisen subsequent to the production of extracellular matrices that bind cells together Sachs , as such matrices create unique opportunities for cell—cell communication Brodsky The essential biochemical machinery for effective cell—cell communication, it turns out, is found in mitochondria.
Some modern cyanobacteria that have both chloroplasts and mitochondria are known to form self-colonies as well as symbiotic colonies with fungi and yeast Badger et al. Since it is clear that such organisms probably possessed iodotyrosines the precursors of THs , it should come as no surprise that the primary sites of action for THs, at least in vertebrates, are in mitochondria.
Therefore, although this remains to be demonstrated, I suggest it is possible that iodinated tyrosines, as evolutionary and biochemical precursors of THs, may up-regulate the production of ATPase in non-vertebrate mitochondria. The most important sources of natural iodine are the oceans. About Much of it is deposited on land where it may become part of the biocycle. There are some iodine-containing minerals, such as alutarite, found in Chile and iodargyte, found in Colorado, Nevada and New Mexico.
World-wide industrial production of iodine is about Iodine is extracted from natural brines and oil brines, which have up to ppm of the element or form chilean nitrate deposits.
Known reserves of easily accessible iodine amount is around 2 million tonnes. Many medicines and cleansers for skin wounds contain iodine. Iodine is a building material of thyroid hormones that are essential for growth, the nervous system and the metabolism. Humans that eat little to no bread can experience iodine shortages. The function of the thyroid gland will than slow down and the thyroid gland will start swelling up. This phenomenon is called struma. This condition is rare now as table salt is dosed with a little iodide.
Large quantities of iodine can be dangerous because the thyroid gland will labour too hastily. This affects the entire body; it causes disturbed heartbeats and loss of weight. Iodine is classified as a halogen — a subset of very chemically reactive elements Group 17 on the periodic table that exist in the environment as compounds rather than as pure elements.
The other halogens include fluorine F , chlorine Cl , bromine Br and astatine At. The term halogen means "salt-producing. Iodine is the least reactive of the halogens as well as the most electropositive, meaning it tends to lose electrons and form positive ions during chemical reactions. It is also the heaviest and the least abundant of the stable halogens. There are 30 known isotopes of iodine, but only one is naturally occurring I Iodine has several commercial applications and can be found in a variety of pharmaceuticals, disinfectants, inks and dyes, catalysts, photography chemicals and animal feed supplements.
It plays a particularly prominent role in medicine. For example, iodine compounds are commonly used as sterilizing and wound-cleansing solutions and as internal contrasting agents in imaging techniques such as computed tomography CT scans, radiography and fluoroscopy. The radioactive isotope iodine is also used to treat cancer in the thyroid gland. About The remaining 0. Iodine is the 61st element in terms of abundance, making it not only one of the least abundant nonmetallic elements on Earth but also one of the rarest elements needed for life.
Although iodine is not particularly abundant, it can be found in trace amounts nearly everywhere: water, soil, rocks, plants, animals and humans.
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