Sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula S. Elemental sulfur is a bright yellow, crystalline solid at room temperature. Sulfur is the tenth most abundant element by mass in the universe and the fifth most on Earth. Though sometimes found in pure, native form, sulfur on Earth usually occurs as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in ancient times, being mentioned for its  uses in ancient India, ancient Greece, China, and ancient Egypt. Historically and in literature sulfur is also called brimstone, which means "burning stone". Today, almost all elemental sulfur is produced as a byproduct of removing sulfur-containing contaminants from natural gas and petroleum. The greatest commercial use of the element is the production of sulfuric acid for sulfate and phosphate fertilizers, and other chemical processes. Sulfur is used in matches, insecticides, and fungicides. Many sulfur compounds are odoriferous, and the smells of odorized natural gas, skunk scent, grapefruit, and garlic are due to organosulfur compounds. Hydrogen sulfide gives the characteristic  odor to rotting eggs and other biological processes. Sulfur is an essential element for all life, but almost always in the form of organosulfur compounds or metal sulfides. Three amino acids (cysteine, cystine, and methionine) and two vitamins (biotin and thiamine) are organosulfur compounds. Many cofactors also contain sulfur, including glutathione, thioredoxin, and iron–sulfur proteins. Disulfides, S–S bonds, confer mechanical strength and insolubility of the protein keratin, found in outer skin, hair, and feathers. Sulfur is one of the core chemical elements needed for biochemical functioning and is an elemental macronutrient for all living organisms. Characteristics. Physical properties . Sulfur forms several polyatomic molecules. The best-known allotrope is octasulfur, cyclo-S. The point group of cyclo-S is D and its dipole moment is 0 D. Octasulfur is a soft, bright-yellow solid that is odorless, but impure samples have an odor similar to that of matches. It melts at , boils at and sublimates easily. At , below its melting temperature, cyclo-octasulfur changes from α-octasulfur to the β-polymorph. The structure of the S ring is virtually unchanged by this phase change, which affects the intermolecular interactions. Between its melting and boiling temperatures, octasulfur changes its allotrope  again, turning from β-octasulfur to γ-sulfur, again accompanied by a lower density but increased viscosity due to the formation of polymers. At higher temperatures, the viscosity decreases as depolymerization occurs. Molten sulfur assumes a dark red color above . The density of sulfur is about 2 g/cm, depending on the allotrope; all of the stable allotropes are excellent electrical insulators. Chemical properties. Sulfur burns with a blue flame with formation of sulfur dioxide, which has a suffocating and irritating odor. Sulfur is insoluble in water but soluble in carbon disulfide and, to a lesser extent, in other nonpolar  organic solvents, such as benzene and toluene. The first and second ionization energies of sulfur are 999.6 and 2252 kJ/mol, respectively. Despite such figures, the +2 oxidation state is rare, with +4 and +6 being more common. The fourth and sixth ionization energies are 4556 and 8495.8 kJ/mol, the magnitude of the figures caused by electron transfer between orbitals; these states are only stable with strong oxidants such as fluorine, oxygen, and chlorine. Sulfur reacts with nearly all other elements with the exception of the noble gases, even with the notoriously unreactive metal iridium (yielding iridium disulfide). Some  of those reactions need elevated temperatures. Isotopes. Sulfur has 23 known isotopes, four of which are stable: S (), S (), S (), and S (). Other than S, with a half-life of 87 days and formed in cosmic ray spallation of Ar, the radioactive isotopes of sulfur have half-lives less than 3 hours. When sulfide minerals are precipitated, isotopic equilibration among solids and liquid may cause small differences in the δS values of co-genetic minerals. The differences between minerals can be used to estimate the temperature of equilibration. The δC and δS of coexisting carbonate minerals and sulfides can be  used to determine the pH and oxygen fugacity of the ore-bearing fluid during ore formation. In most forest ecosystems, sulfate is derived mostly from the atmosphere; weathering of ore minerals and evaporites contribute some sulfur. Sulfur with a distinctive isotopic composition has been used to identify pollution sources, and enriched sulfur has been added as a tracer in hydrologic studies. Differences in the natural abundances can be used in systems where there is sufficient variation in the S of ecosystem components. Rocky Mountain lakes thought to be dominated by atmospheric sources of sulfate have been found to have characteristic  S values from lakes believed to be dominated by watershed sources of sulfate. Natural occurrence. S is created inside massive stars, at a depth where the temperature exceeds 2.5×10 K, by the fusion of one nucleus of silicon plus one nucleus of helium. As this nuclear reaction is part of the alpha process that produces elements in abundance, sulfur is the 10th most common element in the universe. Sulfur, usually as sulfide, is present in many types of meteorites. Ordinary chondrites contain on average 2.1% sulfur, and carbonaceous chondrites may contain as much as 6.6%. It is normally present as  troilite (FeS), but there are exceptions, with carbonaceous chondrites containing free sulfur, sulfates and other sulfur compounds. The distinctive colors of Jupiter's volcanic moon Io are attributed to various forms of molten, solid, and gaseous sulfur. It is the fifth most common element by mass in the Earth. Elemental sulfur can be found near hot springs and volcanic regions in many parts of the world, especially along the Pacific Ring of Fire; such volcanic deposits are currently mined in Indonesia, Chile, and Japan. These deposits are polycrystalline, with the largest documented single crystal measuring 22×16×11 cm. Historically, Sicily was  a major source of sulfur in the Industrial Revolution. Lakes of molten sulfur up to ~200 m in diameter have been found on the sea floor, associated with submarine volcanoes, at depths where the boiling point of water is higher than the melting point of sulfur. Native sulfur is synthesised by anaerobic bacteria acting on sulfate minerals such as gypsum in salt domes. Significant deposits in salt domes occur along the coast of the Gulf of Mexico, and in evaporites in eastern Europe and western Asia. Native sulfur may be produced by geological processes alone. Fossil-based sulfur deposits from  salt domes were once the basis for commercial production in the United States, Russia, Turkmenistan, and Ukraine. Currently, commercial production is still carried out in the Osiek mine in Poland. Such sources are now of secondary commercial importance, and most are no longer worked. Common naturally occurring sulfur compounds include the sulfide minerals, such as pyrite (iron sulfide), cinnabar (mercury sulfide), galena (lead sulfide), sphalerite (zinc sulfide), and stibnite (antimony sulfide); and the sulfate minerals, such as gypsum (calcium sulfate), alunite (potassium aluminium sulfate), and barite (barium sulfate). On Earth, just as upon Jupiter's moon Io, elemental sulfur occurs  naturally in volcanic emissions, including emissions from hydrothermal vents. The main industrial source of sulfur is now petroleum and natural gas. Compounds. Common oxidation states of sulfur range from −2 to +6. Sulfur forms stable compounds with all elements except the noble gases. Allotropes. Sulfur forms over 30 solid allotropes, more than any other element. Besides S, several other rings are known. Removing one atom from the crown gives S, which is more of a deep yellow than the S. HPLC analysis of "elemental sulfur" reveals an equilibrium mixture of mainly S, but with S and small amounts of S.  Larger rings have been prepared, including S and S. Amorphous or "plastic" sulfur is produced by rapid cooling of molten sulfur—for example, by pouring it into cold water. X-ray crystallography studies show that the amorphous form may have a helical structure with eight atoms per turn. The long coiled polymeric molecules make the brownish substance elastic, and in bulk this form has the feel of crude rubber. This form is metastable at room temperature and gradually reverts to crystalline molecular allotrope, which is no longer elastic. This process happens within a matter of hours to days, but can  be rapidly catalyzed. Polycations and polyanions. Sulfur polycations, S, S and S are produced when sulfur is reacted with mild oxidising agents in a strongly acidic solution. The colored solutions produced by dissolving sulfur in oleum were first reported as early as 1804 by C.F. Bucholz, but the cause of the color and the structure of the polycations involved was only determined in the late 1960s. S is deep blue, S is yellow and S is red. The radical anion S gives the blue color of the mineral lapis lazuli. Sulfides. Treatment of sulfur with hydrogen gives hydrogen sulfide. When  dissolved in water, hydrogen sulfide is mildly acidic: Hydrogen sulfide gas and the hydrosulfide anion are extremely toxic to mammals, due to their inhibition of the oxygen-carrying capacity of hemoglobin and certain cytochromes in a manner analogous to cyanide and azide (see below, under "precautions"). Reduction of elemental sulfur gives polysulfides, which consist of chains of sulfur atoms terminated with S centers: This reaction highlights a distinctive property of sulfur: its ability to catenate (bind to itself by formation of chains). Protonation of these polysulfide anions produces the polysulfanes, HS where x= 2, 3, and 4. Ultimately, reduction of  sulfur produces sulfide salts: The interconversion of these species is exploited in the sodium–sulfur battery. Oxides, oxoacids, and oxoanions. The principal sulfur oxides are obtained by burning sulfur: Multiple sulfur oxides are known; the sulfur-rich oxides include sulfur monoxide, disulfur monoxide, disulfur dioxides, and higher oxides containing peroxo groups. Sulfur forms sulfur oxoacids, some of which cannot be isolated and are only known through the salts. Sulfur dioxide and sulfites () are related to the unstable sulfurous acid (HSO). Sulfur trioxide and sulfates () are related to sulfuric acid (HSO). Sulfuric acid and SO combine to give oleum, a  solution of pyrosulfuric acid (HSO) in sulfuric acid. Thiosulfate salts (), sometimes referred as "hyposulfites", used in photographic fixing (hypo) and as reducing agents, feature sulfur in two oxidation states. Sodium dithionite (), contains the more highly reducing dithionite anion (). Halides and oxyhalides. Several sulfur halides are important to modern industry. Sulfur hexafluoride is a dense gas used as an insulator gas in high voltage transformers; it is also a nonreactive and nontoxic propellant for pressurized containers. Sulfur tetrafluoride is a rarely-used organic reagent that is highly toxic. Sulfur dichloride and disulfur dichloride are important industrial chemicals. Sulfuryl chloride and chlorosulfuric  acid are derivatives of sulfuric acid; thionyl chloride (SOCl) is a common reagent in organic synthesis. Pnictides. An important S–N compound is the cage tetrasulfur tetranitride (). Heating this compound gives polymeric sulfur nitride (), which has metallic properties even though it does not contain any metal atoms. Thiocyanates contain the group. Oxidation of thiocyanate gives thiocyanogen, with the connectivity . Phosphorus sulfides are numerous, the most important commercially being the cages and . Metal sulfides. The principal ores of copper, zinc, nickel, cobalt, molybdenum, and other metals are sulfides. These materials tend to be dark-colored semiconductors that are not readily attacked  by water or even many acids. They are formed, both geochemically and in the laboratory, by the reaction of hydrogen sulfide with metal salts. The mineral galena (PbS) was the first demonstrated semiconductor and was used as a signal rectifier in the cat's whiskers of early crystal radios. The iron sulfide called pyrite, the so-called "fool's gold", has the formula FeS. Processing these ores, usually by roasting, is costly and environmentally hazardous. Sulfur corrodes many metals through tarnishing. Organic compounds. Some of the main classes of sulfur-containing organic compounds include the following: Compounds with carbon–sulfur  multiple bonds are uncommon, an exception being carbon disulfide, a volatile colorless liquid that is structurally similar to carbon dioxide. It is used as a reagent to make the polymer rayon and many organosulfur compounds. Unlike carbon monoxide, carbon monosulfide is stable only as an extremely dilute gas, found between solar systems. Organosulfur compounds are responsible for some of the unpleasant odors of decaying organic matter. They are widely known as the odorant in domestic natural gas, garlic odor, and skunk spray. Not all organic sulfur compounds smell unpleasant at all concentrations: the sulfur-containing monoterpenoid (grapefruit mercaptan) in small  concentrations is the characteristic scent of grapefruit, but has a generic thiol odor at larger concentrations. Sulfur mustard, a potent vesicant, was used in World War I as a disabling agent. Sulfur–sulfur bonds are a structural component used to stiffen rubber, similar to the disulfide bridges that rigidify proteins (see biological below). In the most common type of industrial "curing" or hardening and strengthening of natural rubber, elemental sulfur is heated with the rubber to the point that chemical reactions form disulfide bridges between isoprene units of the polymer. This process, patented in 1843, made rubber a major industrial  product, especially in automobile tires. Because of the heat and sulfur, the process was named vulcanization, after the Roman god of the forge and volcanism. History. Antiquity. Being abundantly available in native form, sulfur was known in ancient times and is referred to in the Torah (Genesis). English translations of the Christian Bible commonly referred to burning sulfur as "brimstone", giving rise to the term "fire-and-brimstone" sermons, in which listeners are reminded of the fate of eternal damnation that await the unbelieving and unrepentant. It is from this part of the Bible that Hell is implied to "smell  of sulfur" (likely due to its association with volcanic activity). According to the Ebers Papyrus, a sulfur ointment was used in ancient Egypt to treat granular eyelids. Sulfur was used for fumigation in preclassical Greece; this is mentioned in the "Odyssey". Pliny the Elder discusses sulfur in book 35 of his "Natural History", saying that its best-known source is the island of Melos. He mentions its use for fumigation, medicine, and bleaching cloth. A natural form of sulfur known as () was known in China since the 6th century BC and found in Hanzhong. By the 3rd century, the Chinese  had discovered that sulfur could be extracted from pyrite. Chinese Daoists were interested in sulfur's flammability and its reactivity with certain metals, yet its earliest practical uses were found in traditional Chinese medicine. A Song dynasty military treatise of 1044 AD described various formulas for Chinese black powder, which is a mixture of potassium nitrate (), charcoal, and sulfur. It remains an ingredient of black gunpowder. Indian alchemists, practitioners of the "science of chemicals" (), wrote extensively about the use of sulfur in alchemical operations with mercury, from the eighth century AD onwards. In the tradition, sulfur is called "the smelly  (, ). Early European alchemists gave sulfur a unique alchemical symbol, a triangle at the top of a cross (🜍). The astrological symbol for 2 Pallas, a crossed spear (⚴), has been published in various forms, including one that looks like the symbol for sulfur. In traditional skin treatment, elemental sulfur was used (mainly in creams) to alleviate such conditions as scabies, ringworm, psoriasis, eczema, and acne. The mechanism of action is unknown—though elemental sulfur does oxidize slowly to sulfurous acid, which is (through the action of sulfite) a mild reducing and antibacterial agent. Modern times. Sulfur appears in a column 	Sulfur
23154287	of fixed (non-acidic) alkali in a chemical table of 1718. Antoine Lavoisier used sulfur in combustion experiments, writing of some of these in 1777. Sulfur deposits in Sicily were the dominant source for more than a century. By the late 18th century, about 2,000 tonnes per year of sulfur were imported into Marseille, France, for the production of sulfuric acid for use in the Leblanc process. In industrializing Britain, with the repeal of tariffs on salt in 1824, demand for sulfur from Sicily surged upward. The increasing British control and exploitation of the mining, refining, and transportation of the 	Sulfur
23154288	sulfur, coupled with the failure of this lucrative export to transform Sicily's backward and impoverished economy, led to the Sulfur Crisis of 1840, when King Ferdinand II gave a monopoly of the sulfur industry to a French firm, violating an earlier 1816 trade agreement with Britain. A peaceful solution was eventually negotiated by France. In 1867, elemental sulfur was discovered in underground deposits in Louisiana and Texas. The highly successful Frasch process was developed to extract this resource. In the late 18th century, furniture makers used molten sulfur to produce decorative inlays. Molten sulfur is sometimes still used for 	Sulfur
23154289	setting steel bolts into drilled concrete holes where high shock resistance is desired for floor-mounted equipment attachment points. Pure powdered sulfur was used as a medicinal tonic and laxative. With the advent of the contact process, the majority of sulfur today is used to make sulfuric acid for a wide range of uses, particularly fertilizer. In recent times, the main source of sulfur has become petroleum and natural gas. This is due to the requirement to remove sulfur from fuels in order to prevent acid rain, and has resulted in a surplus of sulfur. Spelling and etymology. Sulfur" is  derived from the Latin word ', which was Hellenized to ' in the erroneous belief that the Latin word came from Greek. This spelling was later reinterpreted as representing an /f/ sound and resulted in the spelling ', which appears in Latin toward the end of the Classical period. The true Greek word for sulfur, , is the source of the international chemical prefix "thio-". In 12th-century Anglo-French, it was '. In the 14th century, the erroneously Hellenized Latin ' was restored in Middle English '. By the 15th century, both full Latin spelling variants "sulfur" and "sulphur" became common in English. The parallel "f ~ph" spellings continued in Britain until the 19th century, when the word was standardized as "sulphur". On the other hand, "sulfur" was the form chosen in the United States, whereas Canada uses both. The IUPAC adopted the spelling "sulfur" in 1990 or 1971, depending on the source cited, as did the Nomenclature Committee of the Royal Society of Chemistry in 1992, restoring the spelling "sulfur" to Britain. Oxford Dictionaries note that "in chemistry and other technical uses ... the "-f-" spelling is now the standard form for this and related words in British as well as US contexts, and is  increasingly used in general contexts as well." Production. Sulfur may be found by itself and historically was usually obtained in this form; pyrite has also been a source of sulfur. In volcanic regions in Sicily, in ancient times, it was found on the surface of the Earth, and the "Sicilian process" was used: sulfur deposits were piled and stacked in brick kilns built on sloping hillsides, with airspaces between them. Then, some sulfur was pulverized, spread over the stacked ore and ignited, causing the free sulfur to melt down the hills. Eventually the surface-borne deposits played out, and miners  excavated veins that ultimately dotted the Sicilian landscape with labyrinthine mines. Mining was unmechanized and labor-intensive, with pickmen freeing the ore from the rock, and mine-boys or "carusi" carrying baskets of ore to the surface, often through a mile or more of tunnels. Once the ore was at the surface, it was reduced and extracted in smelting ovens. The conditions in Sicilian sulfur mines were horrific, prompting Booker T. Washington to write "I am not prepared just now to say to what extent I believe in a physical hell in the next world, but a sulphur mine in  Sicily is about the nearest thing to hell that I expect to see in this life." Elemental sulfur was extracted from salt domes (in which it sometimes occurs in nearly pure form) until the late 20th century. Sulfur is now produced as a side product of other industrial processes such as in oil refining, in which sulfur is undesired. As a mineral, native sulfur under salt domes is thought to be a fossil mineral resource, produced by the action of anaerobic bacteria on sulfate deposits. It was removed from such salt-dome mines mainly by the Frasch process. In this  method, superheated water was pumped into a native sulfur deposit to melt the sulfur, and then compressed air returned the 99.5% pure melted product to the surface. Throughout the 20th century this procedure produced elemental sulfur that required no further purification. Due to a limited number of such sulfur deposits and the high cost of working them, this process for mining sulfur has not been employed in a major way anywhere in the world since 2002. Today, sulfur is produced from petroleum, natural gas, and related fossil resources, from which it is obtained mainly as hydrogen sulfide. Organosulfur compounds, undesirable  impurities in petroleum, may be upgraded by subjecting them to hydrodesulfurization, which cleaves the C–S bonds: The resulting hydrogen sulfide from this process, and also as it occurs in natural gas, is converted into elemental sulfur by the Claus process. This process entails oxidation of some hydrogen sulfide to sulfur dioxide and then the comproportionation of the two: Owing to the high sulfur content of the Athabasca Oil Sands, stockpiles of elemental sulfur from this process now exist throughout Alberta, Canada. Another way of storing sulfur is as a binder for concrete, the resulting product having many desirable properties  (see sulfur concrete). Sulfur is still mined from surface deposits in poorer nations with volcanoes, such as Indonesia, and worker conditions have not improved much since Booker T. Washington's days. The world production of sulfur in 2011 amounted to 69 million tonnes (Mt), with more than 15 countries contributing more than 1 Mt each. Countries producing more than 5 Mt are China (9.6), US (8.8), Canada (7.1) and Russia (7.1). Production has been slowly increasing from 1900 to 2010; the price was unstable in the 1980s and around 2010. Applications. Sulfuric acid. Elemental sulfur is used mainly as a  precursor to other chemicals. Approximately 85% (1989) is converted to sulfuric acid (HSO): In 2010, the United States produced more sulfuric acid than any other inorganic industrial chemical. The principal use for the acid is the extraction of phosphate ores for the production of fertilizer manufacturing. Other applications of sulfuric acid include oil refining, wastewater processing, and mineral extraction. Other important sulfur chemistry. Sulfur reacts directly with methane to give carbon disulfide, which is used to manufacture cellophane and rayon. One of the uses of elemental sulfur is in vulcanization of rubber, where polysulfide chains crosslink organic polymers. Large quantities  of sulfites are used to bleach paper and to preserve dried fruit. Many surfactants and detergents (e.g. sodium lauryl sulfate) are sulfate derivatives. Calcium sulfate, gypsum, (CaSO·2HO) is mined on the scale of 100 million tonnes each year for use in Portland cement and fertilizers. When silver-based photography was widespread, sodium and ammonium thiosulfate were widely used as "fixing agents". Sulfur is a component of gunpowder ("black powder"). Fertilizer. Sulfur is increasingly used as a component of fertilizers. The most important form of sulfur for fertilizer is the mineral calcium sulfate. Elemental sulfur is hydrophobic (not soluble in water ) and cannot be used directly by plants. Over time, soil bacteria can convert it to soluble derivatives, which can then be used by plants. Sulfur improves the efficiency of other essential plant nutrients, particularly nitrogen and phosphorus. Biologically produced sulfur particles are naturally hydrophilic due to a biopolymer coating and are easier to disperse over the land in a spray of diluted slurry, resulting in a faster uptake. The botanical requirement for sulfur equals or exceeds the requirement for phosphorus. It is an essential nutrient for plant growth, root nodule formation of legumes, and immunity and defense systems. Sulfur  deficiency has become widespread in many countries in Europe. Because atmospheric inputs of sulfur continue to decrease, the deficit in the sulfur input/output is likely to increase unless sulfur fertilizers are used. Atmospheric inputs of sulfur decrease because of actions taken to limit acid rains. Fungicide and pesticide. Elemental sulfur is one of the oldest fungicides and pesticides. "Dusting sulfur", elemental sulfur in powdered form, is a common fungicide for grapes, strawberry, many vegetables and several other crops. It has a good efficacy against a wide range of powdery mildew diseases as well as black spot. In organic production , sulfur is the most important fungicide. It is the only fungicide used in organically farmed apple production against the main disease apple scab under colder conditions. Biosulfur (biologically produced elemental sulfur with hydrophilic characteristics) can also be used for these applications. Standard-formulation dusting sulfur is applied to crops with a sulfur duster or from a dusting plane. Wettable sulfur is the commercial name for dusting sulfur formulated with additional ingredients to make it water miscible. It has similar applications and is used as a fungicide against mildew and other mold-related problems with plants and soil. Elemental sulfur powder  is used as an "organic" (i.e., "green") insecticide (actually an acaricide) against ticks and mites. A common method of application is dusting the clothing or limbs with sulfur powder. A diluted solution of lime sulfur (made by combining calcium hydroxide with elemental sulfur in water) is used as a dip for pets to destroy ringworm (fungus), mange, and other dermatoses and parasites. Sulfur candles of almost pure sulfur were burned to fumigate structures and wine barrels, but are now considered too toxic for residences. Pharmaceuticals. Sulfur (specifically octasulfur, S) is used in pharmaceutical skin preparations for the treatment of acne  and other conditions. It acts as a keratolytic agent and also kills bacteria, fungi, scabies mites, and other parasites. Precipitated sulfur and colloidal sulfur are used, in form of lotions, creams, powders, soaps, and bath additives, for the treatment of acne vulgaris, acne rosacea, and seborrhoeic dermatitis. Many drugs contain sulfur; early examples being antibacterial sulfonamides, known as "sulfa drugs". Sulfur is a part of many bacterial defense molecules. Most β-lactam antibiotics, including the penicillins, cephalosporins and monobactams contain sulfur. Biological role. Sulfur is an essential component of all living cells. It is the eighth most abundant element in  the human body by weight, about equal in abundance to potassium, and slightly greater than sodium and chlorine. A human body contains about 140 grams of sulfur. It is vital for the production of insulin, keratin and collagen. Transferring sulfur between inorganic and biomolecules. The sulfur cycle was the first of the biogeochemical cycles to be discovered. In the 1880s, while studying Beggiatoa (a bacterium living in a sulfur rich environment), Sergei Winogradsky found that it oxidized hydrogen sulfide (HS) as an energy source, forming intracellular sulfur droplets. Winogradsky referred to this form of metabolism as inorgoxidation (oxidation of inorganic  compounds). Another contributor, who continued to study it was Selman Waksman. Primitive bacteria that live around deep ocean volcanic vents oxidize hydrogen sulfide for their nutrition, as discovered by Robert Ballard. Sulfur oxidizers can use as energy sources reduced sulfur compounds, including hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g., tetrathionate). They depend on enzymes such as sulfur oxygenase and sulfite oxidase to oxidize sulfur to sulfate. Some lithotrophs can even use the energy contained in sulfur compounds to produce sugars, a process known as chemosynthesis. Some bacteria and archaea use hydrogen sulfide in place of water as  the electron donor in chemosynthesis, a process similar to photosynthesis that produces sugars and utilizes oxygen as the electron acceptor. Sulfur-based chemosynthesis may be simplifiedly compared with photosynthesis: HS +CO → sugars + S HO + CO → sugars + O The photosynthetic green sulfur bacteria and purple sulfur bacteria and some lithotrophs use elemental oxygen to carry out such oxidization of hydrogen sulfide to produce elemental sulfur (S), oxidation state= 0. The giant tube worm is an example of a large organism that uses hydrogen sulfide (via bacteria) as food to be oxidized. There are sulfate-reducing  bacteria, that, by contrast, "breathe sulfate" instead of oxygen. They use organic compounds or molecular hydrogen as the energy source. They use sulfur as the electron acceptor, and reduce various oxidized sulfur compounds back into sulfide, often into hydrogen sulfide. They can grow on other partially oxidized sulfur compounds (e.g. thiosulfates, thionates, polysulfides, sulfites). A common myth exists, that hydrogen sulfide produced by these bacteria is responsible for some of the smell of intestinal gases (flatus) and decomposition products. Often flatus smells otherwise than hydrogen sulfide (which has rotten eggs smell), but the presence of these bacteria leads to a  side effect out of the use of iron supplement, blackening of teeth and feces, that is caused by iron sulfide produced by those bacteria. There are studies pointing that many deposits of native sulfur in places that were the bottom of the ancient oceans have biological origin. These studies indicate that this native sulfur have been obtained through biological activity, but what is responsible for that (sulfur-oxidizing bacteria or sulfate-reducing bacteria) is still unknown for sure. Sulfur is absorbed by plants roots from soil as sulfate and transported as a phosphate ester. Sulfate is reduced to sulfide via  sulfite before it is incorporated into cysteine and other organosulfur compounds. While the plants' role in transferring sulfur to animals by food chains is more or less understood, the role of sulfur bacteria is just getting investigated. Protein and organic cofactors. In plants and animals, the amino acids cysteine and methionine contain most of the sulfur, and the element is present in all polypeptides, proteins, and enzymes that contain these amino acids. In humans, methionine is an essential amino acid that must be ingested. However, save for the vitamins biotin and thiamine, cysteine and all sulfur-containing compounds in the  human body can be synthesized from methionine. The enzyme sulfite oxidase is needed for the metabolism of methionine and cysteine in humans and animals. Disulfide bonds (S-S bonds) between cysteine residues in peptide chains are very important in protein assembly and structure. These covalent bonds between peptide chains confer extra toughness and rigidity. For example, the high strength of feathers and hair is due in part to the high content of S-S bonds with cysteine and sulfur. Eggs are high in sulfur to nourish feather formation in chicks, and the characteristic odor of rotting eggs is due to  hydrogen sulfide. The high disulfide bond content of hair and feathers contributes to their indigestibility and to their characteristic disagreeable odor when burned. Homocysteine and taurine are other sulfur-containing acids that are similar in structure, but not coded by DNA, and are not part of the primary structure of proteins. Many important cellular enzymes use prosthetic groups ending with -SH moieties to handle reactions involving acyl-containing biochemicals: two common examples from basic metabolism are coenzyme A and alpha-lipoic acid. Two of the 13 classical vitamins, biotin, and thiamine, contain sulfur, with the latter being named for its  sulfur content. In intracellular chemistry, sulfur operates as a carrier of reducing hydrogen and its electrons for cellular repair of oxidation. Reduced glutathione, a sulfur-containing tripeptide, is a reducing agent through its sulfhydryl (-SH) moiety derived from cysteine. The thioredoxins, a class of small proteins essential to all known life, use neighboring pairs of reduced cysteines to work as general protein reducing agents, with similar effect. Methanogenesis, the route to most of the world's methane, is a multistep biochemical transformation of carbon dioxide. This conversion requires several organosulfur cofactors. These include coenzyme M, CHSCHCHSO, the immediate precursor to  methane. Metalloproteins and inorganic cofactors. Metalloproteins in which the active site is a transition metal complex bound to sulfur atoms are essential components of enzymes involved in electron transfer processes. Examples include blue copper proteins and nitrous oxide reductase. The function of these enzymes is dependent on the fact that the transition metal ion can undergo redox reactions. Other examples include iron–sulfur clusters as well as many copper, nickel, and iron proteins. Most pervasive are the ferrodoxins, which serve as electron shuttles in cells. In bacteria, the important nitrogenase enzymes contains an Fe–Mo–S cluster and is a  catalyst that performs the important function of nitrogen fixation, converting atmospheric nitrogen to ammonia that can be used by microorganisms and plants to make proteins, DNA, RNA, alkaloids, and the other organic nitrogen compounds necessary for life. Precautions. Elemental sulfur is non-toxic, as are most of the soluble sulfate salts, such as Epsom salts. Soluble sulfate salts are poorly absorbed and laxative. When injected parenterally, they are freely filtered by the kidneys and eliminated with very little toxicity in multi-gram amounts. Aluminium sulfate is used in the purification of drinking water, wastewater treatment plants and papermaking. When sulfur  burns in air, it produces sulfur dioxide. In water, this gas produces sulfurous acid and sulfites; sulfites are antioxidants that inhibit growth of aerobic bacteria and a useful food additive in small amounts. At high concentrations these acids harm the lungs, eyes, or other tissues. In organisms without lungs such as insects or plants, sulfite in high concentration prevents respiration. Sulfur trioxide (made by catalysis from sulfur dioxide) and sulfuric acid are similarly highly acidic and corrosive in the presence of water. Sulfuric acid is a strong dehydrating agent that can strip available water molecules and water components from sugar  and organic tissue. The burning of coal and/or petroleum by industry and power plants generates sulfur dioxide (SO) that reacts with atmospheric water and oxygen to produce sulfuric acid (HSO) and sulfurous acid (HSO). These acids are components of acid rain, lowering the pH of soil and freshwater bodies, sometimes resulting in substantial damage to the environment and chemical weathering of statues and structures. Fuel standards increasingly require that fuel producers extract sulfur from fossil fuels to prevent acid rain formation. This extracted and refined sulfur represents a large portion of sulfur production. In coal-fired power plants, flue gases  are sometimes purified. More modern power plants that use synthesis gas extract the sulfur before they burn the gas. Hydrogen sulfide is as toxic as hydrogen cyanide, and kills by the same mechanism (inhibition of the respiratory enzyme cytochrome oxidase), though hydrogen sulfide is less likely to cause surprise poisonings from small inhaled amounts because of its disagreeable odor. Hydrogen sulfide quickly deadens the sense of smell and a victim may breathe increasing quantities without noticing the increase until severe symptoms cause death. Dissolved sulfide and hydrosulfide salts are toxic by the same mechanism.