Where Sulfur is Found in Nature and Its Biological Role

Sulfur ranks among the most abundant elements on Earth and throughout the solar system, showing up in places as different as the planet’s deep core, the spray of volcanic vents, ocean plankton, and the proteins holding your hair together. It is the tenth most common element by mass in the universe and the fifth most common on Earth, where much of it is locked away underground in mineral deposits and dissolved in seawater as sulfate. Biologically, sulfur sits at the heart of processes that keep cells alive, from shuttling electrons during energy production to shaping the three-dimensional structure of proteins. Its story spans geology, atmospheric chemistry, microbiology, agriculture, and human health in ways that make it one of the most interconnected elements in nature.

Inside the Earth

A large fraction of Earth’s sulfur never sees the surface. Geochemical models suggest that the depletion of sulfur and related elements in Earth’s outer rocky layers can be explained if sulfur, rather than silicon, is the light component mixed with iron in the planet’s core.¹ That would make the core one of the largest sulfur reservoirs in the solar system, far outstripping anything at the surface.

Closer to the crust, sulfur appears in two broad families of minerals. Sulfide minerals pair sulfur with metals: think of pyrite (iron sulfide, often called “fool’s gold”), galena (lead sulfide), and cinnabar (mercury sulfide). Sulfate minerals combine sulfur with oxygen and a metal, producing things like gypsum and barite. These minerals accumulate in sedimentary rocks, hydrothermal ore deposits, and evaporite basins left behind by ancient seas. The tendency of sulfur to form low-solubility sulfates and stable sulfide minerals means it is more readily stored in sediments than an element like nitrogen, which tends to escape into the atmosphere as a gas.²

Volcanoes, Hot Springs, and the Atmosphere

Volcanoes are one of the most dramatic natural sources of sulfur. Eruptions blast sulfur dioxide and hydrogen sulfide into the atmosphere, and fumaroles on volcanic flanks coat surrounding rock with bright yellow elemental sulfur. These emissions are not just local curiosities. Sulfur dioxide injected into the stratosphere during major eruptions can scatter sunlight and cool global temperatures for months. Carbonyl sulfide, a more chemically stable sulfur gas, drifts from the lower atmosphere into the stratosphere as well, and together with volcanic sulfur dioxide these sources dominate stratospheric sulfur chemistry.²

Hot springs and geothermal fields are closely related environments. The sulfur in these systems typically originates from magmatic gases rising through fractured rock. When hydrogen sulfide reaches the surface and meets oxygen, it can deposit native sulfur or dissolve into acidic hot-spring water. These acidic, sulfur-rich pools have become important study sites for extremophile microorganisms, which we will get to shortly.

Sulfur in the Oceans and Its Surprising Link to Clouds

Seawater contains high concentrations of dissolved sulfate, making the ocean a vast sulfur reservoir. But the more interesting story involves what marine life does with that sulfur. Certain phytoplankton produce a compound called dimethylsulfoniopropionate (DMSP), which breaks down to release dimethyl sulfide (DMS) into seawater and eventually into the atmosphere. DMS is the source of that distinctive “ocean smell” on a breezy coast, and it represents a major transfer of sulfur from sea to air.²

Once airborne, DMS oxidizes to form tiny sulfate aerosol particles. These particles can act as seeds around which water vapor condenses, producing cloud droplets. Field observations in the Arctic have confirmed that when atmospheric DMS concentrations rise, the growth of particles to sizes that can serve as cloud condensation nuclei accelerates in a measurable, stepwise fashion.³ Observational work near Iceland has tied these atmospheric DMS levels back to blooms of specific DMSP-rich phytoplankton groups like coccolithophores and dinoflagellates, showing how ocean biology directly shapes sulfur emissions.⁴

The broader hypothesis is that phytoplankton could, through DMS, influence cloud cover and therefore the amount of sunlight reaching the ocean surface, which in turn affects how much phytoplankton can grow. This feedback loop, if strong enough, could matter for climate models.⁵ Whether the effect is large enough to act as a meaningful thermostat remains debated, but the chemical chain from ocean plankton to cloud brightness is well established.

The Global Sulfur Cycle on Land

On land, sulfur moves through weathering, biological uptake, and runoff. Rain dissolves sulfate minerals in rock, and rivers carry that sulfate toward the ocean. But not all of it arrives. Research in a glacier-fed watershed in Norway found that a significant amount of the sulfate produced by weathering remains trapped in terrestrial environments through processes like pyrite burial in freshwater sediments. This land-based sulfur sink may have expanded and contracted over geological time in response to events like the spread of forests on land.⁶

Sulfur also cycles through the biosphere on land. Plants absorb sulfate from the soil through their roots and incorporate it into amino acids and other organic molecules. When plant material decays, soil microbes break those organic sulfur compounds back down, returning sulfate to the soil or releasing volatile sulfur gases. The land portion of the sulfur cycle is less studied than its oceanic counterpart, and it is increasingly clear that scientists have underestimated how much sulfur gets trapped on continents rather than washing out to sea.

Microbes That Run on Sulfur

Some of the most ancient metabolic strategies on Earth revolve around sulfur. Sulfate-reducing bacteria are anaerobic microorganisms found in marine sediments, waterlogged soils, sewage systems, and the guts of animals. They use sulfate the way we use oxygen: as the molecule that accepts electrons at the end of their energy-producing metabolic chain. The waste product is hydrogen sulfide, the gas responsible for the rotten-egg smell of marshes and sewage. These organisms are extremely versatile. When sulfate runs out, many can switch to fermenting organic acids and alcohols, producing hydrogen and acetate instead, and can even partner with methane-producing archaea to keep their metabolism going.⁷

Comparative genomics suggests that the core enzyme machinery for this kind of sulfur metabolism, the dissimilatory sulfite reductase pathway, originated in archaea and initially functioned to reduce sulfite rather than sulfate. Over evolutionary time, additional protein components were acquired that allowed microorganisms to use sulfate itself as an electron acceptor. The ability to run the pathway in reverse, oxidizing sulfur compounds for energy rather than reducing them, evolved independently at least twice in different bacterial lineages.⁸ This deep evolutionary history hints at just how central sulfur metabolism was in the early biosphere.

Why Cells Need Sulfur

Every living cell uses sulfur, and its roles go well beyond a single function. Two of the twenty standard amino acids, cysteine and methionine, contain sulfur. Methionine is the amino acid that kicks off the construction of virtually every protein in your body. Cysteine, meanwhile, can form bonds with other cysteine units in the same protein. These disulfide bonds act like molecular staples that lock a protein into its correct three-dimensional shape, and they are especially common in proteins that operate outside cells, such as antibodies and many hormones.⁹

Sulfur also appears in essential vitamins and cofactors. The biosynthesis of thiamin (vitamin B1) and biotin (vitamin B7) both require sulfur atoms donated from cysteine through multi-step enzymatic pathways. In thiamin synthesis, three enzymes work together to transfer a sulfur atom onto a small carrier peptide before it is incorporated into the vitamin. In biotin synthesis, an iron-sulfur cluster on the biotin synthase enzyme serves as the direct sulfur donor.¹⁰

Perhaps the most fundamental sulfur-containing structures in biology are iron-sulfur clusters. These tiny inorganic assemblies of iron and sulfur atoms sit inside proteins and are best known for carrying electrons through the mitochondrial respiratory chain, the process that generates most of your cellular energy. Within Complex I alone, a chain of seven iron-sulfur clusters connects the site where electrons enter to the site where they leave, with each cluster handing off an electron to the next.¹¹ Iron-sulfur clusters also play critical roles in Complexes II and III of the same chain.¹² Without sulfur, the electron transport chain simply would not work, and aerobic life as we know it would be impossible.

Hydrogen Sulfide as a Signaling Molecule

Hydrogen sulfide is toxic at high concentrations, but at very low levels the body produces it on purpose. Your cells synthesize small amounts of H₂S, and it functions as a signaling molecule in the brain and cardiovascular system, helping regulate blood vessel dilation, inflammation, and defenses against oxidative damage.¹³ One of its key protective tricks is boosting production of glutathione, the cell’s main antioxidant. Research has shown that H₂S enhances transport of cysteine into cells, ramping up glutathione synthesis and directing that glutathione toward mitochondria, where oxidative damage tends to be heaviest. Under conditions of oxidative stress, this effect can increase glutathione production by roughly fourfold.¹⁴

The dose matters enormously in the gut, too. At low concentrations, H₂S stabilizes the mucus lining of the colon and helps resolve inflammation. But when sulfate-reducing gut bacteria produce excessive amounts, that same molecule can damage the mucus layer and promote inflammation.¹⁵ The colonic lining gets H₂S from two sources: the bacteria in the gut lumen and the epithelial cells themselves, and higher luminal concentrations from microbial activity are the main concern.¹⁶ Diets very high in sulfur-containing amino acids (from red meat, for instance) can tip the balance by feeding those sulfate-reducing bacteria more substrate.

Sulfur in Plants and Agriculture

Plants absorb sulfur mainly as sulfate through their roots and convert it into cysteine, methionine, and a range of specialized sulfur compounds. One important group is the glucosinolates, the pungent compounds responsible for the sharp flavors of broccoli, mustard, and cabbage. In the model plant Arabidopsis, when sulfate uptake is impaired, glucosinolate levels drop in leaves and stems, but the plant prioritizes keeping glucosinolate levels steady in its seeds, likely because these compounds serve a dual role in sulfur storage and defense against herbivores.¹⁷

In agriculture, sulfur deficiency has become an increasing concern as industrial sulfur dioxide emissions have fallen, which reduces the amount of sulfur deposited onto fields by rain. Even mild sulfur deficiency can affect crop quality without visibly reducing yield. In wheat, for example, low sulfur shifts protein composition toward sulfur-poor storage proteins, degrading bread-making quality. More severe deficiency reduces Rubisco, the enzyme responsible for capturing carbon dioxide during photosynthesis, leading to yellowing of young leaves and stunted growth.¹⁸ Sulfur fertilization can boost grain yield, and it may also help replenish sulfur removed from soil by repeated harvesting.¹⁹ In rice, sulfur and zinc fertilization increased grain yield across multiple varieties compared with unfertilized controls, though nitrogen had the largest effect on grain protein.²⁰

Plants also use sulfur-containing peptides called phytochelatins to cope with heavy metals in contaminated soil. These short peptide chains bind to metals like cadmium and arsenic, sequestering them so they cause less damage to the cell. Phytochelatins serve a function in plants analogous to what metallothioneins do in animals: keeping toxic metals under control.²¹

Sulfur-Powered Life at Hydrothermal Vents

Some of the most alien ecosystems on Earth exist at deep-sea hydrothermal vents, where superheated, sulfide-rich water gushes from cracks in the ocean floor. In complete darkness, with no photosynthesis possible, entire communities are sustained by bacteria that oxidize hydrogen sulfide for energy. The giant tubeworm Riftia pachyptila is the iconic resident of Pacific vents. It has no mouth, no gut, and no anus. Instead, it houses sulfur-oxidizing bacteria inside a specialized organ, and those bacteria use sulfide as their main fuel to fix carbon and feed their host.²² Vestimentiferan tubeworms more broadly colonize both hydrothermal vents and cold seeps worldwide, relying entirely on their sulfur-oxidizing endosymbionts for nutrition.²³

Snails have gotten in on the arrangement too. The deep-sea snail Gigantopelta aegis hosts two bacterial endosymbionts simultaneously: a sulfur oxidizer and a methane oxidizer, giving it access to two chemical energy sources at once.²⁴ These vent ecosystems are living proof that sulfur can anchor entire food webs without a single photon of sunlight.

Bioleaching and Acid Mine Drainage

Where sulfide minerals are exposed to air and water, either naturally or through mining, acidophilic microorganisms take over. These acid-loving bacteria and archaea generate energy by oxidizing iron and sulfur in sulfide ores, and the sulfuric acid they produce as a byproduct dissolves the surrounding rock. In nature, this process creates acid mine drainage, the rusty, toxic runoff from old mines. But the same chemistry has been harnessed industrially as “bioleaching,” a low-input method for extracting copper, gold, and other metals from low-grade ores.²⁵ Species like Acidithiobacillus ferrooxidans can carry out this iron and sulfur oxidation even at surprisingly low temperatures, meaning that acid mine drainage is not just a problem in warm climates.²⁶

Acid Rain and Its Decline

For much of the twentieth century, burning coal and smelting ores pumped massive quantities of sulfur dioxide into the atmosphere. That SO₂ converted to sulfuric acid in clouds and fell as acid rain, damaging forests, acidifying lakes, and corroding buildings. The problem was eventually addressed through coordinated policy in Europe and North America, particularly under the UNECE Convention on Long-range Transboundary Air Pollution. Emissions dropped substantially, and ecosystem impacts have decreased.²⁷ An ironic side effect of that success is the agricultural sulfur deficiency mentioned earlier. Crops that once received free sulfur from polluted rainfall now need it applied as fertilizer.

Sulfur in Human Genetic Disorders

Because sulfur amino acids are woven into so many metabolic pathways, genetic defects in sulfur metabolism can cause serious disease. The best-known example is homocystinuria, in which the body cannot properly process homocysteine, an intermediate in the conversion of methionine to cysteine. Depending on the specific enzyme affected, these disorders can cause a range of problems including blood clots, bone abnormalities, connective tissue problems, and neurological symptoms. Known genetic defects span several steps of sulfur amino acid processing, from methionine breakdown to homocysteine recycling to cysteine and hydrogen sulfide breakdown, and symptoms can appear anywhere from infancy to late adulthood.²⁸ Study of these inborn errors has also offered broader insights, since elevated homocysteine is a recognized risk factor for cardiovascular disease in the general population.²⁹

Sulfur Beyond Earth

Sulfur’s story does not end at the edge of our atmosphere. Jupiter’s moon Io is the most volcanically active body in the solar system, and its eruptions are powered by sulfur. Because sulfur is volatile enough to drive explosive volcanism yet abundant enough not to be depleted over billions of years, it became the dominant volcanic gas on Io after other common volatiles were exhausted. The moon’s surface is painted in vivid yellows, reds, and oranges by different sulfur compounds.³⁰ Other bodies in the solar system have their own sulfur stories: Europa’s subsurface ocean likely contains dissolved sulfur compounds, and the thick clouds of Venus are laced with sulfuric acid droplets.

Back on early Earth, iron-sulfur minerals may have been catalysts for some of the first chemical reactions that led to life. Iron-sulfur proteins are among the most ancient protein families known, driving electron transfer and carbon dioxide fixation in organisms spanning every domain of life. The idea that iron-sulfur mineral surfaces at hydrothermal vents could have promoted the chemistry needed to build biological molecules remains one of the leading hypotheses for the origin of life.³¹ If that hypothesis holds, sulfur was not just essential for life as it exists today. It may have been essential for life to get started in the first place.