Where Sulfur is Found in Nature and Its Biological Role

Sulfur is a chemical element with the symbol S and atomic number 16. It is a nonmetal known for its bright yellow crystalline appearance in its elemental form. While some of its compounds are known for a distinct smell, like that of rotten eggs, pure sulfur is odorless. This element is found widely across the globe in many different forms, from pure deposits to complex molecules within living organisms.

Elemental Sulfur and its Natural Deposits

Pure, elemental sulfur can be found in various forms, known as allotropes, each with unique structural properties. The most common form at room temperature is orthorhombic sulfur, which consists of crown-shaped S8 molecules arranged in a crystal lattice. When heated above 95.5°C, it transforms into another allotrope called monoclinic sulfur, which has a different crystalline structure.

Significant deposits of elemental sulfur are often located in areas with specific geological characteristics, particularly volcanic regions and around hot springs. In these settings, hydrogen sulfide gas emitted from deep within the Earth reacts with oxygen in the atmosphere to form solid sulfur. Another source is found in sedimentary rock layers associated with salt domes, such as those along the U.S. Gulf Coast. These deposits are believed to have formed from the action of anaerobic bacteria on sulfate minerals like gypsum.

The presence of these large, underground deposits led to the development of specific extraction methods. The Frasch process involves injecting superheated water into the deposit to melt the sulfur. Compressed air is then used to push the molten sulfur to the surface. This method allows for the recovery of very pure sulfur from deep underground.

Sulfur Compounds in the Geosphere and Hydrosphere

Beyond its pure elemental form, sulfur is abundant in a wide array of inorganic compounds distributed throughout the Earth’s crust and water systems. Sulfide minerals are a major category, including pyrite (FeS₂), often called “fool’s gold,” galena (PbS), the primary ore of lead, and sphalerite (ZnS), a main source of zinc. These minerals form in environments with low oxygen, often associated with hydrothermal activity where hot, metal-rich fluids circulate.

Sulfate minerals represent another large group of sulfur-bearing compounds. Gypsum (CaSO₄·2H₂O) is a common sulfate that forms from the evaporation of mineral-rich water in ancient seas and lakes, leading to vast sedimentary beds. Barite (BaSO₄) and Epsom salt (MgSO₄·7H₂O) are other examples of sulfate minerals found in various geological settings. The formation of these compounds is often linked to the weathering of sulfide minerals, which releases sulfur that can then recombine with other elements.

In the hydrosphere, sulfur is predominantly present as dissolved sulfate ions (SO₄²⁻), particularly in the oceans, where it is one of the most abundant ions. This oceanic sulfate originates from several sources, including the weathering and erosion of rocks and minerals on land, which is then carried to the sea by rivers. Volcanic emissions, both on land and underwater, also release sulfur compounds that contribute to the sulfate content of water bodies.

The Biogeochemical Sulfur Cycle

The movement and transformation of sulfur through Earth’s systems are described by the biogeochemical sulfur cycle. Sulfur is constantly changing its form as it moves between different reservoirs, driven by both geological forces and biological activity.

Microorganisms are central to many of these transformations. For example, in environments without oxygen, such as deep sediments or waterlogged soils, sulfate-reducing bacteria use sulfate for respiration, converting it into hydrogen sulfide (H₂S). Conversely, other bacteria can oxidize hydrogen sulfide and elemental sulfur back into sulfate, making it available for plants and other organisms to use.

The cycle also includes atmospheric components. Volcanic eruptions release large quantities of sulfur dioxide (SO₂) and hydrogen sulfide into the atmosphere. Biological decay in wetlands and oceans also releases sulfur gases. These atmospheric compounds can travel long distances before being deposited back on Earth’s surface through rain and dust, a process known as acid deposition.

Biological Significance of Sulfur

Sulfur is an element for all known forms of life, incorporated into a variety of biomolecules. Its most prominent role is as a component of the amino acids methionine and cysteine. Because these amino acids are building blocks of proteins, sulfur is present in countless proteins. Cysteine residues can form disulfide bridges, which are covalent bonds that help to stabilize the three-dimensional structure of proteins.

The element is also found in several coenzymes and vitamins. Coenzyme A contains sulfur and plays a part in the synthesis and oxidation of fatty acids and the oxidation of pyruvate in cellular respiration. The vitamins thiamine (B1) and biotin (B7) contain sulfur atoms and are involved in a variety of metabolic reactions. Another sulfur-containing molecule, glutathione, helps protect cells from damage caused by reactive oxygen species.

In some ecosystems, sulfur metabolism is the foundation of the food web. Around deep-sea hydrothermal vents, where sunlight cannot reach, certain bacteria and archaea perform chemosynthesis. They harness chemical energy by oxidizing hydrogen sulfide and other sulfur compounds that spew from the vents. This process allows them to produce organic matter, forming the base of a food chain that supports a diverse community of organisms, including tubeworms and giant clams.