Sulfur is one of the more abundant elements in the universe, ranking approximately tenth by mass. On Earth, this element is a widespread constituent of various materials, participating in complex chemical interactions across the planet. Its presence is fundamental to the physical environment, contributing significantly to the composition of rocks, water bodies, and the atmosphere. Its complex chemistry allows it to move and transform continuously, existing in various states from highly oxidized to highly reduced. This dynamic behavior establishes sulfur as a significant nutrient that supports life while simultaneously influencing large-scale planetary processes.
Forms and Major Natural Reservoirs
Sulfur exists in several distinct chemical states, which dictate how it interacts with its surroundings. The two most common inorganic forms are the oxidized sulfate ($\text{SO}_4^{2-}$) and the reduced sulfide ($\text{H}_2\text{S}$ or $\text{S}^{2-}$). Sulfate is typically soluble in water, making it the primary dissolved form found in oceans and fresh water bodies. Sulfide, often occurring as hydrogen sulfide gas or incorporated into metal sulfides like pyrite ($\text{FeS}_2$), is characteristic of oxygen-poor environments.
The largest reservoir of sulfur on Earth is locked away within the Earth’s crust, primarily in sedimentary rocks and mineral deposits. Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) and anhydrite ($\text{CaSO}_4$) are common forms of stored sulfate within these geological structures. These deposits represent sulfur that has been chemically immobilized over vast stretches of geological time.
The world’s oceans represent the second largest storage compartment, holding vast amounts of sulfur primarily as dissolved sulfate ions. The concentration of sulfate in the modern ocean is approximately 2,700 parts per million, providing a stable pool. A smaller fraction of sulfur is present in the atmosphere as gaseous compounds, including sulfur dioxide ($\text{SO}_2$) and hydrogen sulfide, which are important for short-term transport.
The Biogeochemical Sulfur Cycle
The movement of sulfur between its major reservoirs is driven largely by biological processes, forming a complex biogeochemical cycle that continuously transforms sulfur compounds between oxidized and reduced states. The incorporation of sulfur into organic molecules, known as immobilization, occurs when plants and microorganisms take up sulfate from the soil or water.
When organisms die, the organic sulfur they contain is released back into the environment through decomposition, a process called mineralization. This typically results in the formation of reduced sulfur compounds, such as hydrogen sulfide, particularly under low-oxygen conditions. The fate of these reduced compounds is determined by specialized microorganisms.
Bacteria play a role in oxidizing reduced sulfur compounds back into sulfate, often using the energy released from this chemical transformation to fuel their growth (chemosynthesis). Certain bacteria can convert hydrogen sulfide directly into elemental sulfur or sulfate in environments ranging from deep-sea vents to marsh sediments. This oxidation step returns the sulfur to the form most readily available for uptake by plants and algae.
Conversely, dissimilatory sulfate reduction converts sulfate back into sulfide. This action is carried out by anaerobic bacteria, which use sulfate instead of oxygen to break down organic matter in environments like waterlogged soils and deep ocean sediments. The resulting hydrogen sulfide can then react with metals, such as iron, to form stable mineral sulfides, completing a localized loop within the global movement.
Sulfur’s Essential Role in Biological Systems
Once absorbed by organisms, sulfur assumes a structural role within living cells, acting as a foundational component for many biological molecules. The element is incorporated directly into two amino acids: cysteine and methionine. Methionine serves as a starter molecule for protein synthesis, while cysteine’s reactive structure allows it to form specific chemical links that stabilize protein shapes.
A primary function of sulfur in proteins is the formation of disulfide bonds, which are strong covalent links between two cysteine residues within a single polypeptide chain or between two different chains. These bonds stabilize the three-dimensional structure of many proteins, including structural proteins like keratin in hair and the hormone insulin. The correct folding and stability imparted by these bonds are necessary for enzyme function and recognition.
Beyond proteins, sulfur is part of several non-protein organic compounds that mediate energy transfer and metabolic reactions. It is incorporated into cofactors like Coenzyme A, which is central to the metabolism of fats and carbohydrates. Sulfur atoms are also present in several B vitamins, including biotin and thiamine, where they participate in specific enzyme reactions.
Organisms typically acquire the sulfur they require from the environment as oxidized sulfate. This sulfate must then be reduced internally before it can be used to synthesize the sulfur-containing amino acids and other biomolecules. This internal reduction process uses energy, highlighting the investment organisms make to incorporate this element.
Geological and Atmospheric Influence
Sulfur cycling is not solely driven by biological and sedimentary processes; large-scale geological events also contribute to the global movement. Volcanic eruptions and geothermal vents represent major natural sources that inject sulfur compounds directly into the atmosphere and oceans. Volcanoes primarily release sulfur dioxide ($\text{SO}_2$) and hydrogen sulfide ($\text{H}_2\text{S}$) into the upper atmosphere.
In the deep ocean, hydrothermal vents release plumes of reduced sulfur compounds, supporting unique chemosynthetic ecosystems. Over geological timescales, the reaction of dissolved sulfide with iron in sediments leads to the formation of iron sulfide minerals, most commonly pyrite. This mineral formation locks sulfur away in the lithosphere, acting as a long-term sink for the element.
The injection of sulfur dioxide into the atmosphere affects climate systems. When $\text{SO}_2$ reaches the stratosphere, it reacts with water vapor to form tiny droplets of sulfuric acid, known as sulfate aerosols. These aerosols are highly reflective, scattering incoming solar radiation back into space, which can cause a temporary cooling effect on the planet’s surface following a major eruption.
Another atmospheric contribution comes from the marine environment in the form of Dimethyl Sulfide (DMS). This gas is produced by marine phytoplankton as a byproduct of metabolism. When released into the air, DMS oxidizes to form sulfate aerosols, which serve as condensation nuclei, playing a natural role in cloud formation over the open ocean.