The sulfur cycle is a fundamental biogeochemical process that describes the movement and transformation of the element sulfur through the Earth’s atmosphere, hydrosphere, lithosphere, and biosphere. This cycling of sulfur is important because the element is a basic building block for life, incorporated into the structure of many biological molecules. Sulfur is a component of the amino acids cysteine and methionine, which are used to construct all proteins. The formation of disulfide bonds, which are sulfur-sulfur linkages within proteins, is responsible for determining the three-dimensional structure and function of proteins. The sulfur cycle involves the continuous changing of the element between various oxidation states, which supports diverse life forms and regulates environmental chemistry.
Natural Storage Locations for Sulfur
The vast majority of the Earth’s sulfur is stored in the lithosphere, primarily locked within rocks and sediments for long geological timescales. These subterranean reservoirs contain sulfur in mineral forms such as gypsum (a sulfate mineral) and pyrite (a sulfide mineral). Sulfur is released from these rocks into the active cycle slowly through chemical weathering of exposed mineral surfaces or through volcanic and geothermal activity.
The hydrosphere represents the second largest sulfur reservoir, where it exists mainly as dissolved sulfate ($\text{SO}_4^{2-}$) in the ocean. This sulfate is highly stable and readily available for marine organisms and for eventual sedimentation back into the lithosphere. A much smaller, but highly dynamic, pool of sulfur resides in the atmosphere in various gaseous forms, including sulfur dioxide ($\text{SO}_2$) and hydrogen sulfide ($\text{H}_2\text{S}$). These gases are released from decomposition, volcanic emissions, and sea spray.
The Four Main Stages of Sulfur Transformation
The movement of sulfur between its organic and inorganic forms is achieved through four primary transformation stages.
Mineralization
Decomposer organisms break down organic molecules, such as sulfur-containing amino acids in dead plants and animals. This action converts the organic sulfur back into an inorganic form, frequently releasing it as hydrogen sulfide ($\text{H}_2\text{S}$) gas.
Oxidation
Reduced sulfur compounds, like $\text{H}_2\text{S}$ or elemental sulfur ($\text{S}^0$), are converted back into sulfate ($\text{SO}_4^{2-}$). This step is important because sulfate is the form most commonly used by plants and microbes. The $\text{H}_2\text{S}$ produced during mineralization can enter the atmosphere or be subjected to this biological conversion in the soil.
Assimilation
Plants and microorganisms absorb the inorganic sulfate from the soil or water. Organisms then chemically reduce the sulfate to sulfide ($\text{S}^{2-}$), which is the required state for synthesizing new organic sulfur compounds like proteins and vitamins.
Reduction
This final major transformation is dissimilatory sulfate reduction, which occurs mainly in oxygen-deprived environments. Specialized microorganisms use sulfate ($\text{SO}_4^{2-}$) as an electron acceptor during anaerobic respiration. This process results in the production of large quantities of hydrogen sulfide ($\text{H}_2\text{S}$), which accumulates in anaerobic sediments and provides a substrate for the oxidation phase.
Microbes as Drivers of the Sulfur Cycle
Microorganisms are the primary agents facilitating the transformations within the sulfur cycle, possessing the enzymatic machinery required for changing the element’s oxidation state. The two major functional groups of bacteria involved are the sulfate-reducing bacteria and the sulfur-oxidizing bacteria. These microbes manage the balance between the oxidized (sulfate) and reduced (sulfide) forms of sulfur across all environments.
Sulfate-reducing bacteria (SRB), such as those from the Desulfovibrio genus, thrive in oxygen-poor habitats like deep-sea sediments and waterlogged soils. They drive the dissimilatory reduction stage by using sulfate to metabolize organic matter, releasing hydrogen sulfide as a waste product. Conversely, sulfur-oxidizing bacteria utilize reduced sulfur compounds for energy, transforming them back into sulfate, which makes the element available for plant assimilation.
Human Influence on Sulfur Movement
Human activities have significantly altered the movement of sulfur, increasing the annual flux of the element into the atmosphere far beyond natural levels. The most substantial disruption comes from the combustion of fossil fuels, particularly coal, which releases large amounts of stored sulfur into the air as sulfur dioxide ($\text{SO}_2$) gas.
Once in the atmosphere, sulfur dioxide reacts with water vapor to form sulfuric acid ($\text{H}_2\text{SO}_4$), which returns to the Earth’s surface as acid rain. This precipitation lowers the pH of lakes and soils, damaging aquatic ecosystems and degrading man-made structures. Industrial emissions, agricultural use of sulfur-containing fertilizers, and mining activities continue to influence the global distribution and transformation of sulfur.