The sulfur cycle is the movement of sulfur through the Earth’s four major systems: the atmosphere, the hydrosphere (water bodies), the lithosphere (rocks and soil), and the biosphere (living things). This biogeochemical cycle transforms and recycles sulfur, an element necessary for all life. Sulfur is an essential component of many biological molecules, and its cycling is linked to the chemistry of the atmosphere and oceans. The transformations in this cycle are driven largely by microbial activity, which changes the element’s chemical form and availability.
Major Sulfur Reservoirs
The lithosphere acts as the largest, most long-term reservoir, storing the vast majority of the Earth’s sulfur in sedimentary rocks and mineral deposits. These geological reserves include mineral sulfides, such as pyrite (iron sulfide), and sulfate evaporite minerals like gypsum (calcium sulfate). Sulfur remains bound within these formations for millions of years until geological uplift and weathering processes slowly release it back into the environment.
The world’s oceans represent the second largest reservoir, holding immense quantities of sulfur mainly as dissolved sulfate (\(\text{SO}_4^{2-}\)). Sulfate is the most stable form of sulfur in the oxygenated marine environment and is continuously supplied by rivers carrying weathered material from land. Atmospheric reservoirs are much smaller but highly active, containing sulfur dioxide (\(\text{SO}_2\)), hydrogen sulfide (\(\text{H}_2\text{S}\)) gases, and sulfate aerosols. The biosphere holds a minor amount of the total global sulfur, which is rapidly cycled through decomposition of living organisms and soil organic matter.
Key Transformations and Processes
The movement of sulfur between these reservoirs involves a series of chemical changes where the sulfur atom alters its oxidation state, ranging from the most reduced form of -2 in sulfides to the most oxidized form of +6 in sulfate. Microorganisms are the primary agents that facilitate these transformations, driving the entire cycle.
Mineralization, or decomposition, is an initial step in the biosphere that occurs when organic matter breaks down. Microbes convert organic sulfur compounds found in proteins into inorganic forms, notably hydrogen sulfide (\(\text{H}_2\text{S}\)). This release, often recognized by its rotten-egg smell, recycles sulfur back into the soil and water for reuse.
Assimilation is the process by which plants and microorganisms take up inorganic sulfur compounds. Plants absorb sulfate (\(\text{SO}_4^{2-}\)) ions from the soil or water and reduce them to incorporate sulfur into organic molecules, such as the amino acids cysteine and methionine. This reduced organic sulfur is then passed up the food chain to animals.
The conversion of reduced sulfur compounds back into oxidized forms is known as oxidation, carried out by specialized chemoautotrophic bacteria. These microbes gain energy by oxidizing hydrogen sulfide (\(\text{H}_2\text{S}\)) and elemental sulfur (S) into sulfate (\(\text{SO}_4^{2-}\)), often where oxygen is present. This step replenishes the supply of sulfate, the primary form plants absorb for assimilation.
The reverse process, sulfate reduction, is performed by a different group of bacteria in oxygen-depleted (anaerobic) environments like deep-sea sediments. These sulfate-reducing bacteria use sulfate as an electron acceptor instead of oxygen to metabolize organic matter. This dissimilative reduction converts the oxidized sulfate back into highly reduced hydrogen sulfide, completing the sulfur loop.
Ecological Significance
Sulfur’s cycling is central to the functioning of all ecosystems, starting with its role as a fundamental macronutrient. It is an indispensable component of the amino acids cysteine and methionine, meaning sulfur is integrated into nearly all proteins. The formation of disulfide bonds involving cysteine determines the three-dimensional folding and ultimate function of many proteins.
The availability of sulfur in the soil directly influences primary productivity, or plant growth. Plants require adequate sulfur to synthesize proteins and activate enzymes; a lack of bioavailable sulfate can limit crop yields and ecosystem biomass. Microbial breakdown through mineralization ensures a continuous nutrient supply, linking soil health to decomposer activity.
The natural sulfur cycle also plays a part in climate regulation through dimethyl sulfide (DMS). DMS is produced in the ocean when bacteria break down dimethylsulfoniopropionate (DMSP), synthesized by phytoplankton. Once released, DMS is oxidized into sulfate aerosols, microscopic particles that act as cloud condensation nuclei. This process links marine biological activity to cloud formation, influencing the reflection of solar radiation.
Human Impacts on the Sulfur Cycle
Human industrial activities have dramatically accelerated the movement of sulfur from geological reservoirs into the atmosphere, changing the natural balance of the cycle. The largest human contribution comes from the combustion of fossil fuels, particularly coal and oil, which contain sulfur impurities. Burning these fuels releases sulfur rapidly as sulfur dioxide (\(\text{SO}_2\)) gas, a major air pollutant.
Mining and various industrial processes also contribute by rapidly extracting and processing sulfur-containing minerals. This bypasses the slow, natural process of weathering, mobilizing vast amounts of sulfur that would otherwise remain locked in the lithosphere. These activities increase the overall pool of oxidized sulfur compounds in the atmosphere, vastly increasing the annual flux.
The most well-known consequence of this excess atmospheric sulfur is the formation of acid rain. Sulfur dioxide reacts with water vapor and other chemicals, forming sulfuric acid (\(\text{H}_2\text{SO}_4\)), which falls to the Earth as acidic precipitation. Acid rain lowers the pH of freshwater lakes and streams, harming aquatic ecosystems and fish populations. It also causes chemical degradation of forests and damages man-made structures.