The sulfur cycle is a biogeochemical process describing the movement of sulfur through the Earth’s atmosphere, hydrosphere, and lithosphere, cycling through various chemical forms. Sulfur is fundamental to all life, incorporated into biomolecules such as the amino acids cysteine and methionine, which are necessary for protein structure and many cofactors. Sulfur exists in multiple oxidation states, ranging from the most reduced state, sulfide (\(\text{S}^{2-}\)), to the most oxidized state, sulfate (\(\text{SO}_4^{2-}\)). These changes in oxidation state, which drive the entire cycle, are almost exclusively mediated by microorganisms.
Microbial Reduction of Sulfur Compounds
Microbial reduction uses oxidized sulfur compounds as electron acceptors, a metabolic strategy allowing certain microbes to generate energy in the absence of oxygen. The most widespread process is Dissimilatory Sulfate Reduction (DSR), which occurs primarily in anaerobic environments like deep-sea sediments, waterlogged soils, and anoxic water columns. This process is carried out by Sulfate-Reducing Bacteria (SRBs) and archaea.
SRBs utilize sulfate (\(\text{SO}_4^{2-}\)) as a terminal electron acceptor in anaerobic respiration. The sulfate must first be activated using ATP to form adenosine 5′-phosphosulfate (APS). APS is then reduced in a series of enzymatic steps, ultimately yielding hydrogen sulfide (\(\text{H}_2\text{S}\)) as the final metabolic product. This conversion requires eight electrons to fully reduce the sulfate ion.
The resulting hydrogen sulfide is highly reactive and toxic to most aerobic life. It can precipitate heavy metals like iron to form insoluble metal sulfides, which often color sediments black. The process of DSR is ancient, with evidence suggesting its existence as far back as 3.5 billion years ago, making it one of the earliest forms of microbial metabolism on Earth.
Microbial Oxidation of Sulfur Compounds
Microbial oxidation acts as the counter-balance to reduction, returning reduced sulfur compounds back to the oxidized state and completing the major redox loop of the cycle. This process is carried out by Sulfur-Oxidizing Bacteria (SOBs) and archaea, which use the oxidation of reduced sulfur compounds as their energy source. These organisms are often chemolithoautotrophs, deriving energy from inorganic chemicals and fixing carbon dioxide into organic matter.
SOBs utilize compounds like hydrogen sulfide (\(\text{H}_2\text{S}\)), elemental sulfur (\(\text{S}^0\)), and thiosulfate as electron donors. In aerobic conditions, oxygen is the electron acceptor, and sulfide oxidation proceeds through intermediate species like elemental sulfur before being fully oxidized to sulfate (\(\text{SO}_4^{2-}\)). The genus Thiobacillus includes many well-known aerobic SOBs that contribute to this process.
Oxidation can also occur under anaerobic conditions, typically mediated by phototrophic sulfur bacteria, such as green and purple sulfur bacteria. These microbes perform anoxygenic photosynthesis, using light energy to drive the oxidation of reduced sulfur compounds instead of water. The electrons released from the sulfur compounds are used to fix carbon dioxide, which links the sulfur cycle directly to the global carbon cycle.
Integration of Sulfur into Organic Matter
Assimilatory Sulfate Reduction (ASR) is the pathway used by most organisms for growth and biomass production, contrasting with dissimilatory processes focused on energy. Plants, fungi, and a wide variety of prokaryotes perform ASR to incorporate inorganic sulfate into their cellular material. This process differs from DSR because the reduced sulfur is retained within the cell to form organic molecules.
In ASR, sulfate is reduced to sulfide, which is then immediately incorporated into the amino acids cysteine and methionine. This pathway is tightly regulated, ensuring that only enough sulfate is reduced to meet the organism’s nutritional requirements. The initial activation of sulfate in ASR often involves the formation of 3′-phosphoadenosine-5′-phosphosulfate (PAPS), which differentiates it enzymatically from the DSR pathway.
The reverse process, the decomposition of organic sulfur, is microbially driven and known as mineralization. When organisms die, microbes degrade the sulfur-containing organic molecules, releasing the sulfur back into the environment. This decomposition typically yields hydrogen sulfide (\(\text{H}_2\text{S}\)), providing a source of reduced sulfur that can re-enter the microbial oxidation or reduction loops.
Global Impact of Microbial Sulfur Cycling
The microbial transformations within the sulfur cycle have far-reaching consequences for global geochemistry and climate regulation. Aerobic sulfur oxidation, carried out by bacteria like Acidithiobacillus, can lead to the formation of sulfuric acid (\(\text{H}_2\text{SO}_4\)) when oxidizing metal sulfides. This acid production is the primary cause of Acid Mine Drainage (AMD), a severe environmental problem that mobilizes heavy metals and acidifies surrounding waterways.
Microbial sulfur cycling also plays a defining role in the ocean through the production of volatile sulfur compounds like Dimethyl Sulfide (DMS). DMS is generated primarily by the enzymatic cleavage of Dimethylsulfoniopropionate (DMSP), a compound produced by marine algae and some bacteria. Marine heterotrophic bacteria are the major degraders of DMSP, converting it into DMS.
DMS is the most abundant biogenic volatile sulfur compound in the ocean and readily vents into the atmosphere. Once airborne, DMS undergoes oxidation to form sulfate aerosols. These aerosols serve as cloud condensation nuclei, influencing cloud formation, planetary reflectivity, and regional climate patterns. The constant microbial interplay between reduction, oxidation, and organic incorporation thus regulates the availability of a fundamental nutrient and affects global atmospheric processes.