What Is the Sulfur Cycle and Why Is It Important?

The sulfur cycle is the continuous movement of sulfur atoms through Earth’s rocks, oceans, atmosphere, and living organisms. Like the better-known carbon and nitrogen cycles, it is one of the fundamental biogeochemical loops that keep the planet habitable. Sulfur is essential for building proteins, it influences how much sunlight reaches the ground, and it has shaped the chemistry of the atmosphere for billions of years. What makes the sulfur cycle particularly interesting is how thoroughly it connects systems that seem unrelated: ocean plankton to cloud formation, volcanic eruptions to summer temperatures, clean-air regulations to crop yields.

How Sulfur Moves Through the Environment

Sulfur exists in many chemical forms, and the cycle’s basic logic is straightforward: sulfur gets released from rocks and sediments, passes through water and air, gets taken up by living things, and eventually returns to the ground or ocean floor. The details, though, involve a web of chemical and biological transformations happening simultaneously in different environments.

In rocks and sediments, sulfur sits mainly as sulfide minerals like pyrite or as sulfate salts like gypsum. Weathering and erosion slowly dissolve these minerals, delivering sulfate to rivers and eventually the ocean. Once in water, sulfate is one of the most abundant dissolved ions in seawater. Volcanic eruptions inject sulfur dioxide directly into the atmosphere, sometimes high into the stratosphere, where it can linger for months. Living organisms, especially microbes, are responsible for many of the conversions between sulfur’s chemical forms. Some bacteria reduce sulfate to sulfide; others oxidize sulfide back to sulfate. Plants pull sulfate from soil and incorporate it into organic molecules. When organisms die and decompose, their sulfur re-enters the cycle.

Why Living Things Need Sulfur

Every cell in your body contains sulfur. It shows up in two of the twenty amino acids that make up proteins: methionine and cysteine. These sulfur-containing amino acids do far more than fill slots in protein chains. They help maintain the cell’s ability to handle toxic compounds, free radicals, and reactive oxygen species, essentially serving as part of the cell’s chemical defense system.¹ When sulfur amino acid supply runs low, protein synthesis suffers, and that shortage can come either from not eating enough of them or from the body diverting them to make other critical molecules like glutathione, an antioxidant the body ramps up during stress.²

Sulfur also matters at a signaling level. Hydrogen sulfide, a gas you would recognize by its rotten-egg smell, turns out to be an important signaling molecule in the human body, with effects on cardiovascular and neurological function.³ Research has shown that hydrogen sulfide helps brain cells communicate and can relax blood vessels, leading to its classification as both a neuromodulator and a smooth muscle relaxant.⁴ The same molecule that smells terrible at high concentrations plays a constructive role at trace amounts inside cells.

Plants and the Soil Sulfur Supply

Plants take up sulfur as sulfate dissolved in soil water, pull it into their roots, and then reduce it step by step inside their cells until it becomes cysteine, the first organic sulfur compound they produce.⁵ From cysteine, the sulfur gets incorporated into a wide range of other molecules the plant needs. This reduction process happens mainly in the chloroplasts, where a chain of enzymatic steps converts sulfate through sulfite and finally to sulfide before it is built into cysteine.⁶

Where soil sulfur comes from matters more than you might expect. For decades, crops in much of the industrialized world got a significant amount of their sulfur for free: sulfur dioxide from coal-burning power plants would settle onto fields as atmospheric deposition. When clean-air regulations cut those emissions, the free sulfur supply dropped, and farmers in the US Midwest and across Europe began noticing sulfur deficiency in their crops. Fertilizer use has risen to compensate.⁷ One economic analysis estimated that the drop in airborne sulfate from the US Acid Rain Program lowered corn and soybean revenue by roughly $1 to $1.5 billion per year, with farmer responses to replace the lost sulfur being limited and slower than ideal.⁸ It is a striking example of how one environmental problem (acid rain) was inadvertently subsidizing another system (agriculture), and fixing the first created a new challenge.

Microbes as the Engine of the Cycle

If the sulfur cycle has a workforce, it is microorganisms. Sulfate-reducing bacteria and archaea are found in soils, ocean sediments, wetlands, and deep underground. These organisms use sulfate the way we use oxygen: as the substance that accepts electrons at the end of their energy-generating chemistry. Sulfate-reducing prokaryotes are deeply intertwined with terrestrial carbon and sulfur cycles, and researchers continue to discover new sulfate-reducing species and unexpected interactions with soil fungi.⁹

On the other end of the spectrum, sulfur-oxidizing bacteria convert sulfide back to sulfate, completing the microbial sulfur loop. Some of these organisms thrive in extreme environments. At deep-sea hydrothermal vents, chemosynthetic sulfur-oxidizing bacteria harvest energy from sulfide-rich water pouring out of the seafloor. This process, rather than sunlight, provides the primary food source for the dense communities of tube worms, clams, and shrimp clustered around the vents.¹⁰ The discovery of these vent ecosystems in the late 1970s reshaped how biologists think about what life requires: not necessarily sunlight, but a chemical energy gradient, and sulfur often provides it.

The Ocean-to-Cloud Connection

One of the most consequential parts of the sulfur cycle plays out between the surface ocean and the atmosphere. Marine phytoplankton produce a compound called dimethylsulfoniopropionate, or DMSP, which serves various functions inside the cell. When DMSP breaks down, it releases dimethyl sulfide, or DMS, a volatile gas that escapes into the air above the ocean. Measurements of Antarctic phytoplankton have found that most species tested could produce DMSP, with production rates and concentrations varying enormously between species.¹¹

Once DMS reaches the atmosphere, it gets oxidized into sulfur-containing particles. These particles can scatter incoming sunlight directly, and they also serve as seeds around which water vapor condenses to form cloud droplets, known as cloud condensation nuclei.¹² More cloud condensation nuclei generally means more, brighter clouds that reflect more sunlight back to space, which cools the surface below. Modeling studies have shown that DMS emissions, combined with ammonia, can drive new particle formation over the ocean that grows into cloud-relevant sizes within three to four days.¹³

The chemistry is more complicated than it first appeared. Global airborne sampling has revealed that more than 30 percent of DMS emitted over the ocean gets converted into a previously unknown compound called hydroperoxymethyl thioformate, or HPMTF, which acts as a major reservoir for marine sulfur in the atmosphere.¹⁴ This finding means atmospheric models that ignore this pathway are getting the link between ocean biology and cloud formation wrong. In short, plankton in the ocean help determine how cloudy it gets over open water, and the sulfur cycle is the mechanism connecting the two.

Volcanoes and Temperature

Volcanic eruptions are the sulfur cycle’s most dramatic atmospheric event. When a large eruption shoots sulfur dioxide into the stratosphere, the gas converts to tiny sulfate aerosol particles that spread around the globe and reflect sunlight, cooling the surface. Both explosive eruptions and proposed geoengineering schemes based on stratospheric sulfur injections work on this same principle.¹⁵

Historical analysis using ice cores and tree rings has shown that some of the coldest summers in the Northern Hemisphere over the past few thousand years trace to volcanic eruptions that injected sulfur high into the atmosphere. Interestingly, eruptions at higher latitudes appear to pack a bigger cooling punch per unit of sulfur than tropical eruptions, and earlier estimates of how much sulfur some of these historical eruptions injected may have been too high by as much as a factor of two. The implication is that Northern Hemisphere summer temperatures are more sensitive to volcanic sulfur loading than previously assumed.¹⁶

When the Cycle Goes Wrong

Human activity has massively altered the sulfur cycle. As early as the 1970s, researchers estimated that human sulfur emissions were about half of what nature produced globally, with projections that by the year 2000, human contributions would roughly match natural ones, and in the Northern Hemisphere alone would exceed them.¹⁷ The most visible consequence was acid rain. Sulfur dioxide released by burning fossil fuels reacts in the atmosphere to form sulfuric acid, which falls with precipitation. Research has shown that mineral dust particles in the atmosphere can catalyze the conversion of sulfur dioxide to sulfuric acid even when only molecular oxygen is available, meaning acid rain can form under a broad range of atmospheric conditions, not just in the presence of specialized oxidants.¹⁸

Mining creates another disruption. When sulfide-rich rocks are exposed to air and water during mining operations, the sulfides oxidize, generating sulfuric acid that leaches into waterways. This acid mine drainage lowers the pH of rivers and streams, dissolves metals from the rock, and allows those metals to bioaccumulate up the food chain.¹⁹ Abandoned mines can continue leaching acid for centuries, making it one of the most persistent environmental legacies of resource extraction.

Sulfur, Methane, and Wetlands

Wetlands are a surprisingly active stage for sulfur cycle drama, largely because of what sulfate-reducing microbes prevent. When sulfate is available in a wetland, sulfate-reducing bacteria outcompete methane-producing organisms for the same food sources. Since sulfate reduction is thermodynamically favored over the fermentation pathways that lead to methane, it effectively lowers the total amount of methane a wetland produces, reducing the flux of this potent greenhouse gas to the atmosphere.²⁰ This is a case where the sulfur cycle directly intersects with climate: the availability of one sulfur compound in a wetland determines how much of a greenhouse gas that wetland emits.

Sulfur as a Window into Deep Time

The sulfur cycle has been running for billions of years, and it has left chemical fingerprints that geologists can read. Sulfur comes in several stable isotope variants, and biological processes preferentially use lighter isotopes. By measuring the ratio of sulfur isotopes in ancient rocks, researchers can infer what kinds of microbial metabolisms were operating in a particular environment and when. Analyses of organic sulfur preserved in 3.45-billion-year-old stromatolites have provided direct evidence of microbial sulfur metabolism operating that early in Earth’s history.²¹

Lab experiments have added nuance to how scientists interpret these isotopic signatures. Modern sulfate-reducing bacteria from similar environments can produce a wide range of sulfur isotope patterns, suggesting that evolutionary adaptation, not just environmental conditions, shapes the isotope fingerprint a community leaves behind.²² This complicates the straightforward reading of ancient rocks but also makes the tool more powerful once the effects of adaptation are accounted for.

Over the sweep of geologic time, the sulfur cycle has been deeply coupled with the carbon cycle in controlling how much oxygen fills the atmosphere. The burial of organic matter and pyrite in sediments removes carbon and sulfur from the surface environment and releases oxygen. When those same sediments eventually weather and oxidize, they consume oxygen. This interplay between the carbon and sulfur cycles has been one of the principal controls on atmospheric oxygen levels throughout the last 540 million years.²³

Sulfur Deep Underground and Beyond Earth

The sulfur cycle does not stop at the surface. Deep in Earth’s crust, sulfate-reducing microbes survive in groundwater that has been isolated from the surface for millions to billions of years, sustained by energy from the radioactive decay of elements in surrounding rocks. Radiolysis of water produces hydrogen, which these microorganisms use alongside sulfate derived from the oxidation of sulfide minerals in the rock matrix.²⁴ This deep biosphere is almost entirely independent of the sun.

The same logic has been applied to Mars. Modeling suggests that radiolysis could produce enough energy to support sulfate-reducing microbes in Martian groundwater, if such water exists. The sulfur cycle, in this view, is not just an Earth story. It offers a framework for thinking about where life could persist on other worlds, running on chemical energy in the dark, mediated by sulfur compounds, with no sunlight required.²⁴

Even deeper, the sulfur cycle operates within Earth’s mantle. Sulfate carried down by subducting ocean plates can oxidize the overlying mantle rock, increasing its oxygen content by several orders of magnitude. This mantle-level sulfur chemistry drives the formation of gold-rich ore deposits in subduction zones, connecting the deep sulfur cycle to the concentration of economically important metals near the surface.²⁵ The same element that cloud-seeding plankton release into the marine atmosphere also shapes the chemistry of rocks a hundred kilometers underground.