The sulfur cycle
The sulfur cycle is the movement of sulfur among rocks, soils, water, organisms, the atmosphere, and sediments through chemical and microbial transformations.
What the sulfur cycle is
The sulfur cycle is a biogeochemical cycle that moves sulfur through Earth systems. Sulfur can be locked in rocks, dissolved as sulfate, reduced to sulfide, built into living tissue, emitted as gases, buried in sediments, or returned to circulation by weathering and microbial activity.
No single simple loop
Unlike a classroom diagram with one neat arrow path, sulfur follows many routes. Some sulfur stays in minerals for long periods. Some moves quickly through soils, wetlands, oceans, volcanic gases, air pollution, or microbial communities in oxygen-poor sediments.
Rocks, weathering, and water
A large share of Earth sulfur is stored in rocks and minerals such as sulfides and sulfates. Weathering releases sulfur to soils, rivers, and oceans. In water, sulfate is often an important dissolved form, while sediments can store reduced sulfur minerals over geologic time.
Microbes change sulfur forms
Microorganisms drive many sulfur transformations. Some oxidize sulfide or elemental sulfur to sulfate, sometimes gaining energy in the process. Others reduce sulfate or sulfur compounds in oxygen-poor environments. These reactions link sulfur cycling to carbon, oxygen, iron, and nitrogen chemistry.
Assimilation into life
Plants, algae, fungi, and microbes assimilate sulfur into biomolecules. Sulfur becomes part of cysteine, methionine, glutathione, iron-sulfur clusters, coenzymes, and many specialized compounds. Animals then obtain sulfur mainly by eating organic sulfur in food.
Atmospheric sulfur
Sulfur can enter the atmosphere from volcanoes, sea spray, wildfires, wetlands, biological emissions, and human activities such as burning sulfur-containing fuels. In the air, sulfur compounds can form sulfate particles or sulfuric acid, influencing haze, clouds, and acid deposition.
Hydrothermal vents and sediments
At hydrothermal vents, reduced sulfur compounds from Earth interior fluids can support chemosynthetic communities. In marine and freshwater sediments, sulfate reduction and sulfide oxidation help shape oxygen levels, metal chemistry, mineral formation, and the availability of nutrients.
Human changes
Mining, fossil fuel combustion, fertilizer use, wastewater, and industrial emissions have altered sulfur flows. Air-quality controls reduced sulfur dioxide pollution in many regions, while agriculture now pays closer attention to sulfur as a plant nutrient where atmospheric deposition has declined.
Why it matters
The sulfur cycle connects geology, microbes, climate, agriculture, pollution, ocean chemistry, and deep-sea ecosystems. It helps explain acid rain, sulfide-rich sediments, sulfur nutrition in crops, mineral deposits, vent food webs, and the chemistry of oxygen-poor environments.