The sulfur cycle is the biogeochemical cycle that describes how sulfur moves through the atmosphere, hydrosphere, lithosphere, and biosphere, changing between different chemical forms as it circulates through Earth systems. Sulfur is an essential element for all living organisms because it is a key component of certain amino acids, proteins, enzymes, and vitamins. Unlike the carbon cycle and nitrogen cycle, most sulfur is stored in rocks and ocean sediments rather than the atmosphere, making geological processes especially important. Weathering, volcanic eruptions, microbial activity, and human activities all influence how sulfur moves through the environment.
Key Takeaways: Sulfur Cycle
- The sulfur cycle is the movement of sulfur through Earth’s rocks, water, air, and living organisms.
- Most sulfur is stored in rocks, minerals, and marine sediments rather than the atmosphere.
- Sulfur cycles through several oxidation states, from sulfide (S²⁻) to sulfate (SO₄²⁻).
- Microorganisms play the central role in converting sulfur between different chemical forms.
- Plants absorb sulfur mainly as sulfate ions from the soil.
- Sulfur is essential for amino acids such as cysteine and methionine, as well as many enzymes and vitamins.
- Volcanoes, weathering, sea spray, and decomposition are major natural sulfur sources.
- Human activities, especially burning fossil fuels, have significantly altered the sulfur cycle and contributed to acid rain.
- Modern pollution controls have reduced sulfur dioxide emissions in many countries, but sulfur pollution remains a global issue.
Why the Sulfur Cycle Is Important
The sulfur cycle is essential because sulfur is required by every living organism. Without sulfur, cells could not produce many proteins or maintain proper enzyme function.
The sulfur cycle also:
- Maintains soil fertility by replenishing plant-available sulfate.
- Supports marine and terrestrial food webs.
- Influences atmospheric chemistry and cloud formation.
- Helps regulate Earth’s climate through sulfur-containing aerosols.
- Controls the formation of economically important mineral deposits.
- Plays a major role in the chemistry of wetlands, oceans, and deep-sea hydrothermal vents.
- Interacts closely with the carbon, oxygen, iron, and nitrogen cycles.
Because sulfur changes oxidation state so readily, it serves as an important energy source for many microorganisms.
History of the Sulfur Cycle
Humans have known about sulfur since ancient times. It was one of the earliest recognized elements because of its bright yellow color, volcanic origin, and strong odor when sulfur-containing compounds are burned.
Important milestones include:
| Time | Discovery |
|---|---|
| Ancient civilizations | Sulfur mined for medicines, fumigation, and gunpowder |
| 1777 | Antoine Lavoisier recognized sulfur as an element rather than a compound |
| 1800s | Geologists linked sulfur deposits to volcanic activity and sedimentary rocks |
| Late 1800s | Scientists discovered sulfur-oxidizing and sulfur-reducing bacteria |
| Early 1900s | Biogeochemical cycling became recognized as an integrated Earth process |
| 1960s–1980s | Acid rain research revealed the major influence of sulfur dioxide pollution |
| Modern era | Satellite observations and isotope studies provide global measurements of sulfur movement |
Today, researchers combine atmospheric chemistry, microbiology, geology, and climate science to understand the sulfur cycle.
What Is the Sulfur Cycle?
The sulfur cycle is the continuous circulation of sulfur among Earth’s major reservoirs:
- Atmosphere
- Biosphere
- Hydrosphere
- Lithosphere
Unlike carbon or nitrogen, sulfur spends relatively little time in the atmosphere. Instead, it is largely stored in:
- Sulfide minerals
- Sulfate minerals
- Ocean sediments
- Sedimentary rocks
Sulfur continuously changes chemical form through oxidation and reduction reactions.
Common sulfur forms include:
| Form | Example |
|---|---|
| Elemental sulfur | S₈ |
| Sulfide | H₂S, FeS |
| Sulfate | SO₄²⁻ |
| Sulfur dioxide | SO₂ |
| Sulfuric acid | H₂SO₄ |
| Organic sulfur | Proteins, amino acids |
Major Reservoirs of Sulfur
Most sulfur is stored in the lithosphere.
Approximate distribution:
| Reservoir | Relative amount |
|---|---|
| Sedimentary rocks | Very large |
| Ocean sulfate | Very large |
| Mineral deposits | Large |
| Soil | Moderate |
| Living organisms | Small |
| Atmosphere | Very small |
Although atmospheric sulfur represents only a tiny fraction of Earth’s sulfur, it strongly influences air quality and climate.
How the Sulfur Cycle Works
The sulfur cycle consists of several interconnected processes.
Rocks
↓ weathering
Sulfate in soil
↓
Plants
↓
Animals
↓
Decomposition
↓
Organic sulfur
↓
Microbial transformations
↙ ↘
H₂S Sulfate
↘ ↙
Atmosphere
↓
Rain
↓
Soils and oceans
1. Weathering of Rocks
The cycle begins when rain, groundwater, and erosion break down sulfur-containing rocks.
Examples include:
- Gypsum (CaSO₄·2H₂O)
- Pyrite (FeS₂)
- Anhydrite (CaSO₄)
Weathering releases sulfate into soils and streams.
2. Plant Uptake
Plants absorb sulfur almost exclusively as sulfate ions (SO₄²⁻).
Inside plants, sulfate is converted into sulfur-containing organic molecules, including:
- Cysteine
- Methionine
- Glutathione
- Coenzyme A
- Biotin
- Thiamine
Animals obtain sulfur by eating plants or other animals.
3. Transfer Through Food Webs
Sulfur moves through ecosystems as organisms consume one another.
Eventually:
- organisms die,
- leaves fall,
- wastes accumulate.
These materials return sulfur to soils and water.
4. Decomposition
Bacteria and fungi break down dead organic matter.
Organic sulfur compounds become:
- sulfate
- hydrogen sulfide
- elemental sulfur
depending on environmental conditions.
5. Microbial Transformations
Microorganisms perform most sulfur conversions.
Major microbial processes include:
Sulfur oxidation
Bacteria convert:
H₂S → S → SO₄²⁻
Examples include Acidithiobacillus species.
Sulfate reduction
Anaerobic bacteria convert:
SO₄²⁻ → H₂S
These bacteria thrive in:
- wetlands
- marine sediments
- oxygen-poor soils
- sewage systems
Examples include Desulfovibrio.
Sulfur disproportionation
Some bacteria simultaneously oxidize and reduce intermediate sulfur compounds.
These reactions are important in marine sediments.
6. Atmospheric Transport
Natural sulfur enters the atmosphere from:
- volcanoes
- sea spray
- wetlands
- forest emissions
- ocean plankton
Major atmospheric sulfur compounds include:
- sulfur dioxide (SO₂)
- hydrogen sulfide (H₂S)
- dimethyl sulfide (DMS)
Atmospheric sulfur eventually returns through:
- rainfall
- snowfall
- dry deposition
7. Burial and Geological Recycling
Some sulfur becomes buried in sediments.
Over millions of years it becomes:
- pyrite
- gypsum
- petroleum sulfur
- coal sulfur
Plate tectonics eventually expose these materials again through uplift and erosion.
Sources of Sulfur
Natural sulfur comes from many sources.
Geological sources
- Volcanoes
- Hot springs
- Hydrothermal vents
- Weathering rocks
- Mineral deposits
Biological sources
- Decomposition
- Marine plankton
- Wetlands
- Sulfur bacteria
Atmospheric sources
- Sea spray
- Ocean emissions
- Wildfires
Human sources
- Coal combustion
- Oil refining
- Metal smelting
- Fertilizer production
- Industrial manufacturing
Sulfur Oxidation States
One reason sulfur participates in so many reactions is that it exists in many oxidation states.
| Compound | Oxidation state |
|---|---|
| Hydrogen sulfide (H₂S) | –2 |
| Pyrite (FeS₂) | –1 |
| Elemental sulfur | 0 |
| Sulfur dioxide (SO₂) | +4 |
| Sulfate (SO₄²⁻) | +6 |
Microorganisms exploit these changes to obtain energy.
The Sulfur Cycle and Other Biogeochemical Cycles
The sulfur cycle is closely connected to several other Earth cycles.
Carbon cycle
Sulfate-reducing bacteria decompose organic matter without oxygen.
Oxygen cycle
Oxygen availability determines whether sulfur becomes sulfate or sulfide.
Iron cycle
Iron combines with sulfide to form minerals such as pyrite.
Nitrogen cycle
Many anaerobic microorganisms participate in both sulfur and nitrogen transformations.
These interactions make sulfur an important regulator of ecosystem chemistry.
Human Effects on the Sulfur Cycle
Humans have altered the sulfur cycle dramatically since the Industrial Revolution.
Major impacts include:
Burning fossil fuels
Coal and petroleum contain sulfur.
Burning them releases sulfur dioxide into the atmosphere.
Acid Rain
Atmospheric sulfur dioxide reacts with oxygen and water:
SO₂ + O₂ + H₂O → H₂SO₄
Sulfuric acid contributes to acid rain, which:
- acidifies lakes
- damages forests
- erodes buildings
- harms aquatic life
- alters soil chemistry
Mining
Mining exposes sulfide minerals to oxygen.
Pyrite oxidation produces sulfuric acid, causing acid mine drainage, one of the most serious forms of water pollution associated with mining.
Agriculture
Modern fertilizers often add sulfur because decades of reduced atmospheric sulfur deposition have left some agricultural soils sulfur deficient.
Air Pollution Controls
Many countries have greatly reduced sulfur dioxide emissions through:
- low-sulfur fuels
- flue-gas desulfurization (“scrubbers”)
- emissions regulations
- cleaner industrial processes
These measures have significantly reduced acid rain in North America and Europe.
Evolution of the Sulfur Cycle
Earth’s sulfur cycle has changed dramatically over geological time.
Early Earth
More than 3 billion years ago:
- atmospheric oxygen was scarce,
- hydrogen sulfide was much more common,
- sulfur-metabolizing microbes dominated many ecosystems.
Some of the earliest forms of life likely relied on sulfur compounds rather than oxygen for energy.
Great Oxidation Event
Around 2.4 billion years ago, photosynthetic organisms increased atmospheric oxygen.
As oxygen rose:
- sulfate became much more abundant,
- oxidative weathering increased,
- new sulfur minerals formed,
- microbial communities diversified.
Modern Earth
Today’s sulfur cycle reflects interactions among:
- oxygen-rich atmosphere
- oceans
- continents
- biological activity
- plate tectonics
- human industry
Scientists reconstruct ancient sulfur cycles using sulfur isotope ratios preserved in sedimentary rocks, which provide evidence for changing atmospheric oxygen levels and microbial activity through Earth’s history.
Examples of the Sulfur Cycle
Examples of parts of the cycle include:
- Volcanic sulfur dioxide forming sulfate aerosols.
- Marsh bacteria producing hydrogen sulfide.
- Plants absorbing sulfate from agricultural soils.
- Ocean plankton releasing dimethyl sulfide that contributes to cloud formation.
- Pyrite oxidation producing acid mine drainage.
- Sulfur bacteria around hydrothermal vents supporting entire ecosystems without sunlight.
Common Misconceptions
“Sulfur is mainly an atmospheric element.”
False. Most sulfur is stored in rocks, sediments, and the oceans.
“Plants absorb elemental sulfur.”
False. Plants primarily absorb sulfur as sulfate (SO₄²⁻).
“Sulfur only comes from volcanoes.”
Volcanoes are important natural sources, but weathering, decomposition, oceans, wetlands, and human activities also release sulfur.
“All sulfur compounds smell bad.”
Hydrogen sulfide has a strong rotten-egg odor, but many sulfur compounds, including sulfate minerals, are odorless.
“Acid rain is caused only by sulfur.”
Nitrogen oxides also contribute significantly to acid rain.
Frequently Asked Questions
What is the sulfur cycle?
It is the movement of sulfur through Earth’s atmosphere, water, rocks, soil, and living organisms.
Why is sulfur important?
Sulfur is required to make proteins, enzymes, vitamins, and many essential biological molecules.
Where is most sulfur found?
Most sulfur is stored in sedimentary rocks, minerals, and ocean sulfate.
What organisms drive the sulfur cycle?
Bacteria and archaea perform most sulfur oxidation and reduction reactions.
How do plants obtain sulfur?
Plants absorb sulfate ions (SO₄²⁻) from the soil through their roots.
What causes acid rain?
Sulfur dioxide and nitrogen oxides react with water in the atmosphere to form sulfuric and nitric acids.
Does sulfur affect climate?
Yes. Sulfate aerosols scatter sunlight and can influence cloud formation and Earth’s energy balance. Marine emissions of dimethyl sulfide (DMS) also contribute to cloud condensation nuclei, linking the sulfur cycle to climate processes.
Is the sulfur cycle faster than the carbon cycle?
No. Much of Earth’s sulfur resides in long-lived geological reservoirs, so many parts of the sulfur cycle operate over thousands to millions of years, although atmospheric and biological sulfur can cycle much more rapidly.
Glossary
Acid mine drainage: Acidic water formed when sulfide minerals are exposed to oxygen and water.
Acid rain: Precipitation made acidic by sulfuric and nitric acids.
Anaerobic: Occurring without oxygen.
Biogeochemical cycle: The movement of chemical elements through living organisms and Earth’s physical systems.
Dimethyl sulfide (DMS): A sulfur-containing gas produced mainly by marine plankton that influences cloud formation.
Hydrogen sulfide (H₂S): A toxic sulfur gas with a characteristic rotten-egg odor.
Oxidation: A chemical process in which sulfur loses electrons and generally forms compounds with higher oxidation states.
Pyrite: An iron sulfide mineral (FeS₂) commonly called fool’s gold.
Reduction: A chemical process in which sulfur gains electrons and moves to a lower oxidation state.
Sulfate (SO₄²⁻): The primary form of sulfur absorbed by plants.
Sulfide: A reduced sulfur compound containing sulfur in a negative oxidation state.
Sulfur dioxide (SO₂): A gas produced naturally by volcanoes and by burning sulfur-containing fuels.
Weathering: The physical and chemical breakdown of rocks that releases sulfur and other elements.
Interesting Facts About the Sulfur Cycle
- Sulfur can exist in oxidation states ranging from –2 to +6, giving it one of the widest ranges of any biologically important element.
- The characteristic odor of rotten eggs comes from hydrogen sulfide—not elemental sulfur.
- Sulfate is the second most abundant dissolved anion in seawater after chloride.
- Deep-sea hydrothermal vent ecosystems are fueled largely by sulfur-oxidizing microbes rather than sunlight.
- Some volcanic eruptions inject enough sulfur dioxide into the stratosphere to cool global temperatures for one to several years by forming reflective sulfate aerosols.
- The sulfur cycle is one of the oldest metabolic cycles on Earth; sulfur-based microbial metabolism likely evolved before oxygenic photosynthesis.
References and Further Reading
- Canfield, D.E.; Raiswell, R. (1999). “The evolution of the sulfur cycle”. American Journal of Science. 299 (7–9): 697–723. doi:10.2475/ajs.299.7-9.697
- Machel, H.G.; Krouse, H.R.; Sassen, R. (1995). “Products and distinguishing criteria of bacterial and thermochemical sulfate reduction”. Applied Geochemistry. 10 (4): 373–389. doi:10.1016/0883-2927(95)00008-8
- Pham, M.; Müller, J.F.; et al. (1996). “A 3D model study of the global sulphur cycle: Contributions of anthropogenic and biogenic sources”. Atmospheric Environment. 30 (10–11): 1815–1822. doi:10.1016/1352-2310(95)00390-8
- Schlesinger, W.H. (1997). Biogeochemistry an Analysis of Global Change (2nd ed.). San Diego, California: Academic Press. ISBN 9780126251555.
- Sievert, Stefan M.; Hügler, Michael; Taylor, Craig D.; Wirsen, Carl O. (2008). Dahl, Christiane; Friedrich, Cornelius G. (eds.). “Sulfur Oxidation at Deep-Sea Hydrothermal Vents”. Microbial Sulfur Metabolism. Berlin, Heidelberg: Springer: 238–258. doi:10.1007/978-3-540-72682-1_19. ISBN 978-3-540-72682-1.