Table of Contents
Overview of the Sulfur Cycle
Sulfur is an essential element for life, present in amino acids (cysteine, methionine), many enzymes, and some coenzymes. The sulfur cycle describes how sulfur moves between atmosphere, hydrosphere, lithosphere, and biosphere, changing its chemical form along the way.
Unlike the carbon and nitrogen cycles, a large proportion of sulfur is stored in rocks and sediments. Biological processes are strongly linked to redox reactions, in which sulfur changes oxidation state between reduced forms (e.g. sulfide) and oxidized forms (e.g. sulfate).
Key features specific to the sulfur cycle:
- Strong connection to anoxic (oxygen-poor) environments, such as sediments and wetlands
- Important role in odor and toxicity (e.g. hydrogen sulfide, $H_2S$)
- Close coupling to iron cycle and climate-relevant gases (e.g. dimethyl sulfide from oceans)
Major Sulfur Reservoirs and Forms
Main Reservoirs
- Sedimentary rocks and minerals
- The largest sulfur reservoir
- Present in minerals like gypsum ($CaSO_4 \cdot 2H_2O$) and pyrite ($FeS_2$)
- Sulfur becomes available through weathering, volcanic activity, and mining
- Oceans
- Contain large amounts of dissolved sulfate ($SO_4^{2-}$), the dominant oxidized form
- Marine organisms use sulfate and organic sulfur compounds
- Soils and sediments
- Contain both inorganic sulfur (sulfate, sulfide) and organic sulfur bound in biomass and humus
- Often sites of intense microbial sulfur transformations
- Atmosphere
- Contains sulfur gases in low concentrations:
- $SO_2$ (sulfur dioxide) from volcanoes, industry, combustion
- $H_2S$ (hydrogen sulfide) from anaerobic decomposition
- Organosulfur compounds such as dimethyl sulfide (DMS)
Important Chemical Forms
Key inorganic forms and their typical environments:
- Sulfate: $SO_4^{2-}$
- Most oxidized, stable in oxygen-rich water and soils
- Primary plant-available form
- Sulfite: $SO_3^{2-}$
- Intermediate oxidation state
- Often transient; formed during oxidation or reduction
- Elemental sulfur: $S^0$
- Intermediate, can be stored in cells of some bacteria or in sediments
- Often a product or substrate of microbial metabolism
- Thiosulfate: $S_2O_3^{2-}$ and other polythionates
- Intermediate compounds in microbial sulfur transformations
- Sulfide: $S^{2-}$ and $H_2S$
- Strongly reduced form
- Typical of anoxic sediments, bogs, sewage, and some groundwater
Organic forms:
- Organic sulfur in biomass
- Sulfur-containing amino acids and other cell components
- Volatile organosulfur compounds
- e.g. dimethyl sulfide (DMS, $(CH_3)_2S$), methyl mercaptan ($CH_3SH$); important in odor and atmospheric chemistry
Key Processes in the Sulfur Cycle
1. Weathering and Release from Rocks
- Chemical and physical weathering of sulfur-containing rocks releases sulfate and sulfide into:
- Soils (where they may be taken up by plants or further transformed)
- Rivers and eventually the oceans
- Volcanic activity and geothermal emissions:
- Emit $SO_2$, $H_2S$, and particulate sulfur
- These gases can be oxidized in the atmosphere to sulfate aerosols, contributing to acid deposition
2. Sulfate Assimilation by Organisms
- Plants, algae, and many microorganisms take up sulfur primarily as sulfate:
- $SO_4^{2-}$ is absorbed from soil solution (terrestrial) or water (aquatic)
- Inside cells, sulfate is reduced and incorporated into organic molecules, especially amino acids
The basic direction of this process is:
$$ SO_4^{2-} \rightarrow \text{organic S (in biomass)} $$
Consumers (animals, many microbes) obtain sulfur by eating biomass, not by taking up sulfate directly (with some exceptions among microbes).
3. Mineralization of Organic Sulfur
When organisms die or excrete waste, organic sulfur compounds undergo decomposition:
- In oxygen-rich environments (aerobic decomposition):
- Organic sulfur is mineralized to sulfate, often via sulfite:
$$ \text{Organic S} \rightarrow SO_3^{2-} \rightarrow SO_4^{2-} $$ - In oxygen-poor environments (anaerobic decomposition):
- Organic matter degradation can release hydrogen sulfide:
$$ \text{Organic S} \rightarrow H_2S $$
This mineralization returns inorganic sulfur to the environment, closing the loop with assimilation.
4. Microbial Sulfate Reduction
In anoxic conditions where oxygen and often nitrate are absent (e.g. waterlogged soils, marine sediments, sewage):
- Sulfate-reducing bacteria use sulfate as a terminal electron acceptor in their energy metabolism:
$$ SO_4^{2-} + \text{organic matter (or }H_2) \rightarrow H_2S + \text{CO}_2 + H_2O $$
Key features:
- Generates $H_2S$, which:
- Is toxic to many organisms
- Has a characteristic “rotten egg” smell
- Plays a major role in:
- Coastal and marine sediments
- Peatlands and rice paddies
- Anaerobic digesters (wastewater treatment, biogas plants)
5. Sulfide Oxidation
Hydrogen sulfide and other reduced sulfur compounds are thermodynamically unstable in the presence of oxygen or other oxidants and tend to be oxidized back to sulfate.
Abiotic oxidation
- Chemical oxidation:
- $H_2S$ can react with oxygen in air or water:
$$ H_2S + 2 \, O_2 \rightarrow H_2SO_4 $$ - Intermediate products may include elemental sulfur or sulfite
- Can contribute to material corrosion (e.g. in sewage systems) due to sulfuric acid formation.
Biological oxidation
- Sulfur-oxidizing bacteria and archaea use reduced sulfur as an energy source:
- Examples of substrates: $H_2S$, $S^0$, $S_2O_3^{2-}$
- Overall direction:
$$ H_2S \; (\text{or } S^0, S_2O_3^{2-}) + O_2 \rightarrow SO_4^{2-} + \text{energy} $$ - Some groups (e.g. colorless sulfur bacteria, purple and green sulfur bacteria under specific conditions) play key roles in:
- Sulfur cycling at redox interfaces (where oxic and anoxic zones meet, e.g. at sediment–water boundaries)
- Chemolithoautotrophy (using inorganic sulfur compounds to fix $CO_2$)
6. Formation of Sulfur Minerals and Long-Term Storage
In sediments, especially marine ones:
- Reaction of sulfide with metals, especially iron, forms insoluble metal sulfides:
- Example:
$$ Fe^{2+} + H_2S \rightarrow FeS + 2 H^+ $$ - Further transformation can yield pyrite ($FeS_2$)
- Over geological timescales, these sulfide minerals and sulfate-bearing minerals build up as:
- Sedimentary rocks (e.g. pyrite-rich shales, evaporites like gypsum)
- Long-term sulfur reservoirs
Later uplift and weathering can re-release this sulfur, feeding back into the biological cycle.
The Atmospheric Sulfur Cycle
Although atmospheric sulfur is a smaller pool, it significantly affects ecology and climate.
Natural Emissions
- From land and water bodies:
- $H_2S$ from wetlands, volcanic/geothermal areas, and anaerobic habitats
- Volatile organosulfur compounds from decomposition and biological activity
- From oceans:
- Dimethyl sulfide (DMS) is produced by marine plankton and released to the atmosphere:
- DMS is oxidized to sulfate aerosols
- These aerosols act as cloud condensation nuclei (CCN), influencing cloud properties and climate
Oxidation and Deposition
In the atmosphere:
- Gaseous sulfur compounds ($SO_2$, DMS, $H_2S$) are oxidized to sulfuric acid and sulfate particles.
- These return to the surface via:
- Wet deposition: rain, snow (often contributing to acidification)
- Dry deposition: direct settling of particles and gas absorption by surfaces
This deposition:
- Supplies sulfate to remote ecosystems (e.g. mountain lakes, boreal forests)
- Can cause environmental stress if acidity is high
Human Influences on the Sulfur Cycle
Human activities have altered the natural sulfur cycle, especially in the atmosphere and soils.
Major Anthropogenic Sources
- Combustion of sulfur-rich fossil fuels (coal, heavy oil):
- Releases large amounts of $SO_2$ and some $H_2S$
- Historically a major source of acid rain
- Metal smelting and industrial processes:
- Emission of $SO_2$ during roasting of sulfide ores (e.g. $CuS$, $ZnS$)
- Agriculture and land use:
- Application of sulfur-containing fertilizers and amendments (e.g. gypsum)
- Drainage of wetlands and peatlands, changing redox conditions and sulfur transformations
- Manure and slurry storage can release $H_2S$
- Fossil fuel extraction and processing:
- Oil and gas often contain sulfur compounds; refining and flaring release $SO_2$ and $H_2S$
Ecological Consequences
Acid Deposition
- $SO_2$ forms sulfuric acid in the atmosphere:
$$ SO_2 + O_2 + H_2O \rightarrow H_2SO_4 $$ - Resulting acid deposition can:
- Acidify soils and surface waters
- Mobilize toxic metals (e.g. aluminum)
- Damage forests and aquatic ecosystems
- Emission control (e.g. flue-gas desulfurization) has reduced this problem in many regions, but it persists in others.
Soil and Water Chemistry
- Elevated sulfate input can:
- Change cation balances (e.g. leaching of calcium, magnesium)
- Influence microbial processes, including sulfate reduction and methane production
- Affect plant nutrition: both deficiencies and excesses of sulfate can impact growth
Interaction with Climate
- Sulfate aerosols from $SO_2$ emissions increase atmospheric aerosol loading:
- Reflect sunlight (cooling effect)
- Seed clouds and change their optical properties
- This cooling partly offsets greenhouse gas warming locally and temporally, complicating climate dynamics.
Ecological Roles of Sulfur Transformations
Redox Gradients and Microbial Niches
Because sulfur can exist in multiple oxidation states, it is central to redox zoning in ecosystems:
- In oxic layers, sulfur is mainly present as sulfate; sulfur-oxidizing microorganisms may thrive where reduced compounds seep in.
- In suboxic and anoxic layers, sulfate reducers and sulfur-disproportionating microbes are active, generating $H_2S$.
- At interfaces (e.g. between oxygenated water and anoxic sediments), strong gradients of $O_2$ and $H_2S$ support specialized microbial communities.
These processes:
- Influence nutrient availability (e.g. binding or releasing metals and phosphorus)
- Shape habitat conditions (e.g. toxicity and odor)
Interaction with Metals and Other Elements
- Sulfide strongly binds metals such as iron, zinc, and copper, influencing:
- Trace metal availability for organisms
- Formation of ore deposits
- Sulfate reduction and oxidation interact with:
- Carbon cycle (use of organic matter and coupling to $CO_2$ production)
- Iron cycle (formation and dissolution of iron sulfides and oxides)
- Methane production (competition between sulfate reducers and methanogens)
Sulfur and Ecosystem Function
- Sulfur availability can limit primary production in some ecosystems, particularly where soils are strongly leached or very young.
- Sulfur transformations contribute to:
- Detoxification or mobilization of harmful substances
- Self-purification in wetlands and sediments (e.g. binding heavy metals in sulfides)
- Characteristic properties of certain habitats (e.g. black, sulfidic mud; smell of marshes and geothermal areas)
Summary of the Sulfur Cycle Pathways
A strongly simplified pathway sequence is:
- Geological release
Rocks → sulfate/sulfide in soil and water - Biological assimilation
Sulfate → organic sulfur in biomass - Decomposition and mineralization
Organic sulfur → sulfate (aerobic) and/or sulfide (anaerobic) - Microbial redox transformations
- Sulfate reduction: $SO_4^{2-} \rightarrow H_2S$
- Sulfide oxidation: $H_2S \rightarrow SO_4^{2-}$ (via intermediates)
- Sedimentation and mineral formation
Sulfide + metals → metal sulfides → new rocks - Atmospheric phase
Emission of sulfur gases → oxidation → deposition as sulfate
This cycle links the biosphere tightly with the geosphere and atmosphere and is profoundly shaped by both microorganisms and human activity.