Sulphur (S) is a vital macronutrient for all living organisms. It is an essential component of amino acids (cysteine, methionine), vitamins (thiamine, biotin), and coenzymes (coenzyme A). The sulphur cycle explains the movement of sulphur between the lithosphere, atmosphere, hydrosphere, and biosphere. This biogeochemical cycle is critical for maintaining ecosystem productivity.
1. Introduction
Sulphur occurs in both organic and inorganic forms.
- Inorganic forms: sulphates (SO₄²⁻), elemental sulphur (S⁰), sulphides (H₂S, FeS₂).
- Organic forms: amino acids, proteins, and other biomolecules.
Sulphur exists in soil, rocks, oceans, plants, animals, and atmosphere, continuously cycling through these compartments. Microorganisms play a central role in transforming sulphur into biologically usable forms.
2. Major Reservoirs of Sulphur
Atmosphere
Contains sulphur gases like sulphur dioxide (SO₂), hydrogen sulphide (H₂S), and dimethyl sulphide (DMS). These gases can combine with water to form acid rain (H₂SO₄), which deposits sulphur back to soil and water.
Lithosphere (Soil and Rocks)
Rocks contain sulphur as pyrite (FeS₂), gypsum (CaSO₄·2H₂O), and elemental sulphur (S⁰). Soils contain sulphates and organic sulphur compounds derived from decomposed plants and animals.
Hydrosphere (Oceans, Rivers, Lakes)
Dissolved sulphates are abundant, mainly in seawater, and play a role in marine food webs. Oceans act as a major sulphur reservoir, slowly exchanging with the atmosphere.
Biosphere
Sulphur is present in all living organisms, mainly in proteins, amino acids, and coenzymes. Plants absorb sulphate, convert it to organic sulphur, and pass it to herbivores and carnivores.
3. Forms of Sulphur
| Form | Nature | Example | Role |
|---|---|---|---|
| Elemental sulphur (S⁰) | Insoluble | Native sulphur in rocks | Intermediate in microbial oxidation |
| Sulphides | Reduced form | H₂S, FeS₂ | Anaerobic microbial reduction |
| Sulphates (SO₄²⁻) | Oxidized, soluble | CaSO₄, MgSO₄ | Plant-available form |
| Organic sulphur | Bound in biomolecules | Methionine, Cysteine | Proteins, vitamins, enzymes |
4. Steps of Sulphur Cycle (Detailed)
4.1 Mineralization (Decomposition of Organic Sulphur)
Organic sulphur from dead plants, animals, and microbial biomass is converted into inorganic forms by microbial decomposition. Microorganisms involved include Bacillus, Pseudomonas, and Desulfovibrio.
Example: R–S–R’ (organic sulphur) → H₂S → SO₄²⁻
This process makes sulphur available for plant uptake.
4.2 Oxidation of Sulphur Compounds
Elemental sulphur (S⁰) or hydrogen sulphide (H₂S) is oxidized by chemolithotrophic bacteria to sulphate (SO₄²⁻). Bacteria involved: Thiobacillus, Beggiatoa.
Reactions:
- H₂S + 2O₂ → H₂SO₄
- S⁰ + 1.5 O₂ + H₂O → H₂SO₄
Sulphate is the primary form absorbed by plants.
4.3 Sulphate Assimilation by Plants
Plants absorb sulphates from the soil through roots. Sulphates are reduced to sulphide inside plant cells and incorporated into amino acids (cysteine, methionine), vitamins (thiamine, biotin), and coenzymes (coenzyme A). This is the biological incorporation step.
4.4 Sulphur in Animals
Animals obtain sulphur by feeding on plants or other animals. Sulphur is incorporated into proteins and vitamins. Excess sulphur is excreted as sulphates (urine) or hydrogen sulphide (H₂S). Dead animals release organic sulphur back into soil through decomposition.
4.5 Microbial Reduction of Sulphates
In anaerobic soils or sediments, sulphate-reducing bacteria (Desulfovibrio) reduce sulphates to hydrogen sulphide (H₂S). Reaction: SO₄²⁻ + Organic matter → H₂S + CO₂ + H₂O. H₂S can escape into the atmosphere or react with metals forming metal sulphides.
4.6 Volatilization (Sulphur Gases to Atmosphere)
Sulphur is released as gases: H₂S, SO₂, and dimethyl sulphide (DMS). Sources include decomposition of organic matter, volcanic eruptions, and industrial emissions (coal, petroleum).
4.7 Atmospheric Deposition
Sulphur returns to soil and water via wet deposition (acid rain) and dry deposition (dust particles). This maintains sulphur availability in ecosystems.
5. Role of Microorganisms
| Microorganism | Function in Sulphur Cycle |
|---|---|
| Thiobacillus, Beggiatoa | Oxidize H₂S/S⁰ → SO₄²⁻ |
| Desulfovibrio | Reduce SO₄²⁻ → H₂S in anaerobic conditions |
| Bacillus, Pseudomonas | Mineralize organic sulphur to inorganic forms |
| Fungi | Decompose organic sulphur |
6. Human Impacts on Sulphur Cycle
- Industrial emissions release SO₂ → acid rain → soil and water acidification.
- Excess fertilizer use alters natural sulphur balance in soil.
- Pollution can cause aquatic ecosystem damage due to sulphur deposition.
7. Summary Table of Sulphur Cycle Steps
| Step | Process | Products/Outcome |
|---|---|---|
| Mineralization | Organic S → Inorganic S | SO₄²⁻ |
| Oxidation | H₂S/S⁰ → SO₄²⁻ | Plant-available sulphate |
| Assimilation | SO₄²⁻ uptake by plants | Organic S in amino acids, proteins |
| Consumption | Animals consume plants | Sulphur in animal proteins |
| Reduction | SO₄²⁻ → H₂S (anaerobic) | H₂S, metal sulphides |
| Volatilization | Release of S gases | H₂S, SO₂, DMS |
| Deposition | Sulphur returns to soil/water | SO₄²⁻ replenishment |
