Introduction

The sulfur cycle is a biogeochemical cycle that describes the movement of sulfur through the lithosphere, hydrosphere, atmosphere, and biosphere.

• Sulfur (S) is the 10th most abundant element on Earth.
• It exists in multiple oxidation states:
  • -2 (H₂S, sulfides)
  • 0 (elemental sulfur)
  • +4 (SO₂)
  • +6 (SO₄²⁻)

👉 This variability makes sulfur highly reactive and important in biological and chemical processes.

🔹 Major Reservoirs of Sulfur

1. Lithosphere (Largest Reservoir)

• Sulfur stored in:
  • Sulfide minerals (FeS₂ – pyrite)
  • Sulfate minerals (CaSO₄ – gypsum)
• Released through:
  • Weathering
  • Volcanic eruptions

2. Hydrosphere

• Oceans contain sulfur mainly as sulfate ions (SO₄²⁻)
• ~90% sulfur is dissolved or in sediments
• Marine organisms use sulfur for biomolecules

3. Atmosphere

• Present in forms:
  • Sulfur dioxide (SO₂)
  • Hydrogen sulfide (H₂S)
  • Dimethyl sulfide (DMS)

Sources:

• Volcanic eruptions
• Fossil fuel combustion
• Marine plankton

4. Biosphere

• Sulfur present in:
  • Amino acids (cysteine, methionine)
  • Vitamins (biotin, thiamine)
• Essential for:
  • Protein structure (disulfide bonds)

🔹 Chemical Forms of Sulfur

🔹 Steps of the Sulfur Cycle

1. Atmospheric Sulfur Transformations

Release:

• Volcanic activity:

$$S + O_2 → SO_2$$

• Organic decay:

$$Organic\ sulfur → H_2S$$

Acid Rain Formation:

$$SO_2 + H_2O → H_2SO_3$$ $$2SO_2 + O_2 → 2SO_3$$ $$SO_3 + H_2O → H_2SO_4$$

👉 Sulfuric acid (H₂SO₄) returns sulfur to Earth via acid rain.

2. Plant Uptake (Assimilation)

Plants absorb sulfur as sulfate:

$$SO_4^{2-} → (plant uptake)$$

Assimilatory Sulfate Reduction:

$$SO_4^{2-} → SO_3^{2-} → H_2S → Amino\ acids$$

👉 Used to form:

• Cysteine
• Methionine

3. Food Chain Transfer

• Plants → Herbivores → Carnivores
• Sulfur becomes part of proteins in animals

4. Decomposition

Dead organisms → broken down by microbes

Release reaction:

$$Organic\ sulfur → H_2S$$

• Occurs in anaerobic conditions

5. Bacterial Transformations

A. Dissimilatory Sulfate Reduction

By anaerobic bacteria (e.g., Desulfovibrio):

$$SO_4^{2-} + 8H^+ + 8e^- → H_2S + 4H_2O$$

👉 Produces hydrogen sulfide (rotten egg smell)

B. Sulfur Oxidation

By chemolithotrophic bacteria (e.g., Thiobacillus):

$$H_2S + O_2 → S + H_2O$$ $$S + O_2 → SO_2$$ $$SO_2 + H_2O → H_2SO_4$$

👉 Converts reduced sulfur → sulfate

C. Phototrophic Bacteria

• Use sulfur instead of water in photosynthesis:

$$CO_2 + 2H_2S → CH_2O + H_2O + 2S$$

6. Sedimentation & Geological Storage

• Sulfur stored as:
  • Metal sulfides (FeS, FeS₂)
  • Sulfates (CaSO₄)
• Long-term storage in rocks

🔹 Role of Microorganisms

Microbes are key drivers of sulfur cycling:

🔹 Importance of Sulfur Cycle

1. Biological Importance

• Protein synthesis
• Enzyme function
• Structural stability (disulfide bonds)

2. Ecological Importance

• Maintains soil fertility
• Supports aquatic ecosystems
• Drives microbial energy cycles

3. Atmospheric Role

• Sulfur aerosols reflect sunlight
• Influence climate cooling

🔹 Human Impacts

1. Fossil Fuel Burning

$$S + O_2 → SO_2$$

Effects:

• Acid rain
• Soil acidification
• Water pollution

2. Industrial Emissions

• Factories release SO₂ and H₂S
• Disrupt natural sulfur balance

3. Acid Rain Effects

• Damages crops
• Leaches nutrients from soil
• Harms aquatic life

4. Fertilizers

• Add excess sulfur
• Disturb microbial processes

🔹 Environmental Problems

• Acid rain (H₂SO₄)
• Air pollution
• Climate change influence
• Toxic hydrogen sulfide accumulation

🔹 Key Takeaways

• Sulfur cycle connects geological, biological, and atmospheric systems
• Sulfur exists in multiple oxidation states
• Microorganisms control most transformations
• Human activities accelerate sulfur emissions → environmental damage

🔹 Quick Exam Summary

• Main forms: H₂S, S, SO₂, SO₄²⁻
• Key processes:
  • Assimilation
  • Decomposition
  • Oxidation
  • Reduction
• Key organisms:
  • Desulfovibrio (reduction)
  • Thiobacillus (oxidation)
• Major issue: Acid rain due to SO₂