Sulfur Cycle

• Number of oxidation states
  • -2(sulfhydryl, R-SH, and sulfide, HS^–), 0(elemental sulfur, S⁰), and +6(sulfate,SO₄^2-) are significant in nature
  • Primarily gypsum (CaSO₄) and sulfide minerals (pyrite, FeS₂) constitute bulk of Earth’s S
  • Significant amount of SO₂ gas enters the sulfur cycle from human activities
    • Primarily burning fossil fuels

Hydrogen Sulfide and Sulfate Reaction

• H₂S major volatile sulfur gas
• SO₄^2-+ 4H₂ → H₂S + 2H₂O + 2OH^–
• H₂S exist below pH-7
• Nonvolatile HS^– and  S^2- exist above pH-7
• In marine sediments rate of sulfate reduction is carbon limited and can be greatly increased by an influx of organic matter
• Hydrogen sulfide is toxic to plants and animals
• Sulfur is detoxified in nature by binding with iron for FeS.

Sulfide and Elemental Sulfur Oxidation-Reduction

• Under oxic conditions sulfide oxidizes at neutral pH
• Sulfur-oxidizing chemolithotrophic bacteria(aerobes) can catalyze oxidation of sulfide
• Microbial sulfide oxidation is significant only in areas where H₂S emerges from anoxic environments and meets air
• Elemental Sulfur is chemically stable but is readily oxidized by sulfur-oxidizing chemolithotrophic bacteria.
• S⁰ is insoluble
  • Bacteria that oxidize must attach to S⁰ crystals to obtain their substrate
  • S⁰ forms sulfuric acid and thus S⁰ oxidation characteristically lowers the pH in the environment
• Reduction of S⁰ to sulfide(anaerobic respiration) is major ecological process
• Organic forms of sulfur
  • Dimethyl sulfide