Sulfur and Iron Biogeochemical Cycles

Introduction to Biogeochemical Cycles

Biogeochemical cycles describe the movement and transformation of elements such as carbon (C), nitrogen (N), sulfur (S), and iron (Fe) through different physical (gas, liquid, solid) and chemical forms in biological and geological matter. These cycles are driven by both biotic (living organisms) and abiotic (physical/chemical) forces.

• Oxidation states: Define the chemical boundaries and reactivity of elements.

• Reservoir sizes and forms: Elements exist in various reservoirs (e.g., atmosphere, lithosphere, biosphere) and in different physical and chemical forms.

• Turnover time: The rate at which elements move between reservoirs.

• Activities: Biological and chemical processes that drive cycling, including rate-limiting steps and those performed by specialized organisms.

• Anthropogenic change: Human activities (e.g., fossil fuel combustion) can alter natural cycles.

• Cycle interactions: Elemental cycles are interconnected and can influence each other.

Sulfur Cycle

Oxidation States and Chemical Forms of Sulfur

Sulfur exists in multiple oxidation states, but three are most dominant in natural environments:

• Sulfide, sulfhydryl (H2S/HS-, cysteine, methionine): Oxidation state -2

• Elemental sulfur (S0): Oxidation state 0

• Sulfate (SO42-): Oxidation state +6

In oxic (oxygen-rich) environments, oxidized forms like sulfate dominate, while reduced forms like sulfide are prevalent in anoxic (oxygen-poor) environments.

Reservoirs and Physical/Chemical Forms of Sulfur

Sulfur is found in various reservoirs and forms:

• Lithic (rock) sulfur: Mainly as pyrite (FeS2) or gypsum (CaSO4), with significant amounts in rocks and sediments.

• Oceanic sulfate: Largest active reservoir, highly involved in the organosulfur cycle.

• Biological sulfur: Important for metal coordination in proteins and redox reactions.

• Atmospheric sulfur: Produced by fossil fuel combustion and volcanic activity, contributing to acid rain.

Sulfur Geochemistry: Key Compounds and States

Sulfur compounds vary in solubility and oxidation state:

• Sulfate (SO42-): Soluble anion, oxidation state +6

• Sulfite (SO32-): Soluble anion, oxidation state +4

• Sulfur dioxide (SO2): Gas, oxidation state +4

• Thiosulfate (S2O32-): Soluble anion, oxidation state -2/+6

• Elemental sulfur (S0): Solid, oxidation state 0

• Hydrogen sulfide (H2S/HS-): Gas/soluble anion, oxidation state -2

Human Impact on the Sulfur Cycle

Human activities have significantly altered the sulfur cycle:

• Fossil fuel combustion: Main source of sulfur pollution, releasing large amounts of SO2 into the atmosphere.

• Volcanic activity: Natural source of sulfur emissions.

• Acid rain: SO2 reacts with atmospheric hydroxyl radicals and O3 to form sulfuric acid, lowering pH and affecting ecosystems.

• Buffering capacity: Sensitivity to acid rain depends on the abundance of carbonate rocks in the environment.

Iron Cycle

Oxidation States and Chemical Forms of Iron

Iron cycles between two main oxidation states in the environment:

• Ferrous iron (Fe2+): Reduced form, soluble in anoxic water and acidic conditions.

• Ferric iron (Fe3+): Oxidized form, forms insoluble minerals (e.g., hematite, ferric oxyhydroxide) in neutral pH, oxic environments.

In oxic environments, ferric iron dominates, while ferrous iron is prevalent in anoxic or acidic conditions.

Ferric Iron (Fe3+) Reduction

Microbial reduction of ferric iron is a key process in anoxic environments:

• Electron donors: Organic acids (e.g., acetate, lactate) or alcohols (e.g., ethanol) are oxidized, transferring electrons to Fe3+.

• Products: Fe2+ (soluble), CO2, and H+.

• Example reaction:

• Microbial examples: Geobacter spp., Shewanella spp.

• Environmental significance: Metal respiration affects the solubility and transport of toxic metals (e.g., Mn, S, As, Cr, U).

Uranium Reduction and Bioremediation

Iron-reducing bacteria can also reduce uranium:

• Uranium redox states: U4+ (reduced, precipitates) and U6+ (oxidized, soluble).

• Bioremediation: Microbial reduction of U6+ to U4+ immobilizes radioactive uranium, reducing environmental risk.

Ferrous Iron (Fe2+) Oxidation

Ferrous iron oxidation is common in acidic, anoxic waters:

• Abiotic oxidation: Rapid at neutral pH, slow at acidic pH.

• Microbial iron oxidizers: Most are acidophiles, such as Acidithiobacillus ferrooxidans.

• Example reaction:

• Products: Ferric iron minerals (rust), acid (acid mine drainage).

• Electron transport chain: Rusticyanin removes electrons from Fe2+, which flow to cytochromes and terminal electron acceptors (O2 or NO3-).

Summary Table: Sulfur and Iron Compounds

Example Applications

• Microbial sulfate reduction: Key process in anaerobic environments, producing hydrogen sulfide.

• Iron oxidation: Important in acid mine drainage and formation of iron minerals.

• Bioremediation: Microbial reduction of toxic metals (e.g., uranium) for environmental cleanup.

Additional info: These notes expand on the brief points in the original slides, providing definitions, examples, and context for key microbiological processes in sulfur and iron cycling.