Comparing Microbial Sulphur and Iron Cycling

What is the main form of sulphur reduced by microorganisms?

Sulphate (SO₄²⁻), reduced to hydrogen sulphide (H₂S) by sulphate-reducing bacteria (SRBs).

Name three genera of sulphate-reducing bacteria (SRBs).

• Desulfovibrio

• Desulfobacter

• Archaeoglobus

What is the end product of microbial sulphide oxidation?

Sulphate (SO₄²⁻), from oxidation of reduced sulphur compounds like H₂S or elemental sulphur.

Name three organisms that oxidise sulphide or elemental sulphur.

• Beggiatoa

• Thiobacillus

• Acidithiobacillus ferrooxidans.

What are key electron donors and acceptors in the sulphur cycle?

• Donors: H₂S, S⁰, thiosulfate.

• Acceptors: O₂, NO₃⁻, SO₄²⁻.

What is the role of sulphur cycling in bioremediation?

SRBs precipitate toxic metals as insoluble metal sulphides (e.g. ZnS, CdS).

What organism is key in iron oxidation at low pH?

• Acidithiobacillus ferrooxidans, an acidophilic chemolithotroph.

What is the end product of Fe(II) oxidation?

Fe(III) — often precipitates as ferric hydroxide (Fe(OH)₃) or jarosite.

Which bacteria reduce Fe(III) to Fe(II)?

Geobacter, Shewanella — dissimilatory iron-reducing bacteria (DIRBs).

What are the three main mechanisms of Fe(III) reduction by microbes?

• Direct contact - direct physical contact between the microbial surface and the Fe(III) oxide surface for the e^- transfer (e.g. Geobacter)

• Electron shuttles (e.g. flavins) - use of metabolic processes to generate electrons, which are transferred to the electron shuttle → donates electrons to Fe(III) compounds (reduction)

• Chelating agents - solubilise Fe(III) from the oxide and then transport it back to the microorganism for reduction

What are major applications of iron-reducing bacteria?

• Magnetite production - reduces Fe(III) → Fe(II) causing magnetite precipitation

• Radionuclide bioremediation - bacteria reduce soluble, mobile radionuclides (e.g. uranium(VI)) to less soluble, less mobile forms (e.g. uranium(IV)). 

• Microbial fuel cells - facilitating electron transfer from their metabolism to the anode, generating electricity

How is sulphur cycling involved in bioleaching?

Acidithiobacillus oxidises sulphur and iron compounds in ores, releasing metals (e.g., Cu, U).

What is the Shell-Budelco process?

A bioreactor-based bioremediation system using SRBs to treat metal/sulphate-contaminated groundwater.

How do sulphur and iron cycles overlap environmentally?

Both occur in redox-stratified environments like marine sediments and influence metal mobility and organic matter degradation.

What form of iron is typically bioavailable for microbial reduction?

Amorphous Fe(III) oxides (e.g., ferrihydrite), more accessible than crystalline forms (e.g., hematite).

Conditions for S oxidation/reduction

• Oxidation: acidic environments, moderate temperatures, and a steady supply of oxygen and carbon dioxide

• Reduction: anaerobic environments, often in the presence of sulphate and a suitable electron donor

Conditions for Fe oxidation/reduction

• Oxidation: acidic, aerobic environments

• Reduction: available electron donor, suitable pH (acidic or alkaline depending on the specific bacteria), presence of Fe(III) minerals