Sulfur-mediated electron shuttling during bacterial iron reduction.

Area of Science:

• Biogeochemistry
• Microbial Ecology
• Environmental Science

Background:

• Microbial reduction of ferric iron [Fe(III)] is a key process in anoxic aquifers.
• Dissimilatory metal-reducing bacteria can utilize alternative electron acceptors like elemental sulfur (S(0)) under varying groundwater pH.
• Alkaline conditions are prevalent in many aquifer systems, influencing microbial respiration.

Purpose of the Study:

• To investigate the interplay between iron and sulfur cycling under alkaline conditions.
• To understand the mechanisms of Fe(III) reduction when direct microbial respiration is limited.

Main Methods:

• Thermodynamic geochemical modeling.
• Bioreactor experiments using the bacterium Shewanella oneidensis MR-1.
• Analysis of microbial and geochemical interactions under controlled alkaline conditions.

Main Results:

• Shewanella oneidensis MR-1 enzymatically reduced S(0) but not goethite (α-FeOOH) under alkaline conditions.
• The sulfide (HS(-)) produced from S(0) reduction abiotically reduced goethite.
• This indicates an indirect Fe(III) reduction pathway mediated by S(0).

Conclusions:

• Elemental sulfur (S(0)) can act as an electron shuttle for Fe(III) reduction in alkaline aquifers.
• S(0)-mediated pathways are significant for iron biogeochemical cycling where direct microbial Fe(III) reduction is inhibited.
• This finding has implications for understanding biogeochemical processes in diverse aquifer environments.