Reduction of Fe(III) (Hydr)oxides with known thermodynamic stability by Geobacter metallireducens

Sulfate-reducing and methanogenic microorganisms become inactive when the concentration of the electron donors drops below a threshold set by the minimum Gibbs free energy required for the bacterial metabolism to be maintained. Thus, their activity is thermodynamically controlled. In this paper we study if the activity of dissimilatory Fe(III) reducing bacteria is also limited by the thermodynamics of the reaction. We synthesized five Fe(III) (hydr)oxides (FHOs) of moderate stability and determined the solubility product (log K-SO (-39.1)-(-41.8)), in order to calculate their standard free energy of formation. K-SO values, estimated from the particle size did not correspond with experimentally determined ones. HCO3- and PIPES-buffered media, containing 45 mM FHO and either 1, 10, or 100 mM acetate were inoculated with Geobacter metallireducens . At the end of bacterial reduction, the Gibbs free energy of the reaction showed significant differences between the different FHOs. The termination of the bacterial activity was consequently not triggered thermodynamically. However, the non-dissolved Fe(II) (HCl-soluble minus soluble Fe(II)) showed an excellent correlation with the surface of the FHOs (15 mumol m(-2) ). It is therefore likely that the termination of the reaction was caused by blocking of the FHO surface with insoluble Fe(II), as has been previously reported in the literature. The ecological significance of both thermodynamic limitation and surface availability limitation is discussed for FHOs of different K SO in environments with approximately neutral pH.