Abstract
In many soils, sediments and groundwaters, ferric iron is a major potential electron acceptor for the oxidation of organic matter. In contrast to other terminal electron acceptors (e.g. nitrate or sulfate), the concentration of Fe3+(aq), is limited by the low solubility of Fe(III) oxyhydroxides under the pH conditions
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in subsurface. The goal of the thesis is to unravel the effects of the physical-chemical properties of Fe(III) oxyhydroxides on dissimilatory iron reduction.
The rates of reduction of different Fe(III) oxyhydroxides are determined as a function of the concentration of solid-phase Fe(III) and the cell density of Shewanella putrefaciens (Chapter II). For a given Fe(III) oxyhydroxide, the iron reduction rate can be described by the Michaelis-Menten rate equation:
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where R is the net rate of iron reduction per unit total volume of the bacteria-mineral suspension, νmax, the maximum cell-normalized iron reduction rate (in µmol Fe(III) h-1 cell-1), B, the cell density of the suspensions, and *Km a half-saturation constant.
The maximum rate, νmax, is interpreted as the iron reducing activity of a S. putrefaciens cell when its membrane-bound Fe(III) reductase sites are saturated by the Fe(III) substrate. νmax values vary over 1.5 orders of magnitude, depending on the Fe(III) mineral. To explain this large variation of νmax, the solubilities of the Fe(III) oxyhydroxides are measured using two methods: (i) pe-pH titrations for fairly soluble, colloidal Fe(III) oxyhydroxides, (Chapter II) and, (ii) a dialysis bag dissolution method for coarser, less soluble minerals (Chapter IV).
νmax and the solubility products exhibit a strong positive correlation :
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Equation (2) defines a linear free energy relationship (LFER), which relates the iron reducing activity of S. putrefaciens to the energetics of the mineral Fe(III) substrate. Although empirical, this relationship hints to a common rate controlling process in the microbial reduction of different Fe(III) oxyhydroxides. In Chapter IV, we show a positive correlation between vmax and the rate constant for detachment of Fe2+ from the mineral (kdes).
In Chapter III, we first investigate the attachment of nanohematite particles onto S. putrefaciens surface. Formally, the partitioning of the nanohematite particles follows a Langmuir isotherm with Mmax, the maximum attachment capacity per cell, and KP an attachment constant. Next, cells with known amounts of attached nanohematite particles are incubated with lactate under anaerobic conditions. The major finding is that the iron reduction rate correlates linearly with the relative coverage of the cell surfaces by nanohematite particles. Using internally consistent parameter values for Mmax, KP, and the first-order Fe(III) reduction rate constant, k, a kinetic model for the microbial reduction of Fe(III) oxyhydroxide colloids is developed. The model reproduces the reduction rates of nanohematite, as well as those of other fine-grained Fe(III) oxyhydroxides by S. putrefaciens. It explains the observed dependency of the Fe(III) half saturation constant, Km*, on the solid-to-cell ratio and, as observed, it predicts that iron reduction rates exhibit saturation with respect to both the cell density and Fe(III) oxyhydroxide substrate.
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