Oxidation of Fe(II) by dissolved O2 at circumneutral pH occurs abiotically or is mediated by microaerophilic Fe(II)-oxidizing bacteria. Under these conditions, abiotic Fe(II) oxidation proceeds rapidly, which raises the question how and to which extent neutrophilic microaerophilic Fe(II)-oxidizing bacteria can compete with chemical reactions and gain metabolic energy from microbial Fe(II) oxidation. In this study, we have virtually investigated the environmental constraints for microaerophilic Fe(II) oxidation in a film layer characterized by diffusive supply of both atmospheric O2 (from the top) and dissolved Fe(II) (from the bottom). A coupled diffusion-reaction model was tested at different pH and alkalinity gradients and biofilm layer thicknesses to investigate their effects on the relative contributions of different reactions (abiotic homogeneous, heterogeneous and microaerophilic Fe(II) oxidation) to the overall Fe(II) oxidation. The simulations demonstrate distinct spatial oxidation rate patterns for all of the considered reactions. Maximum biological rates were on the order of 7*10-10 mol L-1 s-1 and are in the range of experimentally-observed values with minimum rates being close to the thermodynamic limit as defined by ATP synthesis. In the presence of a pH gradient, two distinct zones were observed: an upper zone dominated by abiotic Fe(II) oxidation (pH ~7) and a lower zone dominated by microbial (microaerophilic) Fe(II) oxidation (pH &lt;6.3), while the position and extent of the zones depend on the alkalinity. Such separation is strongly amplified for thinner biofilms (d = 0.2 mm). Overall, our simulations highlight the importance of pH gradients in allowing for the formation of microniches that are suitable to microaerophilic Fe(II)-oxidizers. The simulations define a kinetically-controlled geochemical window for the activity of Fe(II)-oxidizing bacteria that is characterized by three factors: i) a pH low enough (&lt;6.3) to outcompete abiotic processes with pH gradients being controlled by the alkalinity of the solution; ii) sufficiently fast diffusive supply of Fe(II) by e.g. Fe(III)-reducing bacteria or chemical processes with O2 concentrations below 150 μmol L-1; iii) microbial growth at the thermodynamic limit. The kinetic analysis is underpinned by confocal laser scanning microscopy analysis confirming the occurrence of low pH microniches in an Fe-rich biofilm.

Kinetically controlled microniche formation for microaerophilic Fe(II) oxidation at circumneutral pH

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