John Zachara supported early work on basic geochemical and microbiological features of microbial Fe(III) oxide reduction, and later led a campaign to elucidate the molecular mechanisms involved in electron transfer (ET) at the cell-oxide interface (i.e. extracellular electron transfer, EET). This presentation revisits biotic versus abiotic controls on the initial rate and long-term extent of microbial Fe(III) oxide reduction in light of current understanding of enzymatic mechanisms of EET and the thermodynamics of Fe(III) oxide reduction systems. The initial rate analysis hinges on the relative importance of oxide thermodynamic properties, intrinsic rates of EET at the cell-oxide surface, or intracellular generation of reducing equivalents as limiting factors for oxide reduction. The absence of a correlation between surface area-normalized microbial reduction rates and oxide thermodynamic properties, together with the results of experiments with electron shuttles and purified outer-membrane (OM) protein, suggest that rates of intracellular metabolism as well as “steric” hindrances related to cell-mineral interactions limit oxide reduction rates, such that oxide thermodynamic properties impose no systematic rate limitation. Recent electrochemical characterization of Fe(III) oxide-Fe(II) systems suggest that the concentration of Fe(II) in solution, rather than accumulation of sorbed Fe(II), systematically reduces the effective oxide reduction potential, which supports previous thermodynamics-based models of long-term microbial Fe(III) oxide reduction. However, other recent studies demonstrate that sorbed Fe(II) ultimately inhibits ET by saturating reactive defect sites such as Fe vacancies that favor ET. The well-known ability of sorbed Fe(II) to serve as an effective abiotic reductant suggests involvement of complex surface electronic interactions that cannot be explained by classical thermodynamics. We speculate that such interactions, as well as the important role of defects that can obscure the role of bulk phase thermodynamics, are likely to directly impact the short and long-term ability of OM proteins to engage in EET at oxide surfaces.

Biotic versus abiotic controls on microbial Fe(III) oxide reduction revisited

ACS Spring 2022

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