Microbial Fe(III) reduction in subsurface environments

Abstract

Until recently, nonenzymatic processes were generally considered to account for much of the Fe(III) reduction in subsurface environments. However, it is now clear that enzymatic Fe(III) reduction catalyzed by microorganisms which conserve energy to support growth by completely oxidizing organic compounds to carbon dioxide accounts for most of the Fe(III) reduction. Microbial Fe(III) reduction in deep pristine aquifers releases dissolved inorganic carbon into groundwater which may increase aquifer porosity. The Fe(II) released into the groundwater is an important groundwater quality problem in many aquifers. Microbial oxidation of organic contaminants coupled to Fe(III) reduction removes significant amounts of pollutants from many contaminated aquifers. Fe(III) reduction and hence contaminant removal can be accelerated in aquifer sediments with the addition of Fe(III) chelators or humic substances. Both of these amendments alleviate the need for Fe(III) reducers to come into direct physical contact with Fe(III) oxides in order to reduce them. Some Fe(III)-reducing microorganisms can reduce contaminant metals and metalloids such as uranium, technetium, cobalt, chromium and selenium. This metabolism may be useful for remediation of metal-contaminated subsurface environments. Fe(III) reducers and some of the insoluble Fe(II) products of Fe(III) reduction can reducively dechlorinate chlorinated contaminants. Magnetite that is similar to that produced by known Fe(III)-reducing microorganisms has been recovered at depths as great as 6.7 km on Earth and has been observed in a Martian meteorite. Thus, microbial oxidation of organic matter coupled to the reduction of Fe(III) to Fe(II) appears to be a important process in a variety of subsurface environments.