Determining the Terminal Electron-Accepting Reaction in the Saturated Subsurface

Abstract

Microorganisms obtain their energy for metabolism by catalyzing a variety of oxidation-reduction reactions. The distribution of the terminal electron-accepting reactions in an aquifer is dictated by several factors. The sequence of terminal electron-accepting reactions occurs in the order shown in a table, but very long periods can be necessary before the supply of a given electron acceptor is depleted. Determining whether aerobic respiration is the dominant terminal process is relatively straightforward. When oxygen is present, it is the electron acceptor, not only for the thermodynamic reasons but also because it is toxic to the obligately anaerobic processes (iron reduction, sulphate reduction, and methanogenesis) and inhibits the expression and the function of the denitrification enzymes. In many aspects, iron reduction differs from the other terminal electron-accepting reactions. The substrate, Fe(III), is present in many forms, all crystalline solids, and often in large quantities in the saturated subsurface. Concentrations of dissolved hydrogen can serve as an additional indicator of the predominant terminal electron-accepting reaction. An integral part of future characterizations and insights regarding microbial communities in the subsurface must necessarily involve the context of the terminal electron-accepting reactions.