Hydrogen concentrations as an indicator of the predominant terminal electron-accepting reactions in aquatic sediments

Abstract

Factors controlling the concentration of dissolved hydrogen gas in anaerobic sedimentary environments were investigated. Results, presented here or previously, demonstrated that, in sediments, only microorganisms catalyze the oxidation of H 2 coupled to the reduction of nitrate, Mn(IV), Fe(III), sulfate, or carbon dioxide. Theoretical considerations suggested that, at steady-state conditions, H 2 concentrations are primarily dependent upon the physiological characteristics of the microorganism(s) consuming the H 2 and that organisms catalyzing H 2 oxidation, with the reduction of a more electrochemically positive electron acceptor, can maintain lower H 2 concentrations than organisms using electron acceptors which yield less energy from H 2 oxidation. The H 2 concentrations associated with the specified predominant terminal electron-accepting reactions in bottom sediments of a variety of surface water environments were: methanogenesis, 7–10 nM; sulfate reduction, 1–1.5 nM; Fe(III) reduction, 0.2 nM; Mn(IV) or nitrate reduction, less than 0.05 nM. Sediments with the same terminal electron acceptor for organic matter oxidation had comparable H 2 concentrations, despite variations in the rate of organic matter decomposition, pH, and salinity. Thus, each terminal electron-accepting reaction had a unique range of steady-state H 2 concentrations associated with it. Preliminary studies in a coastal plain aquifer indicated that H 2 concentrations also vary in response to changes in the predominant terminal electron-accepting process in deep subsurface environments. These studies suggest that H 2 measurements may aid in determining which terminal electron-accepting reactions are taking place in surface and subsurface sedimentary environments.