The possibility that quinones serve as electron-accepting moieties when microorganisms transfer electrons to humic substances was investigated. Humic substances from a variety of environments had the ability to accept electrons from a humics-reducing microorganism, Geobacter metallireducens. A lignaceous carbon source for the humic substances was not necessary in order for the humic substances to act as electron acceptors. Humic substances extracted from soils typically had a greater electron-accepting capacity than humic substances extracted from aquatic sediments, and sediment humic substances had more electron-accepting capacity than dissolved aquatic humic substances. Electron spin resonance measurements at pH 10 demonstrated that humic substances that had a higher electron-accepting capacity also had a higher free radical content than did humic substances with a lower electron-accepting capacity. The ESR spectra were consistent with semiquinones as the main organic radicals. Microbial reduction of humic substances resulted in an increase in semiquinones that was in proportion to the electron-accepting capacity of the humic substances. These results provide direct evidence that organic radicals in humic substances, which are primarily quinone groups, are reduced when humics-reducing microorganisms transfer electrons to humic substances.

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