Mechanism of microbial humic substance electron shuttling to Fe(III) minerals

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Humic substances (HS) are redox-active organic constituents of natural soils and sediments. They can be reduced by a variety of physiological groups of microorganisms including Fe(III)-reducers, but also by some sulfate-reducers, methanogens and even fermenters. Reduced HS can undergo various redox reactions including electron transfer to Fe(III) (oxyhydr)oxide minerals. This process called HS electron shuttling has been shown to stimulate microbial Fe(III) mineral reduction by increasing the reduction rates and by making some otherwise inaccessible Fe(III) mineral phases accessible for microbial reduction. In spite of extensive research, various open questions are still remaining regarding the mechanism of HS electron shuttling. For instance, it is not clear to which extent commonly used extraction and purification procedures for humic substances affect their redox activity and thus the electron shuttling effect of HS, over which spatial distances humic substance electron shuttling is possible and what is the molecular mechanism underlying electron transfer between microorganisms and Fe(III) minerals over spatial distances, i.e. diffusion of electron shuttles or electron hopping. In order to answer these questions, we propose Fe(III) mineral reduction experiments by the metal-reducing bacterium Shewanella oneidensis in the presence of differently prepared soil organic matter extracts and set-ups of spatially separated cells and Fe(III) minerals. The results from these experiments will allow evaluating and understanding HS redox activity and electron shuttling mechanisms within the complex network of biogeochemical processes in nature.