The transfer of material and energetic substrates for microbial metabolism has historically been viewed as strongly dependent on the diffusion of chemical species within the physicochemical milieu in which the microbial community is active. Ideas that individual organisms and microbial communities may mediate redox reactions despite spatial separation of energetic substrates have now begun to challenge this view. Microbial communities that are electrically integrated in a network of conductive extracellular structures (e.g. microbial nanowires) and redox-active mineral phases may facilitate and exploit the movement of electrons over scales (mm- to cm-scale) far exceeding those of the individual cells (micrometer to meter-scale), referred to as "far-afield extracellular electron transport (EET)." An important implication of farafield EET is that biogeochemical redox reactions may occur despite the spatial separation of reductant, oxidant, and even individual microorganisms themselves. The work proposed here will use an acid mine drainage (AMD)-impacted system to examine the dynamics of electron flow in a "natural" setting. In several settings, when Fe(II)-rich AMD reaches the terrestrial surface aerobic, acidophilic bacteria oxidize Fe(II) to Fe(III). The Fe(III) (hydr)oxides that result from these microbial activities accumulate as 'iron mounds,' which are composed almost exclusively of Fe(III) phases.
It is hypothesized that integrated, conductive networks composed of mineral phases, microbial nanowires, and other conductive cellular material facilitate EET and the transfer of electrons through the iron mound, supporting microbiological oxidation of Fe(II) at depths within the iron mound that could not be sustained simply by diffusion of O2 into the mound. Field-based fine-scale geochemical site characterizations coupled with measurements of geo- and electro-chemical changes and detailed characterizations of electrically conductive microbial structures in laboratory-scale sediment incubations will be used to elucidate the rates, scales, and extents of electron transfer processes mediated by iron mound-associated microbial communities. Multiscale physical modeling of electron transfer processes will be used to support and supplement experimental examinations of electron transfer within this system, and will include modeling of electron flow in simulated microbial nanowires, 'biogeobatteries,' and in larger scale systems like that encountered in an iron mound.
Results of this work will enhance understanding of microbially mediated geochemical processes in iron mounds and AMD treatment approaches. A non-profit AMD treatment company will serve as an unfunded collaborator on this project to facilitate knowledge transfer to AMD treatment practitioners. Funds from this project will aid in the interdisciplinary training of a post-doctoral researcher, graduate, and undergraduate students, while facilitating a strong collaboration between a public university (The University of Akron) and private university (The University of Southern California). Graduate and undergraduate students will be recruited from UA's McNair Scholars program. The iron mound field site will also serve as a field classroom for formal courses at UA and a local school district.
