Collaborative Research: Mechanisms, Modeling and Geochemical Consequences of Electron Flow in Acid Mine Drainage-Induced Sediments

微生物代谢的物质和能量基质的转移历来被认为强烈依赖于微生物群落活跃的物理化学环境中化学物质的扩散。个体生物和微生物群落可能介导氧化还原反应，尽管有能量底物的空间分离，现在已经开始挑战这一观点。在导电细胞外结构（如微生物纳米线）和氧化还原活性矿物相的网络中电集成的微生物群落可能促进和利用电子在尺度（毫米到厘米尺度）上的运动，远远超过单个细胞（微米到米尺度），称为“远场细胞外电子传递（EET）”。farfield EET的一个重要含义是，尽管还原剂、氧化剂甚至单个微生物本身存在空间分离，但生物地球化学氧化还原反应仍可能发生。这里提出的工作将使用酸性矿井排水（AMD）影响系统来检查“自然”环境下电子流的动力学。在一些情况下，当富铁（II）的AMD到达陆地表面需氧时，嗜酸细菌将铁（II）氧化为铁（III）。这些微生物活动产生的铁（III）（氢）氧化物积累成“铁丘”，几乎完全由铁（III）相组成。据推测，由矿物相、微生物纳米线和其他导电细胞材料组成的综合导电网络促进了EET和电子通过铁丘的转移，支持铁丘深处Fe（II）的微生物氧化，而这种氧化不能简单地通过O2扩散到铁丘中来维持。基于现场的精细尺度地球化学现场表征，结合地球和电化学变化的测量，以及实验室尺度沉积物培养中导电微生物结构的详细表征，将用于阐明铁丘相关微生物群落介导的电子传递过程的速率、规模和程度。电子转移过程的多尺度物理建模将用于支持和补充该系统内电子转移的实验检查，并将包括模拟微生物纳米线、“生物电池”和更大规模系统（如在铁丘中遇到的系统）中的电子流建模。这项工作的结果将加强对铁丘微生物介导的地球化学过程和AMD治疗方法的理解。一家非营利性AMD治疗公司将作为该项目的非资助合作伙伴，以促进向AMD治疗从业者的知识转移。该项目的资金将用于博士后研究员、研究生和本科生的跨学科培训，同时促进公立大学（阿克伦大学）和私立大学（南加州大学）之间的紧密合作。研究生和本科生将从UA的麦克奈尔学者项目中招募。铁丘场地也将作为UA和当地学区正式课程的现场教室。

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.