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

腐殖物质（HS）是天然土壤和沉积物中具有氧化还原活性的有机组分。它们可以被各种生理群的微生物还原，包括Fe（III）还原剂，但也可以被一些硫酸盐还原剂、产甲烷菌甚至发酵剂还原。还原HS可以经历各种氧化还原反应，包括电子转移到Fe（III）（oxyhydr）氧化物矿物。这种称为HS电子穿梭的过程已被证明通过增加还原速率和使一些原本不可接近的Fe（III）矿物相可用于微生物还原来刺激微生物Fe（III）矿物还原。尽管广泛的研究，各种开放的问题仍然存在关于HS电子穿梭的机制。例如，目前尚不清楚腐殖物质的常用提取和纯化程序在多大程度上影响其氧化还原活性，从而影响HS的电子穿梭效应，腐殖物质电子穿梭在空间距离上是可能的，以及微生物和Fe（III）矿物之间的电子转移在空间距离上的分子机制是什么，即电子穿梭或电子跳跃的扩散。为了回答这些问题，我们提出了Fe（III）矿物还原实验中的金属还原菌希瓦氏菌oneidensis在不同制备的土壤有机质提取物和空间分离的细胞和Fe（III）矿物的设置的存在下。从这些实验的结果将允许评估和理解HS氧化还原活性和电子穿梭机制内的复杂网络的地球化学过程的性质。

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.