Effect of Low Concentrations of Arsenic on Microbial Iron Reduction

在自然环境中，亚砷酸盐（As(III)）不仅比砷酸盐（As(V)）或有机砷化合物更具流动性，而且毒性也更大。尽管在实验室或显示砷浓度极高的水生系统中获得了丰富的知识，但很少有关于低砷输入对天然水生系统的影响的研究。与来自乔治亚河的沉积物孵育表明，低（10微M）但增加的As(V)浓度刺激了微生物铁还原率的增加，并伴有铁氧化物的二次再结晶。在本项目中，将确定低浓度砷影响铁还原菌对铁氧化物厌氧呼吸的机制。假设As(V)在ADP磷酸化过程中通过砷酸盐取代磷酸盐刺激铁还原菌的能量生成过程。这种效应被认为是由于铁还原细菌在磷酸盐限制条件下表达磷酸盐转运蛋白及其调节因子。最后，亚微摩尔As(V)浓度对自然环境中微生物铁还原的巨大影响被假设是由于催化循环，即微生物解毒产生的As（III）通过自然环境中普遍存在的Mn（IV）氧化物的化学氧化迅速再循环为As(V)。设计了一系列金属还原希瓦氏菌纯培养，利用最先进的分析和分子技术，在一个控制良好的生物系统中研究受小砷(V)浓度影响的主要分子和地球化学过程。这项研究将同时提供关于希瓦氏菌的新信息和在自然环境中可能发生的模型。这项工作还将为在铁还原细菌中添加低浓度As(V)影响的基因和基因产物提供新的信息。虽然矿物氧化物对水生系统中微量金属形态的影响是众所周知的，但微量金属浓度对矿物氧化物转化的影响尚未得到很好的认识。这项工作将提供第一个证据，证明铁呼吸是由有毒化合物增强的，铁氧化物的结构和反应性受到这些过程的影响。由于铁氧化物作为污染物的清除剂起着重要作用，这项工作将对水生系统中其他污染物的命运产生影响。类金属是水生系统中毒性最大的无机污染物之一，但令人惊讶的是，它们在水库中的存在很少受到关注，因为它们的浓度通常很低。这项研究将证明，即使是低浓度的有毒金属也会通过改变生物和地球化学过程来影响这些生态系统。

In natural environments, arsenite (As(III)) is not only more mobile but is also more toxic than arsenate (As(V)) or organoarsenic compounds. Despite a wealth of knowledge gained in the laboratory or aquatic systems displaying remarkably high concentrations of arsenic, little research has been conducted on the effects of low arsenic inputs to natural aquatic systems. Incubations with sediments from a Georgia river revealed that low ( 10 micro M) but increasing As(V) concentrations spurred increases in microbial iron reduction rates accompanied by the secondary recrystallization of Fe oxides. In this project, the mechanism by which low concentrations of arsenic affect anaerobic respiration on Fe oxides by iron-reducing bacteria will be determined. It is hypothesized that As(V) stimulates the energy generation process in iron-reducing bacteria by substitution of phosphate by arsenate during phosphorylation of ADP. This effect is postulated to result from the expression of phosphate transporters and their regulators by iron-reducing bacteria in phosphate-limiting conditions. Finally, the large impact of submicromolar As(V) concentrations on microbial Fe reduction in natural environments is hypothesized to result from a catalytic cycle whereby As(III) produced by microbial detoxification is rapidly recycled to As(V) via chemical oxidation by Mn(IV) oxides present ubiquitously in natural environments. A series of incubations with pure cultures of metal-reducing Shewanella was designed to investigate in a well-controlled biological system the main molecular and geochemical processes affected by small As(V) concentrations using state-of-the art analytical and molecular techniques. This study will simultaneously provide new information on Shewanella and a model for what may occur in natural environments. This work will also provide novel information on the genes and gene products affected by the addition of low concentrations of As(V) in iron-reducing bacteria. While the influence of mineral oxides on trace metal speciation in aquatic systems is well known, the effect of trace metal concentrations on the transformation of mineral oxides is not well recognized. This work will provide the first evidence that Fe respiration is enhanced by a toxic compound and that the structure and reactivity of Fe oxides is affected by these processes. As Fe oxides play a fundamental role as scavengers of contaminants, this work will have ramifications on the fate of other contaminants in aquaticsystems. Metalloids are among the most toxic inorganic pollutants in aquatic systems but, surprisingly, their presence in water reservoirs receives little attention as their concentration is generally low. This study will demonstrate that even low concentrations of a toxic metal affect these ecosystems by altering biological and geochemical processes.