土壤微生物铁循环及其环境意义
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国家自然科学基金项目(No. 41025003, 41201253)资助


Soil microbe mediated iron cycling and its environmental implication
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    摘要:

    铁是土壤中重要的高活性高丰度元素。铁的生物地球化学循环包括铁还原与亚铁氧化两个过程,需要微生物提供基本驱动力,已经成为陆地表层系统研究的国际热点。铁循环控制着土壤有机物矿化、反硝化、甲烷产生、重金属固定等环境过程,是连结土壤养分循环与污染物转化的纽带。从生物地球化学循环的角度,综述了微生物作用下的铁还原、亚铁氧化过程及其主要微生物类群,重点论述了厌氧条件铁还原与亚铁氧化的环境效应及其地球化学机理,以及铁还原与亚铁氧化两个过程的协调及其调控因子。本文将有助于深入理解地球表层的关键环境过程与驱动机制。

    Abstract:

    Microbe drives biogeochemical cycling of elements on Earth. Being the fourth most abundant element on earth and the most frequently utilized transition metal in the biosphere, iron (Fe) naturally undergoes active reactions between ferrous and ferric states in circumneutral-pH or acid environment. Due to instability of dissolved Fe(Ⅱ) and adsorptive capability of insoluble Fe(Ⅲ) compounds, active Fe cycling exerts a strong influence on soil geochemistry. Advances in geo-microbiology have transformed our understanding of the edaphic iron cycling from mere physico-chemical reaction to biogeochemical process over the past three decades. Fe ion, undergoing active oxidation-reduction reactions in all life forms, is required asan integral component in cellular processes. And it has been demonstrated that phylogenetically diverse groups of microbes can grow either aerobically or anaerobically using Fe as electron donor or electron acceptor to generate energy from Fe reduction and Fe oxidation in vitro or in vivo. In recent years, significant progresses have been made toward understanding the biochemical mechanisms of microorganisms catalyzing anaerobic reduction of Fe(Ⅲ) in the circumneutral pH environment. Shewanella and Geobacter are the two model organisms commonly used in studyingmechanisms of Fe-reduction, and the use of insoluble ferric oxyhydroxide minerals as terminal electron acceptors in anaerobic respiration through extracellular electron transfer (dissimilatory Fe(Ⅲ) reduction). Comparatively little information is available on mechanisms of Fe(Ⅱ) oxidation at neutral pH conditions. Microaerobic Fe(Ⅱ)-oxidizers, such as Gallionella andLeptothrix, active at circumneutral pH, could compete with O2 in abiotic oxidation of Fe(Ⅱ), forming Fe(Ⅲ) oxide encrustation specific to the oxic-anoxic interface of soil. Fe-oxidizing microbes are not limited to aerobic habitats, but can also oxidize iron under anaerobicconditions using NO3−[nitrate-dependent Fe(Ⅱ)oxidation], or CO2[phototrophic Fe(Ⅱ)oxidation] as the terminal electron acceptor. The microbial Fe(Ⅱ)-Fe(Ⅲ) wheel promotesvarious environmental or ecosystem processes, such as nutrient cycling and contaminant transformation, at the water-soil interphase. It is worthwhile to note that in anaerobic environments, microbial Fe(Ⅲ) reduction is an important pathwayof anaerobic degradation of organic matter. Besides, dissimilatory Fe(Ⅲ) reduction is a key process governing reduction of humic substances, reductive dechlorination and metals reduction. Furthermore, Fe(Ⅲ)-reducing bacteria successfully outcompetemethanogenenic bacteria for H2 as an energy source, which results in dropping of methane production in soil of high organic matter content. Fe(Ⅱ)-oxidizing microbes have been demonstrated to oxidize both soluble and insoluble Fe(Ⅱ), producing a variety of insoluble Fe(Ⅲ) mineral products. Owing to their high affinity on surface, bacteriogenic iron oxides are ameliorating agents and geochemical barriers for fixing heavy elements, thus generating a major influence on release, transport, immobilization and bioavailability of heavy metals in soil. As a whole, it is apparent that iron biogeochemical cyclingistightly linked to organic matter degradation, denitrification, methane production and metal immobilization, which is one of the most important issues in environmental science.The processes driving iron cycling are not instantaneous, and Fe(Ⅲ) reduction and Fe(Ⅱ) oxidation occur simultaneously in adjacent (micro-scale) locations. Dissimilatory iron-reducing bacteria are found capable of excreting Fe(Ⅲ), resulting in anaerobic reduction of iron oxides in soil. Fe(Ⅱ) species in soils is usually soluble and highly mobile, and able to act as an electron donor for iron oxidizing bacteria. Thus, it is re-oxidized to Fe(Ⅲ), forming secondary iron minerals. So far, it is less understood that the key factors which control Fe-cyclingatcircumneutral pH include local gradients of oxygen, light, nitrate and ferrous iron. And recent researches have demonstrated that environmental organic matter, such as lactate, plays an important role in the transition of Fe(Ⅲ) reduction and Fe(Ⅱ) oxidation. To sum up, in the paper, the authors highlight the process, mechanism and environmental significance of microbe-mediated iron biogeochemical cycling, particularly in circumneutral pH environment that prevailsin soil, and also demonstrate the coupling relationship between iron and other related elements in biogeochemical cycling. Furthermore, the authors discussed key factors controlling shift between Fe(Ⅱ) oxidation and Fe(Ⅲ) reduction. In the end, the authors present their outlook about priority direction of the research on biogeochemical cycling of Fe in soil environment.This review is believed to be conducive to understanding of iron biogeochemical processes in the environment and formation of new strategies for sustainable rational utilization of the soil resources in China.

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胡 敏,李芳柏.土壤微生物铁循环及其环境意义[J].土壤学报,2014,51(4):683-698. DOI:10.11766/trxb201309160418 Hu Min, Li Fangbai. Soil microbe mediated iron cycling and its environmental implication[J]. Acta Pedologica Sinica,2014,51(4):683-698.

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  • 收稿日期:2013-09-16
  • 最后修改日期:2014-04-18
  • 录用日期:2014-04-28
  • 在线发布日期: 2014-04-29
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