华南稻田产甲烷过程对铁氧化物晶型转变的响应机制

Due to their redox reactivity and ubiquitous presence, iron oxides have a substantial influence on biochemical processes in soils and sediments, such as methanogenesis. Previous studies have demonstrated that the presence of poorly crystalline Fe(III) oxides (such as ferrihydrite, Fe(OH)3) increased the contribution of iron reduction to anaerobic degradation of organic matter at the expense of methanogenesis. The suppression of methane production usually results from Fe(III)-reducing microorganisms reducing the level of primary electron donors (acetate, hydrogen) for methane production. However, in recent studies, (semi)conductive minerals of hematite and magnetite were reported to accelerate methane production in paddy soils in which methanogenesis was the predominant terminal electron-accepting process. The accelerated methanogenesis was associated with the role of hematite and magnetite acting as electron conduits for electric syntrophy between Geobacter and Methanosarcina. On the basis of the above-mentioned findings, the form of iron(III) oxides appears to be a dominant influence on methane production in anoxic soils, especially crystallinity and conductivity properties of iron(III) oxides.As known, time would introduce the conversion of unstable iron oxide to more stable iron oxides via abiotic and biotic processes, and physical and biological properties of iron oxide consequently changed compared with the host mineral.To date, the secondary mineralization of iron(III) oxides has not been considered in the competition between methanogenesis and microbial iron reduction. Using paddy soils of South China, the objective of this project is to reveal the response of methanogenesis to the phase transformation of iron(III) oxides; and to study the changing relationship within the system of "organic carbon-Fe(III)-reducing bacteria-iron(III) oxides-methanogens" during the phase transformation of iron(III) oxides. This project is expected to provide new insights into the role of iron(III) oxides in methanogenic processes in paddy soils, and also will make great contributions for the understanding of carbon mineralization and methane emission in paddy soils, significant to global carbon cycling and climate change.

铁氧化物是稻田土壤中最重要、最活跃的固相组分之一，强烈影响稻田产甲烷过程。过去认为Fe(III)作为电子受体竞争基质是其抑制甲烷产生的根本原因。最新研究表明，具有导电性的铁氧化物能介导铁还原菌与产甲烷菌的"种间直接电子传递"，这种电子互营模式比传统的"种间氢转移"更为"经济高效"，对产甲烷过程有明显促进效应。这就提示：铁氧化物导电性可能影响稻田产甲烷效应。但目前的研究局限于产甲烷反应器或纯菌体系，尚未涉及实际土壤过程。本项目以华南红壤稻田为对象，将"铁氧化物晶型/导电性转变"和"产甲烷过程"联系起来，系统研究稻田产甲烷过程对铁氧化物晶型转变的生态响应及微生物机制，探讨"有机碳-铁还原菌-铁氧化物-产甲烷菌"共存体系中铁氧化物晶型转化过程、电子流向及其微生物互营关系，揭示铁氧化物对稻田产甲烷过程的长期效应，为理解稻田有机碳矿化及甲烷排放机制提供新的科学依据，为发展稻田甲烷减排提供技术支撑。

铁氧化物是稻田土壤中最重要、最活跃的固相组分之一，强烈影响稻田产甲烷过程。近期研究表明，导电性铁氧化物可作为电子导体有效介导互营氧化菌与产甲烷菌之间“直接电子传递”，明显加速促进产甲烷过程。本项目以华南红壤稻田为研究对象，将“铁氧化物晶型/导电性转变”和“产甲烷过程”联系起来，系统研究了稻田产甲烷过程对铁氧化物晶型转变的响应规律及其微生物机制。重要的研究结果及科学意义包括：1）利用Geobacter sulfurreducens/Methanosarcina barkeri与Geobacter metallireducens/Methanosarcina barkeri纯菌共培养体系，研究了水铁矿次生成矿过程对乙酸/乙醇产甲烷过程的影响规律，结果表明当水铁矿还原的二次成矿产物为导电性磁铁矿时，其存在能明显促进Geobacter/ Methanosarcina介导的产甲烷过程，当水铁矿还原的二次产物为非导电性蓝铁矿时，其存在不影响Geobacter/ Methanosarcina介导的产甲烷过程。首次提供了磁铁矿激发Geobacter和Methanosarcina之间的“直接电子传递”及促进产甲烷过程的直接证据。2）以磷酸盐作为影响土壤氧化铁晶型转变过程的典型环境因子，研究了无定形水铁矿晶型转化过程对稻田厌氧产甲烷过程（以乙酸、丙酸为底物）的影响，结果表明在磷酸盐作用下，水铁矿转化成不具导电性蓝铁矿，蓝铁矿对稻田产甲烷过程没有显著影响。而无磷酸盐时，水铁矿转化成具有导电性磁铁矿，显著促进稻田产甲烷过程，这表明铁氧化物的晶型和性质是影响产甲烷过程的重要因素，揭示了水铁矿二次成矿与“电子互营”产甲烷过程的响应关系，对传统施加铁肥甲烷减排措施提出了新的研究思路。3）采用Illumina Miseq测序技术分析了水铁矿二次成矿过程中产甲烷微生物的群落结构和多样性，结果表明水铁矿还原及二次成矿过程改变了微生物群落结构。并进一步利用宏基因组、DNA-SIP（13C全标丙酸）结合16S rRNA高通量测序等分子生物学研究手段，研究了磁铁矿介导的丙酸互营产甲烷过程中的丙酸代谢和甲烷代谢过程，证实了丙酸产甲烷过程中互营微生物之间的直接电子传递（DIET），初步揭示了参与磁铁矿介导的产甲烷过程的关键功能微生物，这对丰富环境中DIET功能微生物多样性具有一定的意义。