Geochemical patterns and microbial contribution to iron plaque formation in the rice plant (Oryza sativa) rhizosphere

水稻是世界上一半以上人口的主要食物来源，世界上80%的水稻种植是在水田土壤上进行的。土壤中和土壤-水界面还原条件的建立不仅刺激了微生物的生产和温室气体甲烷的释放。这些设置也为微生物铁（III）还原创造了最佳条件，从而使系统充满还原性亚铁。通过表面活性高的铁质矿物的还原和溶解，吸收的营养物和污染物（如东南亚的砷）将被动员起来，从而可供植物吸收。水稻植物已经进化出一种从根部释放氧气的策略，以防止在高铁环境中铁中毒。向还原性水稻土释放氧气，使水稻根部形成铁膜，最终增加对有毒金属的吸附能力。到目前为止，控制铁斑块形成的地球化学和微生物过程还没有被破译。据推测，铁（II）氧化细菌在沿根的铁（III）矿物形成中起潜在作用。然而，它们在根际的空间分布、它们的丰度模式作为植物生长的功能以及它们控制局部铁氧化还原循环的潜力尚未得到详细的研究。本研究的目的是评估铁（II）氧化细菌和铁（III）矿物沉淀细菌对水稻根系铁斑块形成的贡献。为此，我们首先打算绘制和关联水稻根际中作为水稻生长功能的高分辨率（生物）地球化学和铁矿物学模式。在第二步中，铁（II）氧化和铁（III）还原种群将在植物确定的营养阶段的整个既定氧化还原梯度中进行量化。第三，将在生长实验中对意大利Vercelli原生水稻土中分离细菌的铁（II）氧化率和铁（III）还原率进行量化。最后，微生物对铁斑块形成的贡献将通过一系列实验来评估，在这些实验中，水稻植物在无菌的地球化学控制的环境中种植，该环境中加入了分离的嗜氧微铁（II）氧化细菌。综合数据集将提供时空变化贡献的见解，以及水稻根际微生物铁矿砂生产和溶解的变化。所获得的知识将为研究通过铁斑块形成的污染物（im）动员提供良好的基础，并有可能将水稻种植作为食物输送和土壤修复的并行行动。

Rice is the major food source for more than half of the world population and 80 percent of the worldwide rice cultivation is performed on water logged paddy soils. The establishment of reducing conditions in the soil and across the soil-water interface not only stimulates the microbial production and release of the greenhouse gas methane. These settings also create optimal conditions for microbial iron(III) reduction and therefore saturate the system with reduced ferrous iron. Through the reduction and dissolution of ferric minerals that are characterized by their high surface activity, sorbed nutrients and contaminants (e.g. arsenic in South East Asia) will be mobilized and are thus available for uptake by plants. Rice plants have evolved a strategy to release oxygen from their roots in order to prevent iron toxification in highly ferrous environments. The release of oxygen to the reduced paddy soil causes ferric iron plaque formation on the rice roots and finally increases the sorption capacity for toxic metals. To this date the geochemical and microbiological processes that control the formation of iron plaque are not deciphered. It has been hypothesized that iron(II)-oxidizing bacteria play a potential role in the iron(III) mineral formation along the roots. However, their spatial distribution within the rhizosphere, their abundance pattern as a function of plant growth and their potential to control the local iron redox cycling has never been investigated in detail. The goal of the proposed study is to evaluate the contribution of Fe(II)-oxidizing and thus Fe(III) mineral precipitating bacteria to iron plaque formation on rice roots. To this end, we first intend to map and correlate high resolution (bio)geochemical and iron-mineralogical patterns in the rice plant rhizosphere as a function of rice growth. In a second step, iron(II)-oxidizing and iron(III)-reducing populations will be quantified throughout the established redox gradients at defined vegetative stages of the plant. Thirdly, iron(II) oxidation and iron(III) reduction rates of isolated bacteria from native paddy soil sampled in Vercelli (Italy), will be quantified in growth experiments. And finally, the microbial contribution to iron plaque formation will be evaluated by an experimental series in which the rice plants are cultivated in a sterile geochemically controlled set-up that has been spiked with isolated microaerophilic iron(II)-oxidizing bacteria. The combined dataset will provide insights on the spatially and temporally varying contribution, as well as on the variation of microbial ferric mineral production and dissolution in the rice plant rhizosphere. The acquired knowledge will provide an excellent basis research on contaminant (im)mobilization through iron plaque formation and the potential to apply rice cultivation as food delivering and soil remediation action in parallel.