Coupling and competitiveness of iron-, sulfate-, and CO2-reduction along gradients

生物地球化学梯度与有机物和污染物周转的相关性目前还知之甚少。这项研究旨在识别和量化不同氧化还原过程沿梯度的相互作用。铁还原和硫酸盐还原与产甲烷的相互作用将通过可控的间歇和柱状实验进行研究。将评估制约使用这些电子受体的可及性和能量产量的因素，例如铁氧化物的钝化、硫化氢在铁氧化物上的再次氧化。然后将描述这些限制对特定进程的竞争力的影响。将特别关注以前以铁和硫酸盐还原条件为特征的系统中产甲烷条件的演变。由于产甲烷条件大多是从微生态位演化而来的，因此将建立研究这种微生态位的存在、演化和稳定性的方法。为此，将结合使用吉布斯的自由能计算、同位素分馏和示踪剂测量以及代谢中间体(小池大小)和终端产品(大池大小)的质量平衡。将比较使用微电极(毫米尺度)、用于溶质和气体的微型采样装置(厘米尺度)和质量流动平衡(柱/反应器尺度)在不同尺度上测量的这些参数，以表征有机物降解路径和电子流动的单位体积。特别令人感兴趣的是，氧化还原活性腐殖质对所涉及的终端电子接受过程的竞争力的影响，这些过程既可以作为电子穿梭，也可以直接提供电子接受能力。这将使用获得的光谱的荧光光谱和平行因子分析(PARAFAC)来研究。我们期望这一结果将为改进含水层和沉积物中厌氧过程的反应运移模型提供基础。

The relevance of biogeochemical gradients for turnover of organic matter and contaminants is yet poorly understood. This study aims at the identification and quantification of the interaction of different redox processes along gradients. The interaction of iron-, and sulfate reduction and methanogenesis will be studied in controlled batch and column experiments. Factors constraining the accessibility and the energy yield from the use of these electron acceptors will be evaluated, such as passivation of iron oxides, re-oxidation of hydrogen sulfide on iron oxides. The impact of these constraints on the competitiveness of the particular process will then be described. Special focus will be put on the evolution of methanogenic conditions in systems formerly characterized by iron and sulfate reducing condition. As methanogenic conditions mostly evolve from micro-niches, methods to study the existence, evolution and stability of such micro-niches will be established. To this end, a combination of Gibbs’ free energy calculations, isotope fractionation and tracer measurements, and mass balances of metabolic intermediates (small pool sizes) and end products (large pool sizes) will be used. Measurements of these parameters on different scales using microelectrodes (mm scale), micro sampling devices for solutes and gases (cm scale) and mass flow balancing (column/reactor scale) will be compared to characterize unit volumes for organic matter degradation pathways and electron flow. Of particular interest will be the impact of redox active humic substances on the competitiveness of involved terminal electron accepting processes, either acting as electron shuttles or directly providing electron accepting capacity. This will be studied using fluorescence spectroscopy and parallel factor analysis (PARAFAC) of the gained spectra. We expect that the results will provide a basis for improving reactive transport models of anaerobic processes in aquifers and sediments.