Influence of hydroxamate siderophores on redox-sensitive trace elements and their isotopes and their relevance for the application of redox proxies in paleo-climate reconstruction studies

在自然环境中，植物、微生物和真菌会分泌一系列铁特异性有机螯合剂，称为铁载体。这些分子几乎存在于所有的自然环境中，已知不仅与Fe（III）结合，而且与一系列其他环境重要的微量金属，特别是那些具有高电荷和低离子半径的金属，具有高的复杂稳定常数。这些元素，即Zr， Hf，稀土元素，Th， U等，在地球化学中被称为高场强元素（HFSE），传统上认为在水-岩相互作用过程中是不可移动的，除非氧化态发生特殊变化，这可能导致对氧化还原敏感的HFSE（即U， Ce）及其同位素的行为与对氧化还原不敏感的HFSE不同。这种观测通常用于古气候研究，以重建过去的大气氧水平。我的试点研究表明，铁载体去铁胺B能够与一些氧化还原敏感的HFSE结合，并在从某些岩石基质中浸出时提高它们相对于氧化还原不敏感的HFSE的流动性。在本提案中，我想系统地研究DFOB铁载体对一系列氧化还原敏感微量元素及其同位素的潜在影响。为了实现这一点，我将在天然岩石上进行批量浸出实验，并在有铁载体和其他有机配体存在的情况下进行有氧和无氧条件下的清除实验。微量元素和同位素研究将由XANES/EXAFS同步辐射测量补充，以验证即使在低氧条件下，铁载体是否真的能够氧化氧化敏感的痕量金属，或者是否有其他机制导致氧化还原敏感的HFSE在DFOB铁载体存在下的明显动员。这项研究填补了一个重要的研究空白，因为细胞外生物化合物对自然环境中这些元素和同位素的影响是相当未知的，也不清楚这些生物化合物的潜在存在是否会限制某些古还原代用物的应用，以及沉积物沉积期间或之后的强烈生物活性是否会导致古气候重建研究中的假阳性。这个项目的结果也可能提供一种想法，即在某些情况下，某些氧化还原指标是否可能表明“生命”的存在，而不是氧气的存在，例如在中太古宙或新太古代。为了实现这一目标，我将与微量元素和同位素地球化学以及x射线吸收光谱领域的国际知名专家合作。

In the natural environment, plants, microbes and fungi excrete a range of iron-specific organic chelators called siderophores. These molecules occur in almost all natural settings and are known to not only bind to Fe(III), but also have high complex stability constants with a range of other environmentally important trace metals, esp. to those that have a high charge and low ionic radius. These elements, i.e., Zr, Hf, the rare earth elements, Th, U and others, are referred to as high-field strength elements (HFSE) in geochemistry and are traditionally considered immobile during water-rock interaction unless a specific change in the oxidation state occurs, which may cause a different behavior of the redox-sensitive HFSE (i.e., U, Ce) and their isotopes in comparison to redox-insensitive HFSE. This observation is commonly used in paleoclimate studies to reconstruct past atmospheric oxygen levels. My pilot research indicates that the siderophore desferrioxamine B is able to bind to some of these redox-sensitive HFSE and enhances their mobility relative to the redox-insensitive HFSE during leaching from certain rock matrices. In the present proposal, I want to systematically investigate the potential impact of the DFOB siderophore on a range of redox-sensitive trace elements and their isotopes. In order to achieve this, I will conduct batch leaching experiments on natural rocks and I will also conduct scavenging experiments, both under oxic and anoxic conditions, in presence of siderophores as well as other organic ligands. The trace element and isotope studies will be complemented by XANES/EXAFS synchrotron radiation measurements to verify whether siderophores are actually able to oxidize redox-sensitive trace metals even under low oxygen conditions or whether other mechanisms cause the pronounced mobilization of redox-sensitive HFSE in presence of the DFOB siderophore. This study is filling an important research gap as the effects of extracellular biological compounds on these elements and isotopes in the natural environment are fairly unknown and it is also not clear whether the potential presence of such biogenic compounds may set limitations on the application of certain paleoredox proxies and whether intense biological activity during or after sediment deposition could lead to false-positives in paleoclimate reconstruction studies. The results of this project may also provide an idea on whether certain redox proxies, under certain circumstances, might indicate the presence of ‘life’ rather than the presence of oxygen, e.g., in the Meso- or Neoarchean. To achieve this goal, I will be collaborating with internationally renowned experts in the fields of trace element and isotope geochemistry and x-ray absorption spectroscopy.