Identification of metabolic and enzymatic pathways in the phosphorous cycle using triple oxygen isotope systematics

磷(P)通常是植物生长的限制养分。这一限制减少了食物的生产和陆地碳汇缓解全球变暖的潜力。了解土壤中的磷循环，以了解生物有效磷库是如何维持的，以及生态系统如何对人类干预作出反应，这是至关重要的。这需要深入了解影响植物获得磷的过程、途径、时间尺度和因素。同位素分析经常被用来更好地了解元素循环，但磷是单同位素。因此，PO4的氧同位素被用来识别磷循环中的化学和物理过程。在这里，我建议通过分析PO4的17O/16O比以及更常用的18O/16O来扩展这一指标。这是有益的，主要有两个原因。1)大气O2包括一个负17O异常，它继承于与臭氧形成有关的质量无关的分馏效应。这种异常可以通过生物对空气O2的代谢消耗转移到土壤中的PO4。众所周知，动物体内的水含有负17O异常。我预计这种异常会转移到动物粪便中。例如，鸟粪基肥料应含有可用作天然示踪剂的负17O异常的PO4。同样，代谢活跃的微生物产生异常的PO4，如初步数据集中所示。此外，野火期间的高温促进了空气O2和植物PO4之间的无机氧同位素交换，为研究火灾序列提供了示踪剂。我想测试这些应用。磷是如此重要的元素，因为生物体用它来合成DNA、RNA、ATP和磷脂。然而，这种有机磷酸盐(POG)对植物是不可生物利用的。通常认为，在磷限制条件下，植物释放胞外酶，即所谓的磷酸酶，作为从有机化合物中获取磷的一种策略。正如我们的初步数据集所证明的，这些酶在离开的、现在生物可用的PO4分子上诱导特征的三重氧同位素效应。这种同位素指纹可以用来追踪土壤中的酶活性。要做到这一点，将使用实验装置精确地表征酶指纹。三重氧同位素方法是一种很有前途的工具，可以识别和量化土壤磷循环中的各种过程。如果成功，这种方法也可以应用于例如海洋环境。这项提议是一个更大的海森堡提议的一部分，在那里我计划将这些17O系统学应用到古生物应用中。为此，有必要建立现代三氧同位素系统学，这是这项建议的主要目标之一。

Phosphorus (P) is often the limiting nutrient for plant growth. This limitation reduces the production of food and the potential of the terrestrial carbon sink to mitigate global warming. It is essential to understand P-cycling in soils to understand how the bioavailable P-pool is maintained and how ecosystems respond to human intervention. This requires an in-depth understanding of the processes, pathways, timescales and factors that contribute to the availability of P to plants.Analyses of isotopes are frequently used to better understand element cycling, but P is mono-isotopic. Therefore, oxygen isotopes of PO4 are used to identify chemical and physical processes within the phosphorous cycle. Here I suggest to expand this proxy by analyzing 17O/16O ratios of PO4 in addition to the more commonly used 18O/16O. This is beneficial for two principal reasons. 1) Air O2 comprises a negative 17O anomaly inherited from mass independent fractionation effects associated to the formation of ozone. This anomaly can be transferred to PO4 in soils via metabolic consumption of air O2 by organisms. It is well known that the body water of animals comprises a negative 17O anomaly. I expect that this anomaly is transferred to animal feces. Guano based fertilizer, for instance, should thus comprise PO4 with a negative 17O anomaly that can be used as a natural tracer. Similarly, metabolically active microorganisms generate anomalous PO4 as shown in the preliminary dataset. In addition, high temperatures during wildfires facilitate inorganic oxygen isotope exchange between air O2 and plant PO4 providing a tracer to investigate fire sequences. I want to test these applications.2) Phosphorus is such an essential element because organisms use it to synthesize DNA, RNA, ATP and phospholipids. This organic phosphate (Porg) is, however, not bioavailable for plants. It is generally assumed that under P limiting conditions, plants release extracellular enzymes, so called phosphatases, as a strategy to acquire P from organic compounds. As demonstrated in our preliminary dataset, these enzymes induce characteristic triple oxygen isotope effects on to the leaving, now bioavailable, PO4 molecule. This isotopic fingerprint can be used to trace enzyme activity within soils. To do so, the enzymatic fingerprints will be characterized precisely using experimental setups.The triple oxygen isotope approach is a promising tool to identify and quantify various processes within the soil phosphorus cycle. If successful, this approach can also be applied e.g. in the marine environment. This proposal is part of a larger Heisenberg proposal, where I plan to apply these 17O systematics to paleo-applications. For this purpose, it is essential to work out the modern day triple oxygen isotope systematics, which is one major goal of this proposal.