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

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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.