Isotope Fractionation of O2 Associated with Enzymatic and Photochemical Reactions in Aquatic Environments
Third-party funded project
Project title Isotope Fractionation of O2 Associated with Enzymatic and Photochemical Reactions in Aquatic Environments
Principal Investigator(s) Pati, Sarah
Organisation / Research unit Departement Umweltwissenschaften,
Departement Umweltwissenschaften / Geochemie Stoffkreisläufe (Lehmann)
Project start 01.09.2019
Probable end 31.08.2023
Status Active
Abstract

Molecular oxygen (O2) is one of the most important electron acceptors for enzymatic and abiotic redox reactions in the environment. Enzymes that utilize Oas the terminal electron acceptor catalyze a large number of biological reactions involved in processes of metabolic activity, cellular detoxification, and biosynthesis. Many of these reactions are essential for life, but some of them also contribute to the metabolic and co-metabolic removal of anthropogenic organic contaminants in natural aquatic environments and during wastewater treatment. Abiotic reduction of Ois important for the biogeochemical cycling of elements, such as iron or other trace metals, as well as the photomineralization of organic compounds in sunlit surface water. Measuring systematic changes in the stable isotopic composition (i.e., isotope fractionation) of dissolved Oduring enzymatic reactions has been shown to enable the identification of underlying Oreduction mechanisms. Consequently, measuring isotope fractionation of Ois a promising tool for investigating environmentally relevant redox reactions. However, to ultimately apply this approach to reactions with unknown Oreduction mechanisms, it is important to systematically extend the current knowledge of reaction-specific 18O-kinetic isotope effects (18O-KIEs), which ultimately determine observable isotope fractionation of enzymatic and abiotic reactions of O2 in aquatic environments.The overall objective of this project is to experimentally determine reaction-specific 18O-KIEs associated with selected enzymatic and abiotic O2 reduction reactions and, in turn, to evaluate the potential of O2 isotope analysis as a tool to study environmental redox reactions. The central part of this project will entail the systematic investigation of well-known enzymatic reactions in laboratory experiments to determine the dependence of O2 consumption-associated 18O-KIEs on the type of enzyme, substrate, and catalyzed reaction. Different O2 reduction reactions will be studied in headspace-free reactors containing an oxidase or oxygenase enzyme, native or non-native substrates, and required cofactors and/or co-substrates. Reaction-specific 18O-KIEs will be determined in replicate experiments, where O2 concentrations will be monitored continuously, and the isotopic composition of dissolved O2 will be measured at different time-points. Comparison of these 18O-KIEs with isotope fractionation of O2 during enzymatic oxidations of anthropogenic contaminants with unknown reaction mechanisms will represent the first step in our efforts to evaluate the suitability of this approach to study O2 reduction mechanisms in anthropogenically impacted ecosystems. Similarly, 18O-KIEs will be studied for photochemical reactions involving O2 , including singlet oxygen and superoxide radical formation. The gas chromatography isotope ratio mass spectrometry (GC/IRMS) method used for these experiments will be optimized regarding sample size and the O2 extraction procedure at the beginning of the proposed project.It is expected that results from this project will enable the routine use of isotope analysis of O2 to study environmentally relevant redox reactions in laboratory-scale experiments and provide valuable mechanistic insights into specific enzymatic and abiotic redox reactions. The comprehensive set of 18O-KIEs determined in this work will establish benchmarks for various abiotic and microbial O2 consumption processes in the aquatic environment, which can be used in future studies applying isotope analysis of dissolved O2 , or of organic products containing O-atoms derived from O2, in natural systems.

Financed by Swiss National Science Foundation (SNSF)
   

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12/08/2020