Yeung, Laurence2023-09-012023-09-012023-082023-07-10August 202Yuan, Bing. "Oxygen Isotope Fractionation during Photosynthesis at Different Light Intensities." (2023) Master's Thesis, Rice University. https://hdl.handle.net/1911/115251.https://hdl.handle.net/1911/115251This work is to study the oxygen isotopic fractionation during photosynthesis under different light intensities (60 to 1750 μE·m-2·s-1) and thylakoids extracted from spinach leaves are used as reaction material to suppress respirations and photorespiration. Triple oxygen isotope method is used to estimate the gross oxygen production. The isotope fractionation slope for the plot of ln(δ17O) versus. ln(δ18O)) of the oxygen produced from photosynthesis is 0.5242 ± 0.0039, consistent wih previous measurement. Using the substrate water and produced oxygen as reactant and product, respectively, the 18α values increases from 1.0011 to 1.0041 as light intensity increased from 60 to 1750 μE·m-2·s-1. From our analysis, both the VSMOW values and the isotope fractionation slope of respiration (λ17/18) impact the accuracy of GOP estimation. We suggest using λ17/18 = 0.522 and the VSMOW data reported by Luz et al. in 2011 to estimate GOP through the modified GOP equation by Prokopenko et al. in 2011. Clumped isotope 18O18O of oxygen from photosynthesis has a negative Δ36 which is around 0 to -0.15‰. It means that the formation of O2 during photosynthesis is a non-isotopic equilibrium process starting from the non-equivalent binding of two water molecules, consistent with previous prediction. Δ36 value maybe used to estimate GOP in future because it is not influenced by the substrate medium. From our study, as light intensities rise from 60 to 1750 μE·m-2·s-1, oxygen isotope fractionation (18ε) increases from 1 to 4‰ while the oxygen production yield decreases from 32 to 17 μmol/mg chl. Baes on our model, the increased isotope fractionation and the decreased net O2 production yield during photosynthesis are mainly caused by the excitation of O2 from ground state to the singlet state (1O2) under photoinhibition. About 15-30% oxygen is consumed in this way. Formation of 1O2 has a theoretical isotope fractionation of -6.6‰ at room temperature. When the light intensity is above 1200 μE·m-2·s-1, besides 1O2, a small percentage of O2- (1-3.3%) is formed which further increases oxygen isotope fractionation of collected oxygen.application/pdfengCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Oxygen isotope fractionationphotosynthesisgross oxygen productionclumped isotopeThesis2023-09-01