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© 2018 Elsevier B.V. We have used the desiccation-tolerant lichen Flavoparmelia caperata, containing the green algal photobiont Trebouxia gelatinosa, to examine H/D isotope effects in Photosystem II in vivo. Artifact-free H/D isotope effects on both PSII primary charge separation and water oxidation yields were determined as a function of flash rate from chlorophyll-a variable fluorescence yields. Intact lichens could be reversibly dehydrated/re-hydrated with H2O/D2O repeatedly without loss of O2 evolution, unlike all isolated PSII preparations. Above a threshold flash rate, PSII charge separation decreases sharply in both D2O and H2O, reflecting loss of excitation migration and capture by PSII. Changes in H/D coordinates further slow charge separation in D2O (−23% at 120 Hz), attributed to reoxidation of the primary acceptor QA−. At intermediate flash rates (5–50 Hz) D2O decreases water oxidation efficiency (O2 evolution) by −2–5%. No significant isotopic difference is observed at slow flash rates (<5 >Hz) where charge recombination dominates. Slower D2O diffusion, changes in hydrogen bonding networks, and shifts in the pKa's of ionizable residues may all contribute to these systematic variations of H/D isotope effects. Lichens’ reversible desiccation tolerance allows highly reproducible H/D exchange kinetics in PSII reactions to be studied in vivo for the first time.

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Biochimica et Biophysica Acta - Bioenergetics

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