Desiccation tolerant lichens facilitate in vivo H/D isotope effect measurements in oxygenic photosynthesis |
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Authors: | David J. Vinyard Gennady M. Ananyev G. Charles Dismukes |
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Affiliation: | 1. Department of Chemistry, Princeton University, Princeton, NJ 08544, USA;2. Waksman Institute of Microbiology, Rutgers University, Piscataway, NJ 08854, USA;3. Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, USA |
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Abstract: | 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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Keywords: | Photosystem II Lichens Isotope effects Water oxidation |
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