Desiccation tolerant lichens facilitate in vivo H/D isotope effect measurements in oxygenic photosynthesis.

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 HO/DO repeatedly without loss of O evolution, unlike all isolated PSII preparations. Above a threshold flash rate, PSII charge separation decreases sharply in both DO and HO, reflecting loss of excitation migration and capture by PSII. Changes in H/D coordinates further slow charge separation in DO (-23% at 120 Hz), attributed to reoxidation of the primary acceptor Q. At intermediate flash rates (5-50 Hz) DO decreases water oxidation efficiency (O evolution) by -2-5%. No significant isotopic difference is observed at slow flash rates (<5 Hz) where charge recombination dominates. Slower DO diffusion, changes in hydrogen bonding networks, and shifts in the pK'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.