On the functional site of manganese in photosynthetic oxygen evolution

Abstract

1. 1. Restoration of Hill activity to illuminated Mn-deficient Euglena cultures is complete 4–6 h after Mn addition. 2. 2. The maximum electron transfer rate at infinite light intensity is 2.5 times higher for Mn R (reactivated) than for −Mn (deficient) chloroplasts, and the quantum efficiency is some 3 times greater. The fluorescence yield and quantum efficiency are likewise 4–5 fold higher in +Mn as against −Mn chloroplasts. 3. 3. At 283°K, +Mn chloroplasts have a characteristic fluorescence maximum at 685 nm, while −Mn chloroplasts have an additional shoulder at 693 nm. Reactivation of −Mn cells increases the fluorescence emission of the isolated chloroplasts uniformly over the entire spectrum. 4. 4. At 77°K, reactivation is accompanied by positive shifts in the fluorescence difference spectrum at 685, 700 and 719 nm. The (+Mn) — (−Mn) spectrum is characterized in addition by a trough at about 730 nm. 5. 5. The initial fluorescence yield induced by a weak measuring beam is 50% higher in either −Mn chloroplasts or partially reactivated chloroplasts compared with +Mn chloroplasts. However, the variable excess fluorescence yield in strong illumination is almost absent in −Mn chloroplasts and is high in Mn R and +Mn samples. 6. 6. Hydroxylamine can donate electrons to Photosystem II, by-passing the −Mn block in electron flow and restoring high fluorescence yields. In cases of severe Mn deficiency, restoration of electron flow by hydroxylamine is only partial. 7. 7. The results indicate a structural and a functional role for Mn in O 2 evolution; the latter, at least, residing in the span between water oxidation and the System II photoact.