ABSENCE OF OXYGEN-EVOLVING CAPACITY IN DARK-GROWN CHLORELLA: THE PHOTO ACTIVATION OF OXYGEN-EVOLVING CENTERS*

Abstract— Chlorella cells were cultured in darkness on glucose and examined for pigment composition, capacity to evolve oxygen, and capacity to photo-oxidize artificial electron donors to Photosystems II and I. Evidence was obtained which indicated that such cells lack oxygen-evolving centers and the Mn pool associated with such centers but possess fully active System II and I trapping centers. Brief illumination ( t _1/2˜ 2–3 min) of dark-grown cells resulted in incorporation of Mn into the O₂-evolving centers and the capacity to evolve O₂ (photo-activation). Kinetic data from flashing or continuous light showed that the photoactivation is a multi-quantum process involving several rate limitations and a photosensitive state of limited stability. Measurements of oxygen-yield oscillations indicated that throughout the photoactivation process each newly formed oxygen-yielding center was independent of its neighbors. It is concluded that photoactivation of the Mn-containing oxygen-yielding catalyst is a fundamental process in all photosynthetic oxygen-evolving tissue.