Distribution of excitation energy between photosystem I and photosystem II in red algae. III. Quantum requirements of the induction of a state 2-state 1 transition.

1. The light-induced redistribution of excitation energy between both photosystems (state 1-state 2 phenomenon) is investigated in Halymenia latifolia and in eight other marine red algae by measurements of slow fluorescence kinetics and of O2-exchange in monochromatic and in flashing light. 2. A light 1 pulse (443 nm) of 0.2 s and of medium intensity is sufficient to induce complete transfer from state 2 (maximum energy transfer) to state 1 (minimum energy transfer). At inducing light periods of 3 min, light intensities as low as 2.10(-13) einstein.cm-2.s-1 gave half-maximum effect. This low energy effect is strictly to be distinguished from another, somewhat similar effect restricted to higher light intensities (more than 10(-10) E.cm-2.s-1). 3. The low-energy effect is definitely dose-dependent over a wide range of inducing illumination times. In the mean of all experiments with Halymenia, a photon fluence of 2.7.10(-11) E.cm-2 gave a half-maximum transfer to state 1. The dose-effect curves are always found distinctively S-shaped. 4. On the basis of light flash experiments it is calculated that in Halymenia, Stenogramme and in Phycodrys, 2-4 photons per electron transport chain, absorbed in surplus by Photosystem I, are sufficient to induce a half-maximum transition to state 1. 5. The quantum requirement for the induction of the inverse transition to state 2 starting with state 1 is in the same range; it tends to be slightly higher. 6. The results are interpreted as revealing a close connection between the redox state of the electron transport chain (or of some single component of it) and the probability of energy transfer between Photosystem II and Photosystem I.