State transitions or delta pH-dependent quenching of photosystem II fluorescence in red algae.

Fluorescence changes attributed to state transitions have been shown to exist in phycobilisome-containing organisms. Contradictory conclusions have been derived from studies about the mechanism of state transitions carried out either in cyanobacteria or in red algae. In this paper, fluorescence changes induced by light 1 and light 2 are reinvestigated in a unicellular red alga, Rhodella violacea, by performing 77 K fluorescence spectra and fluorescence yield measurements at room temperature in the presence of uncouplers and inhibitors of the electron transfer. We show that transfer of light 1-adapted cells to light 2 (green light) induces a large quenching of photosystem II which is suppressed by subsequent incubation in light 1 (far-red or blue light). The level of the photosystem I-related fluorescence does not change during these transfers. We demonstrate that the large quenching of photosystem II induced by low intensities of green light is completely suppressed by addition of NH4Cl, an uncoupler that inhibits ATP synthesis by canceling the delta pH across the membrane. DCCD, which is an inhibitor of the ATPase that swells the delta pH, maintains the quenched state even under light 1 illumination. The opposite effects of DCMU and DBMIB on state transitions are demonstrated to be due to a suppression (by DCMU) or maintenance (by DBMIB) of the delta pH and not to change in the redox state of the plastoquinone. We conclude that, in R. violacea, the fluorescence change commonly associated with state 2 transition is in fact a delta pH-dependent quenching. This type of quenching has always been associated with near-saturating light intensities. Here, we show that very low intensities of a light that activates only the photosystem II induce a delta pH across the membrane that is not dissipated since the ATPase is not activated. The delta pH is dissipated only under conditions in which the photosystem I turns, confirming that the thioredoxin must be reduced to activate the ATPase. We suggest that the fluorescence changes, induced by various light conditions, in cyanobacteria and red algae could be associated with different phenomena.