STATE TRANSITIONS OR DELTA-PH-DEPENDENT QUENCHING OF PHOTOSYSTEM-II FLUORESCENCE IN RED ALGAE

Fluorescence changes attributed to state transitions have been shown t o exist in phycobilisome-containing organisms. Contradictory conclusio ns have been derived from studies about the mechanism of state transit ions carried out either in cyanobacteria or in red algae. In this pape r, fluorescence changes induced by light 1 and light 2 are reinvestiga ted in a unicellular red alga, Rhodella violacea, by performing 77 K f luorescence spectra and fluorescence yield measurements at room temper ature in the presence of uncouplers and inhibitors of the electron tra nsfer. We show that transfer of light 1-adapted cells to light 2 (gree n light) induces a large quenching of photosystem II which is suppress ed by subsequent incubation in light 1 (far-red or blue light), The le vel of the photosystem I-related fluorescence does not change during t hese 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 can celing the Delta pH across the membrane. DCCD, which is an inhibitor o f the ATPase that swells the Delta pH, maintains the quenched state ev en under light 1 illumination. The opposite effects of DCMU and DBMIB on state transitions are demonstrated to be due to a suppression (by D CMU) or maintenance (by DBMIB) of the Delta pH and not to a 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, w e show that very low intensities of a light that activates only the ph otosystem II induce a Delta pH across the membrane that is not dissipa ted since the ATPase is not activated. The Delta pH is dissipated only under conditions in which the photosystem I turns, confirming that th e thioredoxin must be reduced to activate the ATPase. We suggest that the fluorescence changes, induced by various light conditions, in cyan obacteria and red algae could be associated with different phenomena.