CHARACTERIZATION OF THE NONPHOTOCHEMICAL QUENCHING OF CHLOROPHYLL FLUORESCENCE THAT OCCURS DURING THE ACTIVE ACCUMULATION OF INORGANIC CARBON IN THE CYANOBACTERIUM SYNECHOCOCCUS PCC-7942

Previous work has shown that the maximum fluorescence yield from PS 2 of Synechococcus PCC 7942 occurs when the cells are at the CO2 compens ation point. The addition of inorganic carbon (C-i), as CO2 or HCO3-, causes a lowering of the fluorescence yield due to both photochemical (q(p)) and non-photochemical (q(N)) quenching. In this paper, we chara cterize the q(N) that is induced by C-i addition to cells grown at hig h light intensities (500 mu mol photons m(-2) s(-1)). The C-i-induced q(N) was considerably greater in these cells than in cells grown at lo w light intensities (50 mu mol photons m(-2) s(-1)), when assayed at a white light (WL) intensity of 250 mu mol photons m(-2) s(-1). In high -light grown cells we measured q(N) values as high as 70%, while in lo w-light grown cells the q(N) was about 16%. The q(N) was relieved when cells regained the CO2 compensation point, when cells were illuminate d by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These character istics indicate that the q(N) was not a form of energy quenching (q(E) ). Supplemental FRL illumination caused significant enhancement of pho tosynthetic O-2 evolution that could be correlated with the changes in q(p) and q(N). The increases in q(p) induced by C-i addition represen t increases in the effective quantum yield of PS 2 due to increased le vels of oxidized Q(A). The increase in q(N) induced by C-i represents a decrease in PS 2 activity related to decreases in the potential quan tum yield. The lack of diagnostic changes in the 77 K fluorescence emi ssion spectrum argue against q(N) being related to classical state tra nsitions, in which the decrease in potential quantum yield of PS 2 is due either to a decrease in absorption cross-section or by increased ' spill-over' of excitation energy to PS 1. Both the C-i-induced q(p) (t (0.5) < 0.5 s) and q(N) (t(0.5) similar or equal to 1.6 s) were rapidl y relieved by the addition of DCMU. The two time constants give furthe r support for two separate quenching mechanisms. We have thus characte rized a novel form of q(N) in cyanobacteria, not related to state tran sitions or energy quenching, which is induced by the addition of C-i t o cells at the CO2-compensation point.