Characterization of the non-photochemical 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 compensation 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 characterize the q(N) that is induced by C-i addition to cells grown at high 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 low 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 low-light grown cells the q(N) was about 16%. The q(N) was relieved when cells regained the CO2 compensation point, when cells were illuminated by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These characteristics indicate that the q(N) was not a form of energy quenching (q(E)). Supplemental FRL illumination caused significant enhancement of photosynthetic 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 represent increases in the effective quantum yield of PS 2 due to increased levels 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 quantum yield. The lack of diagnostic changes in the 77 K fluorescence emission spectrum argue against q(N) being related to classical state transitions, 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 rapidly relieved by the addition of DCMU. The two time constants give further support for two separate quenching mechanisms. We have thus characterized a novel form of q(N) in cyanobacteria, not related to state transitions or energy quenching, which is induced by the addition of C-i to cells at the CO2-compensation point.