Characterization of the non-photochemical quenching of chlorophyll fluorescence that occurs during the active accumulation of inorganic carbon in the cyanobacterium Synechococcus PCC 7942

Abstract

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 μmol 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 μmol photons m⁻² s⁻¹), when assayed at a white light (WL) intensity of 250 μmol 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 O₂ 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≃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.