State transitions and fluorescence quenching in the cyanobacterium Synechocystis PCC 6803 in response to changes in light quality and intensity

• State transitions take place under various light conditions.
• State transitions achieve a conserved level at a variable rate.
• State transition stops as soon as OCP-mediated quenching appearance.
• Appearance, level and rate of the quenching depends on light intensity.
• Red-form OCPs undergo a deactivation under light.

State transition and non-photochemical fluorescence quenching in cyanobacteria are short-term adaptations of photosynthetic apparatus to changes in light quality and intensity, however, the kinetic details and relationship are still not clear. In this work, time-dependent 77 K fluorescence spectra were monitored for cyanobacterium Synechocystis PCC 6803 cells under blue, orange and blue–green light in a series of intensities. The characteristic fluorescence signals indicated state transition taking place exclusively under 430–450 or 580–600 nm light or 480–550 nm light at the intensities ⩽150 μE m−2 s−1 to achieve a conserved level with variable rate constant. Under 480–500 nm or 530–550 nm light at the intensities ⩾160 μE m−2 s−1, state transition took place at first but stopped as soon as the fluorescence quenching appeared. The dependence of appearance, induction period, level and rate constant for the quenching on light intensity suggests that a critical concentration of photo-activated OCPs is necessary and may be achieved by a dynamic equilibrium between the activation and deactivation under light.