Controlling Light Harvesting with Light

When exposed to intense sunlight, all organisms performing oxygenic photosynthesis implement various photo protective strategies to prevent potentially lethal photodamage. The rapidly responding photoprotective mechanisms, occurring in the lightharvesting pigment protein antennae, take effect within tens of seconds, while the dramatic and potentially harmful light intensity fluctuations manifest also on shorter time scales. Here we show that, upon illumination, individual phycobilisomes from Synechocystis PCC 6803, which, in vivo under low-light conditions, harvest solar energy, and have the built-in capacity to switch rapidly and reversibly into light-activated energy-dissipating states. Simultaneously measured fluorescence intensity, lifetime; and spectra, compared with a multicompartmental kinetic model, revealed that essentially any subunit of a phycobilisorne can be quenched, and that the Core complexes were targeted most frequently. Our results provide the first evidence for fluorescence blinking from a biologically-active system at physiological light intensities and suggest that the light-controlled switches to intrinsically available energy-dissipating states are responsible for a novel type of photoprotection in cyanobacteria. We anticipate other photosynthetic organisms to employ similar strategies to respond instantly to rapid solar light intensity fluctuations. A detailed understanding of the photophysics of photosynthetic antenna:complexes is of great interest for bioinspired solar energy technologies.