Light-induced increase in the steady-state chlorophyll fluorescence in cyanobacteria reflects induction of energy dissipation complementary to orange carotenoid protein-dependent thermal dissipation.

Under high-light conditions, the dissipation of excess energy as heat in the light-harvesting antenna is essential for photosynthetic organisms to protect the photosynthetic machinery. In the case of cyanobacteria, however, the induction of the thermal dissipation in the antennae is insufficient to dissipate all excess energy, which is manifested as the increase in the steady-state level of chlorophyll fluorescence (Fs) under high light. To elucidate the underlying cause of the incomplete dissipation of excess light in the antenna, we investigated the impact of depletion and overexpression of orange carotenoid protein (OCP), which is essential to induce thermal dissipation in the antenna, on photosynthesis in Synechocystis sp. PCC 6803. The suppression of the OCP-dependent thermal dissipation resulted in elevated Fs with a constant yield of photosynthesis, suggesting that the light-induced increase in Fs might function as an acclimation mechanism to high light, which compensated for the lower OCP-dependent thermal dissipation. By contrast, over-induction of the OCP-dependent thermal dissipation decreased not only Fs but also the yield of photosynthesis under high light, due to the reduced energy transfer from the antenna to photosystem II. These results indicate that the complete removal of excess energy via the OCP-dependent mechanism has a drawback in photosynthetic efficiency under high-light conditions, and the strategy independent of OCP is employed to cope with excess light without lowering the yield of photosynthesis in cyanobacteria.