Photoprotection in cyanobacteria: regulation of light harvesting.

To cope with a rapidly fluctuating light environment, vascular plants and algae have evolved a photoprotective mechanism that serves to downregulate the transfer of excitation energy in the light-harvesting complexes to the photosynthetic reaction centers. This process dissipates excess excitation energy in the chlorophyll pigment bed by a nonradiative pathway. Since this pathway competes with and therefore quenches chlorophyll fluoresence in a nonphotochemical manner, it has been termed Non-photochemical Quenching (NPQ). For many years, cyanobacteria were not considered capable of performing NPQ as a photoprotective mechanism. Instead, the redistribution of the phycobilisome (PBS) light-harvesting antenna between reaction centers by a process called state transitions was considered the major means of regulating the utilization of harvested light energy. Recently, it was demonstrated that cyanobacteria are able to use NPQ as one component of their photoprotective strategies. Cyanobacteria exhibit significant NPQ during nutrient-replete growth, but it becomes a more prominent means of managing absorbed excitation energy when the cells experience iron starvation. Rapid progress in understanding the molecular mechanism of cyanobacterial NPQ has revealed a process that is very distinct from the functionally analogous process in plants and algae. Cyanobacterial NPQ involves the absorption of blue light by a carotenoid binding protein, termed the Orange Carotenoid Protein, and most likely involves quenching in the PBS core. In this study, we summarize work leading to the discovery of NPQ in cyanobacteria and the elucidation of molecular mechanisms associated with this important photoprotective process.