Mechanisms Modulating Energy Arriving at Reaction Centers in Cyanobacteria

Abstract

Cyanobacteria possess several photoprotective mechanisms involving relatively rapid (<few minutes) changes in the photosynthetic apparatus to dynamically adjust the amount of irradiance arriving at photosystem II (PS II). Photoprotection in cyanobacteria differs from that in plants and algae, with both of the latter possessing transmembrane chlorophyll- and carotenoid-binding light-harvesting complexes. Most cyanobacteria harvest light via a large extra-membrane complex, the phycobilisome (PBS) that contains blue and red phycobiliproteins. Thus far, three prominent and rapid processes have been identified that control effective PS II antenna size in cyanobacteria: state transitions, blue-green-light-induced thermal dissipation of excess energy absorbed by PBS, and PBS decoupling. While the latter two have a mostly photoprotective function, the process of state transitions also optimizes energy distribution between the two photosystems. For blue-green-light-induced energy thermal dissipation, a water soluble photoactive Orange Carotenoid Protein (OCP) is essential. The OCP acts as a light-intensity sensor and energy-dissipation-inducer and is the only photoactive protein known thus far with a carotenoid as its sensor. Strong blue-green light induces structural changes in the OCP that lead to the formation of its “red-active form”. The red OCP, by interacting with the PBS core, increases thermal energy dissipation at the level of antenna and decreases the energy arriving at reaction centers. To recover full antenna (light-harvesting) capacity under low light conditions, a second protein is required, the “Fluorescence Recovery Protein” (FRP) that plays a key role in dislodging the red OCP protein from the PBS and accelerates OCP conversion to the inactive orange form. In this chapter, we review the current understanding of the mechanism of OCP-mediated fluorescence and energy dissipation in cyanobacteria. Despite decades of research on state transitions in cyanobacteria, the underlying mechanism of this phenomenon remains unresolved. Two mechanisms of state transitions occurring at low light intensities have been suggested for cyanobacteria. In the first, a physical movement of PBS or PS I monomers leads to redistribution of energy absorbed by PBS between PS II and PS I. In the second suggested mechanism, excitation-energy spillover regulates redistribution of absorbed light between PS II and PS I. Controversy exists as to whether state transitions involve long-range displacement of proteins (PBS, PS I) and how changes in plastoquinone redox state trigger energy redistribution between the photosystems. Several new reports now indicate that cyanobacteria can modulate energy transfer within PBS or from PBS to PS II reaction centers. Such a “decoupling” could represent an alternative safety valve for excess energy dissipation, especially in those groups of cyanobacteria that do not possess OCP-related NPQ.