Modulating Energy Transfer from Phycobilisomes to Photosystems: State Transitions and OCP-Related Non-Photochemical Quenching

Abstract

It is crucial for living organisms to be able to constantly sense and adapt to environmental changes. Light becomes dangerous for photosynthetic organisms when the energy arriving to the photochemical reaction centers exceeds the energy consumption by multiple cellular processes. This occurs under high irradiance but also under nutrient starvation conditions. In these cases, the entire photosynthetic electron transport chain becomes reduced and reactive oxygen species (ROS) leading to severe damage to the cells. Higher plants, eukaryotic algae and cyanobacteria have developed photoprotective mechanisms that decrease the energy arriving at the photochemical reaction centers or balance the energy arriving at each photosystem by regulating the antenna-reaction center interactions. The mechanisms differ in cyanobacteria from those existing in higher plants and eukaryotic algae due to the special cyanobacterial antenna, the phycobilisome (as distinct from the phycobilsome of red algae). In this chapter, two phycobilisome photoprotective mechanisms will be described: the Orange Carotenoid Protein (OCP)-related Non-Photochemical-Quenching (NPQ) and state transitions. The OCP is a photoactive protein that senses light intensity and induces thermal dissipation of excess excitation energy by interacting with the phycobilisome. The OCP-related NPQ is induced only under high irradiance independently of the redox state of the electron transport chain. In contrast, the redox state of the plastoquinone pool regulates state transitions by inducing a restructure of the photosynthetic apparatus. This leads to redistribution of the energy absorbed by the phycobilisomes between the photosystems and/or to changes in excitation energy spillover between photosystems.