Cyanobacteria exhibit a novel form of non-photochemical quenching (NPQ) at the level of the phycobilisome. NPQ is a process that protects photosystem II (PSII) from possible highlight-induced photo-damage. Although significant advancement has been made in understanding the NPQ, there are still some missing details. This critical review focuses on how the orange carotenoid protein (OCP) and its partner fluorescence recovery protein (FRP) control the extent of quenching. What is and what is not known about the NPQ is discussed under four subtitles; where does exactly the site of quenching lie? (site), how is the quenching being triggered? (trigger), molecular mechanism of quenching (quenching) and recovery from quenching. Finally, a recent working model of NPQ, consistent with recent findings, is been described.
Keywords: Cyanobacteria; Fluorescence recovery protein; Non-photochemical quenching; Orange carotenoid protein; Phycobilisome.