Gaining Insight into Mechanisms of Nonphotochemical Quenching of Chlorophyll Fluorescence in Chlamydomonas reinhardtii via the Observation of Dark-induced State Transitions

Abstract

Photosynthetic organisms are usually exposed to fluctuating light, and therefore they evolved mechanisms enabling fast acclimation to changing light conditions. Among them, two important ones are energy-dependent quenching of excited chlorophyll (qE) and state transitions (ST). qE is a photoprotective mechanism regulated by pH gradient across thylakoid membranes, in which excessive energy is dissipated as heat. ST are rearrangements of antenna systems regulated by the phosphorylation of LHC II complexes. Both of these mechanisms result in changes in NPQ parameters. In the present article, changes of NPQ in the green microalga Chlamydomonas reinhardtii were evaluated in the dark period, after various lengths of actinic light exposure, and after the application of the thiol reducing reagent dithiotreitol and the cyt b6f inhibitor 2' ,4' -dinitrophenyl ether of 2-iodo-4-nitro-thymol. The impact of the length of actinic light exposure on xanthophyll cycle progression in C. reinhardtii was also analysed. The obtained results enabled us to gain more insight into the nonphotochemical quenching of chlorophyll fluorescence in model organism C. reinhardtii, i.e. the role of zeaxanthin-dependent quenching and chlororespiration-induced pH gradient, and the inhibitory action of tested compounds on state transitions in this species.