Photosynthetic organisms have found various smart ways to cope with unexpected changes in light conditions. In many cyanobacteria, the lethal effects of a sudden increase in light intensity are mitigated mainly by the interaction between phycobilisomes (PBs) and the orange carotenoid protein (OCP). The latter senses high light intensities by means of photoactivation and triggers thermal energy dissipation from the PBs. Due to the brightness of their emission, PBs can be characterized at the level of individual complexes. Here, energy dissipation from individual PBs was reversibly switched on and off using only light and OCP. We reveal the presence of quasistable intermediate states during the binding and unbinding of OCP to PB, with a spectroscopic signature indicative of transient decoupling of some of the PB rods during docking of OCP. Real-time control of emission from individual PBs has the potential to contribute to the development of new super-resolution imaging techniques.

Correspondence Author -Tjaart P. J. Krüger, tjaart.kruger@up.ac.za.Abstract-Plasmonic coupling of metallic nanoparticles and adjacent pigments can dramatically increase the brightness of the pigments due to the enhanced local electric field. Here, we demonstrate that the fluorescence brightness of a single plant light-harvesting complex (LHCII) can be significantly enhanced when coupled to single gold nanorods (AuNRs). The AuNRs utilized in this study were prepared via chemical reactions, and the hybrid system was constructed using a simple and economical spin-assisted layer-by-layer technique.Enhancement of fluorescence brightness of up to 240-fold was observed, accompanied by a 109-fold decrease in the average (amplitude-weighted) fluorescence lifetime from approximately 3.5 ns down to 32 ps, corresponding to an excitation enhancement of 63-fold and emission enhancement of up to 3.8-fold. This large enhancement is due to the strong spectral overlap of the longitudinal localized surface plasmon resonance of the utilized AuNRs and the absorption or emission bands of LHCII. This study provides an inexpensive strategy to explore the fluorescence dynamics of weakly emitting photosynthetic light-harvesting complexes at the single molecule level.

No abstract