Distribution of excitation energy in photosynthesis: quantification of fluorescence yields from intact cyanobacteria

Fluorescence emission spectra of intact cyanobacteria in state 1 and state 2 were determined at 77 K. Spectra were obtained from samples held in a rotating cuvette designed to average sample inhomogeneities and allow accurate fluorescence yield comparisons without normalization to intrinsic or added fluorophore emission peaks. Emission spectra obtained for excitation in the chlorophyll a (Chl a) and phycobilisome absorption regions were modelled with a sum of components representing phycobilin and Chl a fluorescence emission bands. Our results show that the transition from state 1 to state 2 is characterized by a large decrease in the amplitude of the fluorescence emission from the photosystem 2 associated Chl a components, and a small increase in the amplitude of the emission from the photosystem 1 associated Chl a component for excitation of either Chl a or the PBS. The results are discussed with respect to three models for regulation of the distribution of excitation energy (light state transition) between photosystem 2 and photosystem 1 in cyanobacteria.