Mechanism of the light state transition in photosynthesis. IV. Picosecond fluorescence spectroscopy of Anacystis nidulans and Porphyridium cruentum in state 1 and state 2 at 77 K

Abstract

The sequential energy-transfer pathway through the phycobilin pigments to chlorophyll a was investigated as a function of the state transition in the cyanobacterium Anacystis nidulans and the red alga Porphyridium cruentum. The fluorescence decay kinetics of the phycobilin pigments and chlorophyll a were determined for cells frozen at 77 K in state 1 and state 2 using a single-photon timing fluorescence spectroscopy apparatus with picosecond resolution. Time-resolved 77 K fluorescence emission spectra were also obtained for both species in state 1 and state 2. In both A. nidulans and P. cruentum the transition to state 1 was accompanied by a large increase in the apparent fluorescent lifetime of chlorophyll a associated with PS II (emission peak at 695 nm). There were smaller increases in the lifetime of the terminal phycobilin emitter (685 nm) in both species and no change in phycocyanin (645 nm) or allophycocyanin (660 nm). Time-resolved spectra showed sequential emission from phycocyanin, allophycocyanin, the terminal phycobilin emitter and chlorophyll a. Spectral red shifts were observed with time for all emission peaks with the exception of the terminal phycobilin emitter. In A. nidulans this peak showed a small blue shift with time. The results are interpreted as evidence for an effective uncoupling of PS II chlorophyll a from subsequent energy transfer to PS I chlorophyll a upon transition to state 1. Our recently proposed model for the mechanism of the state transition in phycobilisome-containing organisms is discussed in terms of a decrease in the energy transfer overlap between PS II chlorophyll a and PS I chlorophyll a in state 1.