Spectrally decomposed dark-to-light transitions in Synechocystis sp. PCC 6803

Abstract

Photosynthetic activity and respiration share the thylakoid membrane in cyanobacteria. We present a series of spectrally resolved fluorescence experiments where whole cells of the cyanobacterium Synechocystis sp. PCC 6803 and mutants thereof underwent a dark-to-light transition after different dark-adaptation (DA) periods. Two mutants were used: (i) a PSI-lacking mutant (ΔPSI) and (ii) M55, a mutant without NAD(P)H dehydrogenase type-1 (NDH-1). For comparison, measurements of the wild-type were also carried out. We recorded spectrally resolved fluorescence traces over several minutes with 100 ms time resolution. The excitation light was at 590 nm so as to specifically excite the phycobilisomes. In ΔPSI, DA time has no influence, and in dichlorophenyl-dimethylurea (DCMU)-treated samples we identify three main fluorescent components: PB-PSII complexes with closed (saturated) RCs, a quenched or open PB-PSII complex, and a PB-PSII 'not fully closed.' For the PSI-containing organisms without DCMU, we conclude that mainly three species contribute to the signal: a PB-PSII-PSI megacomplex with closed PSII RCs and (i) slow PB → PSI energy transfer, or (ii) fast PB → PSI energy transfer and (iii) complexes with open (photochemically quenched) PSII RCs. Furthermore, their time profiles reveal an adaptive response that we identify as a state transition. Our results suggest that deceleration of the PB → PSI energy transfer rate is the molecular mechanism underlying a state 2 to state 1 transition.

Modelling excitation energy transfer and trapping in the filamentous cyanobacterium Anabaena variabilis PCC 7120

“…The importance of the APC680 to PSI EET is consistent with Dong and Zhao (2008) who established that state transitions are present in Anabaena. As a mechanism for the state 2 to state 1 transition (Acuña et al 2018) proposed a deceleration of the APC680 to PSI EET rate.…”