Time-resolved fluorescence study of excitation energy transfer in the cyanobacterium Anabaena PCC 7120

Parveen Akhtar,Avratanu Biswas,Tomas Zakar,Ivo H M Van Stokkum,Nia Petrova,Petar H Lambrev

doi:10.1007/s11120-020-00719-w

Abstract

Excitation energy transfer (EET) and trapping in Anabaena variabilis (PCC 7120) intact cells, isolated phycobilisomes (PBS) and photosystem I (PSI) complexes have been studied by picosecond time-resolved fluorescence spectroscopy at room temperature. Global analysis of the time-resolved fluorescence kinetics revealed two lifetimes of spectral equilibration in the isolated PBS, 30–35 ps and 110–130 ps, assigned primarily to energy transfer within the rods and between the rods and the allophycocyanin core, respectively. An additional intrinsic kinetic component with a lifetime of 500–700 ps was found, representing non-radiative decay or energy transfer in the core. Isolated tetrameric PSI complexes exhibited biexponential fluorescence decay kinetics with lifetimes of about 10 ps and 40 ps, representing equilibration between the bulk antenna chlorophylls with low-energy “red” states and trapping of the equilibrated excitations, respectively. The cascade of EET in the PBS and in PSI could be resolved in intact filaments as well. Virtually all energy absorbed by the PBS was transferred to the photosystems on a timescale of 180–190 ps.

Highlights

Light harvesting in cyanobacteria relies on phycobilisomes (PBS)—the large multimeric water-soluble assemblies of phycobiliproteins (PBPs) and linker proteins that efficiently absorb light in the spectral range between 550 and 650 nm (Harris et al 2018; Bar-Eyal et al 2018)
Absorbed energy is transferred to the photosystems with a very high quantum efficiency (Scott et al 2006; Glazer 1984)—PBS are generally considered as antenna for photosystem II (PSII) but they can supply excitation energy to photosystem I (PSI), either indirectly via “spillover” from PSII or directly (Mullineaux 1994; Liu et al 2013; Chukhutsina et al 2015)
As with other studies on cyanobacteria, we find that the great majority of Chls is connected with PSI and only a small fraction belongs to PSII

Summary

Introduction

Light harvesting in cyanobacteria relies on phycobilisomes (PBS)—the large multimeric water-soluble assemblies of phycobiliproteins (PBPs) and linker proteins that efficiently absorb light in the spectral range between 550 and 650 nm (Harris et al 2018; Bar-Eyal et al 2018). PBS are commonly described as hemidiscoidal structures consisting of PBP trimers (αβ) and hexamers (αβ) assembled together with linker proteins into a central core of allophycocyanin (APC) with radiating rods containing phycocyanin (PC) and in some species phycoerythrin (PE) or phycoerythrocyanin (PEC) (Arteni et al 2009; MacColl 2004). Switalski and Sauer 1984; Choubeh et al 2018), PBS substructures (Sandström et al 1988; Zhao et al 1998), intact PBS (Tian et al 2012; Nganou et al 2016; Zhang et al 1997; van Stokkum et al 2018) as well as in whole cells of various species (Mullineaux and Holzwarth 1991; Tian et al 2011; Acuña et al 2018a, b; Chukhutsina et al 2015), including Anabaena variabilis (Bittersmann et al 1988; Nultsch et al 1990). By time-resolved fluorescence, Acuña et al (2018b) reported a 20 ps EET time from the PBS terminal emitter to PSII, i.e. faster than the EET within the PBS

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