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

Avratanu Biswas,Xinpeng Huang,Petar H Lambrev,Ivo H M Van Stokkum

doi:10.1007/s11120-020-00723-0

Avratanu Biswas, Xinpeng Huang + Show 2 more

Open Access

https://doi.org/10.1007/s11120-020-00723-0

Copy DOI

Abstract

The phycobilisome (PBS) serves as the major light-harvesting system, funnelling excitation energy to both photosystems (PS) in cyanobacteria and red algae. The picosecond kinetics involving the excitation energy transfer has been studied within the isolated systems and intact filaments of the cyanobacterium Anabaena variabilis PCC 7120. A target model is proposed which resolves the dynamics of the different chromophore groups. The energy transfer rate of 8.5 ± 1.0/ns from the rod to the core is the rate-limiting step, both in vivo and in vitro. The PBS-PSI-PSII supercomplex reveals efficient excitation energy migration from the low-energy allophycocyanin, which is the terminal emitter, in the PBS core to the chlorophyll a in the photosystems. The terminal emitter of the phycobilisome transfers energy to both PSI and PSII with a rate of 50 ± 10/ns, equally distributing the solar energy to both photosystems. Finally, the excitation energy is trapped by charge separation in the photosystems with trapping rates estimated to be 56 ± 6/ns in PSI and 14 ± 2/ns in PSII.

Highlights

Photosynthesis is initiated by a network of light-absorbing chromophores capturing the sunlight and funnelling the excitation energy from one light-harvesting complex to another and getting trapped at a photochemical reaction centre (RC), where charge separation takes place
A bottom-up approach has been adopted where we acquire preliminary biophysical properties of the individual systems using target analysis of the individual systems and with the help of simultaneous target analysis, we develop a functional compartmental model of the excitation energy transfer and trapping in Anabaena cells
The absorption spectrum (Fig. 2b) reveals the Chl a Qy absorption maximum at 679 nm, which is typical for photosystem I (PSI) (Andrizhiyevskaya et al 2002)

Summary

Introduction

Photosynthesis is initiated by a network of light-absorbing chromophores capturing the sunlight and funnelling the excitation energy from one light-harvesting complex to another and getting trapped at a photochemical reaction centre (RC), where charge separation takes place. The PBS of Anabaena variabilis PCC 7120 (hereafter called Anabaena) belongs to the same hemidiscoidal family but contains a penta-cylindrical core (Ducret et al 1998; Glauser et al 1992). A cascade of excitation energy transfer (EET) takes place from the rod to the core. An elaborate functional compartmental model has been developed to describe the microscopic rates within the rods, from rod to core and within the core (van Stokkum et al 2018). Each of these compartments contains many pigments. The challenge would be to connect these effective rates that describe EET between compartments to specific routes of EET in a PBS structure, like the recent cryo-EM PBS structure of the red algae, Griffithsia pacifica (Zhang et al 2017)

Objectives

Methods

Results