Energy transfer in purple bacterial photosynthetic units from cells grown in various light intensities

Abstract

Three photosynthetic membranes, called intra-cytoplasmic membranes (ICMs), from wild-type and the ∆pucBA mutant of the purple phototrophic bacterium Rps. palustris were investigated using optical spectroscopy. The ICMs contain identical light-harvesting complex 1-reaction centers (LH1-RC) but have various spectral forms of light-harvesting complex 2 (LH2). Spectroscopic studies involving steady-state absorption, fluorescence, and femtosecond time-resolved absorption at room temperature and at 77 K focused on inter-protein excitation energy transfer. The studies investigated how energy transfer is affected by altered spectral features of the LH2 complexes as those develop under growth at different light conditions. The study shows that LH1 → LH2 excitation energy transfer is strongly affected if the LH2 complex alters its spectroscopic signature. The LH1 → LH2 excitation energy transfer rate modeled with the Förster mechanism and kinetic simulations of transient absorption of the ICMs demonstrated that the transfer rate will be 2-3 times larger for ICMs accumulating LH2 complexes with the classical B800-850 spectral signature (grown in high light) compared to the ICMs from the same strain grown in low light. For the ICMs from the ∆pucBA mutant, in which the B850 band of the LH2 complex is blue-shifted and almost degenerate with the B800 band, the LH1 → LH2 excitation energy transfer was not observed nor predicted by calculations.

Simulating Fluorescence-Detected Two-Dimensional Electronic Spectroscopy of Multichromophoric Systems

“…The low-frequency ring contains twice as many bacteriochlorophyll molecules as the high-frequency ring, and its absorption is red-shifted to 850 nm, resulting in the B850 absorption band. The energy transfer between these two bands is efficient despite weak coupling between chromophores in the two bands. − Variations of this system exist in different bacteria and also depend on the growth conditions. − In this study, we will consider a bacterial system with 27 bacteriochlorophylls, modelled after the wild-type, high-light LH2 from Rhodopseudomonas palustris …”

Differential stability of bacterial photosynthetic apparatus of Rhodobacter alkalitolerans strain JA916T under alkaline and light environment

“…A comparative study of the absorbance spectrum of ICMs shows that in hpH (pH 8.60 ± 0.05), the absorbance intensity is higher in all light conditions compared with npH (pH 6.80 ± 0.05) ( Figures 2A – C ). The absorbance spectrum is also an indication of photosystem integrity and stability of the photosystem in relation to given stress conditions ( Niedzwiedzki et al, 2018 ). When increasing light intensity, the ICMs in both normal and high pH complexes are reduced, but in npH, the complexes are more sensitive than hpH.…”

“…It starts with the absorption of sunlight and subsequently converts the light energy to different forms of hydrocarbon energy. In purple bacteria, photosynthesis is performed by a properly oriented network of densely packed pigment-protein complexes known as light-harvesting comlex2 (LH2) and light harvesting complex1 (LH1), capturing photons and transferring the energy to the reaction center (RC) ( van Grondelle et al, 1994 ; Niedzwiedzki et al, 2018 ). Electron passes from RC to the cytochrome (Cyt) bc1 via quinone/quinol exchange at the Qb site of the RC ( Okamura et al, 2000 ; Francia et al, 2004 ).…”

“…Previously, many species from the genus Rhodobacter have been studied for its basic mechanism of photosynthesis and its photosynthetic membrane protein complexes, i.e., RC-LH1 monomer, dimer, and LH2 housed in chromatophore/intracytoplasmic membranes ( Ng et al, 2011 ; Crouch and Jones, 2012 ). They have also been studied at biophysical level to investigate the electron transfer efficiency using femtosecond transient absorption spectroscopy and metabolite study ( Yen et al, 2010 ; Niedzwiedzki et al, 2018 ). Many species of purple non-sulfur photosynthetic bacteria such as Rhodobacter sphaeroides 2.4.1 , Rhodospirillum rubrum ( Hustede et al, 1993 ; Ryu et al, 2014 ), and Rhodobacter capsulatus KU002 ( Merugu et al, 2012 ) have been studied for the production of hydrogen gas and a bioplastic known as polyhydorxybutyrate (PHB).…”

The Growth‐promoting Mechanism of Unusual Spectroscopic Form of LH2 (LH4) from Rhodopseudomonas palustris CGA009 in Low Light

“…Rps. palustris and affiliated strains) (13,17). To better understand the role of each pucBA gene pair in determining the LH2 phenotype in response to different light conditions, Southal et al (16) recently created a bacterial mutant (DpucBA abce ).…”