title: Efficiency of light harvesting in a photosynthetic bacterium adapted to different levels of light.
abstract: In this study, we use the photosynthetic purple bacterium Rhodobacter sphaeroides to find out how the acclimation of photosynthetic apparatus to growth conditions influences the rates of energy migration toward the reaction center traps and the efficiency of charge separation at the reaction centers. To answer these questions we measured the spectral and picosecond kinetic fluorescence responses as a function of excitation intensity in membranes prepared from cells grown under different illumination conditions. A kinetic model analysis yielded the microscopic rate constants that characterize the energy transfer and trapping inside the photosynthetic unit as well as the dependence of exciton trapping efficiency on the ratio of the peripheral LH2 and core LH1 antenna complexes, and on the wavelength of the excitation light. A high quantum efficiency of trapping over 80% was observed in most cases, which decreased toward shorter excitation wavelengths within the near infrared absorption band. At a fixed excitation wavelength the efficiency declines with the LH2/LH1 ratio. From the perspective of the ecological habitat of the bacteria the higher population of peripheral antenna facilitates growth under dim light even though the energy trapping is slower in low light adapted membranes. The similar values for the trapping efficiencies in all samples imply a robust photosynthetic apparatus that functions effectively at a variety of light intensities.
authors.affiliation: Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia.
authors.forename: Kõu
authors.initials: K
authors.lastname: Timpmann
authors.affiliation: Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia.
authors.forename: Manoop
authors.initials: M
authors.lastname: Chenchiliyan
authors.affiliation: Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia.
authors.forename: Erko
authors.initials: E
authors.lastname: Jalviste
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK.
authors.forename: John A
authors.initials: JA
authors.lastname: Timney
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK.
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
authors.affiliation: Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia; Institute of Molecular and Cell Biology, University of Tartu, Riia 23, Tartu 51010, Estonia. Electronic address: arvi.freiberg@ut.ee.
authors.forename: Arvi
authors.initials: A
authors.lastname: Freiberg
mesh_terms: D000222:Adaptation, Physiological; D005453:Fluorescence; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000502:physiology*; D010788:Photosynthesis; D012242:Rhodobacter sphaeroides / Q000378:metabolism / Q000502:physiology*
chemical_list: D045342:Light-Harvesting Protein Complexes
keywords: Exciton; Light harvesting; Optical spectroscopy; Photosynthesis; Photosynthetic unit; Picosecond excitation energy transfer