title: Energy transfer in light-adapted photosynthetic membranes: from active to saturated photosynthesis.
abstract: In bacterial photosynthesis light-harvesting complexes, LH2 and LH1 absorb sunlight energy and deliver it to reaction centers (RCs) with extraordinarily high efficiency. Submolecular resolution images have revealed that both the LH2:LH1 ratio, and the architecture of the photosynthetic membrane itself, adapt to light intensity. We investigate the functional implications of structural adaptations in the energy transfer performance in natural in vivo low- and high-light-adapted membrane architectures of Rhodospirillum photometricum. A model is presented to describe excitation migration across the full range of light intensities that cover states from active photosynthesis, where all RCs are available for charge separation, to saturated photosynthesis where all RCs are unavailable. Our study outlines three key findings. First, there is a critical light-energy density, below which the low-light adapted membrane is more efficient at absorbing photons and generating a charge separation at RCs, than the high-light-adapted membrane. Second, connectivity of core complexes is similar in both membranes, suggesting that, despite different growth conditions, a preferred transfer pathway is through core-core contacts. Third, there may be minimal subareas on the membrane which, containing the same LH2:LH1 ratio, behave as minimal functional units as far as excitation transfer efficiency is concerned.
authors.affiliation: Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, United Kingdom.
authors.forename: Francesca
authors.initials: F
authors.lastname: Fassioli
authors.forename: Alexandra
authors.initials: A
authors.lastname: Olaya-Castro
authors.forename: Simon
authors.initials: S
authors.lastname: Scheuring
authors.forename: James N
authors.initials: JN
authors.lastname: Sturgis
authors.forename: Neil F
authors.initials: NF
authors.lastname: Johnson
mesh_terms: D000465:Algorithms; D001426:Bacterial Proteins / Q000737:chemistry; D001703:Biophysics / Q000379:methods; D002462:Cell Membrane / Q000378:metabolism; D004735:Energy Transfer; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000502:physiology*; D018625:Microscopy, Atomic Force / Q000379:methods; D008954:Models, Biological; D015233:Models, Statistical; D010777:Photochemistry / Q000379:methods; D010788:Photosynthesis; D011487:Protein Conformation; D012246:Rhodospirillum / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes