title: Comparison of the physical characteristics of chlorosomes from three different phyla of green phototrophic bacteria.
abstract: Chlorosomes, the major antenna complexes in green sulphur bacteria, filamentous anoxygenic phototrophs, and phototrophic acidobacteria, are attached to the cytoplasmic side of the inner cell membrane and contain thousands of bacteriochlorophyll (BChl) molecules that harvest light and channel the energy to membrane-bound reaction centres. Chlorosomes from phototrophs representing three different phyla, Chloroflexus (Cfx.) aurantiacus, Chlorobaculum (Cba.) tepidum and the newly discovered "Candidatus (Ca.) Chloracidobacterium (Cab.) thermophilum" were analysed using PeakForce Tapping atomic force microscopy (PFT-AFM). Gentle PFT-AFM imaging in buffered solutions that maintained the chlorosomes in a near-native state revealed ellipsoids of variable size, with surface bumps and undulations that differ between individual chlorosomes. Cba. tepidum chlorosomes were the largest (133×57×36nm; 141,000nm(3) volume), compared with chlorosomes from Cfx. aurantiacus (120×44×30nm; 84,000nm(3)) and Ca. Cab. thermophilum (99×40×31nm; 65,000nm(3)). Reflecting the contributions of thousands of pigment-pigment stacking interactions to the stability of these supramolecular assemblies, analysis by nanomechanical mapping shows that chlorosomes are highly stable and that their integrity is disrupted only by very strong forces of 1000-2000pN. AFM topographs of Ca. Cab. thermophilum chlorosomes that had retained their attachment to the cytoplasmic membrane showed that this membrane dynamically changes shape and is composed of protrusions of up to 30nm wide and 6nm above the mica support, possibly representing different protein domains. Spectral imaging revealed significant heterogeneity in the fluorescence emission of individual chlorosomes, likely reflecting the variations in BChl c homolog composition and internal arrangements of the stacked BChls within each chlorosome.
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK.
authors.forename: Peter G
authors.initials: PG
authors.lastname: Adams
authors.forename: Ashley J
authors.initials: AJ
authors.lastname: Cadby
authors.forename: Benjamin
authors.initials: B
authors.lastname: Robinson
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tsukatani
authors.forename: Marcus
authors.initials: M
authors.lastname: Tank
authors.forename: Jianzhong
authors.initials: J
authors.lastname: Wen
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
authors.forename: Donald A
authors.initials: DA
authors.lastname: Bryant
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry*; D021961:Cell Membrane Structures / Q000737:chemistry* / Q000648:ultrastructure; D041883:Chlorobium / Q000145:classification* / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism*; D018625:Microscopy, Atomic Force; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence
chemical_list: D001429:Bacteriochlorophylls
keywords: 2-D; 3-D; AFM; Atomic force microscopy; BChl(s); Bacterial photosynthesis; Ca; Cab.; Cba.; Cfx.; Chloracidobacterium; Chloroacidobacterium; Chlorobaculum; Chloroflexus; Chlorosome; EM; FAP; FWHM; Fluorescence microscopy; GSB; LH; Light harvesting; PFT; PeakForce Tapping (AFM); QNM; Quantitative Nanomechanical Mapping; RC; Reaction Centre; TEM; TM; Tapping Mode (AFM); atomic force microscopy; bacteriochlorophyll(s); electron microscopy; filamentous anoxygenic phototrophs; full width at half maximum; green sulphur bacteria; light-harvesting; three-dimensional; transmission electron microscopy; two-dimensional