BACTERIAL PHOTOSYNTHESIS 181 



The concept of the chromatophore as a photochemical organelle 

 is beginning to assume definite form. Until recently it was believed 

 that the pigments of photosynthetic bacteria are freely dispersed 

 throughout the cell in the form of protein complexes. The chromato- 

 phore, in name at least, came into being in 1952, when Pardee, 

 Schachman, and Stanier found that the photosynthetic pigments in 

 extracts of R. rubrum sedimented much more rapidly than antici- 

 pated. The electron micrographs of shadowed specimens of these 

 chromatophores showed thin disks about 1100 A in diameter. It was 

 assumed that the disks were derived from spheres with an average 

 diameter of about 600 A. This value agreed roughly ( Schachman 

 et al., 1952) with the diameter (400 A) calculated by Stokes' rela- 

 tion from the sedimentation constant (200 S) of a purified prepara- 

 tion with a partial specific volume of 0.73 (assumed). The esti- 

 mated molecular weight of these R. rubrum chromatophores was 

 thirty million. At the same time, Thomas (1952) also published 

 electron micrographs of shadowed specimens prepared from crude 

 extracts of several photosynthetic bacteria. These preparations also 

 contained thin disks. Reasoning by analogy with similar prepara- 

 tions from chloroplasts, Thomas described these objects as "lamellae" 

 and as "grana," or stacks of lamellae. Despite the obvious differences 

 in interpretation, these reports had in common the suggestion that 

 the pigments of photosynthetic bacteria are localized in specific 

 structures. Subsequently, when Frenkel (1954) described photo- 

 phosphorylation by subcellular preparations of R. rubrum, it became 

 logical to assume that the pigments of photosynthetic bacteria 

 are integrated structurally and functionally at the submicroscopic 

 level. 



With this point of view, we began a systematic study of the struc- 

 ture and function of the photochemical apparatus of the purple 

 sulfur bacterium, Chromatium strain D. Several properties of this 

 organism made it especially attractive. Many photosynthetic organ- 

 isms have the ability to form ATP bv non-photosynthetic pathways 

 and can grow aerobically in the dark. Chro7natium is an obligate 

 phototroph and an obligate anaerobe (van Niel, 1931, 1935, 1936); 

 thus, in this organism ATP formation seems to be completely light- 

 dependent (see general discussion by van Niel, 1956). The syn- 

 thetic' abilities are well developed, and Chromatium, given light, 

 can live and grow anaerobically in simple salt solutions containing 

 carbonate and hydrogen sulfide. 



