192 MACROMOLECULAR COMPLEXES 



thirds of the celkilar sohds are chromatophores, it is clear that, 

 although chromatophores are present at the surface of the cyto- 

 plasm, the structure with which they are associated is the cell itself. 

 In our opinion, the data obtained in this systematic study are 

 consistent with the view that the chromatophore is the photochemi- 

 cal organelle of Chromatium (Bergeron, 1958). Since the electron 

 micrographs of thin sections of several other photosynthetic bacteria, 

 Rhodospirillum nibriim, Rhodopseudomonas spheroides, and Chlo- 

 robium limicola, also show vesicular inclusions in abundance ( Vatter 

 and Wolfe, 1958), it is possible that this view has some general 

 validity. Since the vesicles in the non-sulfur photosynthetic bacteria 

 are much larger than the Chromatium chromatophore, they may be 

 more complicated. For example, the Rhodospirillum ruhrum chro- 

 matophore, which has been studied extensively, though not as sys- 

 tematically, appears to be more complex (see review by Frenkel, 

 1959). It is also possible, considering the variety of these micro- 

 organisms, that lamellated pigmented structures exist. The electron 

 micrographs of Rhodomicrohium vannielii (Vatter et ah, 1959) sug- 

 gest this possibility. It remains to be established, however, that 

 these lamellae are pigmented. It is also conceivable that in some 

 photosynthetic bacteria the pigments will not be integrated into 

 vesicular structures which can perform the complete sequence of 

 events involved in photophosphorylation.^ 



Interpretation of the Electron-Optical Image 



Both the physicochemical and the electron microscope data indi- 

 cate that the isolated chromatophore is a sphere. The electron mi- 

 croscope observations on the thin sections of the organism also reveal 

 discrete spherical structures; anastomoses or tubular profiles have 

 not been observed. The annular form of the image of the sectioned 

 chromatophore shows that the central region does not scatter 50- 

 kilovolt electrons as efi^ectively as does the cortical region. Since the 

 chromatophores are frequently seen in direct contact, we can infer 

 that, within the limits of resolution, the outer boundarv of the image 



^ The suggestion that photosynthesis can proceed at an even simpler level of 

 structural organization than the chromatophore has been supported by study of the 

 green sulfur bacterium, Chlorohium thiosulfatophilum. In this instance, the photo- 

 synthetic pigments are recovered in a macromolecule with a molecular weight of 

 about 1 million (J. A. Bergeron and R. C. Fuller in "Biological Structure and Func- 

 tion," lUB/IUBS Symposium, 1960, Academic Press, Inc., in press). 



