BACTERIAL PHOTOSYNTHESIS 189 



geron et al., 1957; Bergeron, 1958). The most profitable observa- 

 tions were made with thin sections of methacrylate-embedded pel- 

 lets of preparations which had been fixed for 1 hour at 25° C in 1 

 per cent osmium tetroxide solutions. The fixatives were buffered at 

 pH 7.4 with Veronal-HCl and matched as closely as possible in ion 

 balance and osmolarity with the culture or suspension medium. All 

 substances which reacted with the osmium tetroxide were excluded. 



In such sections (Fig. 6), the bacteria have smooth contours and 

 are bounded by two distinct membranes, the cell wall and the cyto- 

 plasmic membrane. The obvious and predominant intracellular 

 structures are the chromatophores. This abundance, though puz- 

 zling, agrees with the indirect measurements; it was calculated, by 

 comparing the bacteriochlorophyll content of lyophilized cells and 

 of lyophilized chromatophores, that the chromatophores represent 

 about two-thirds of the total cellular solids. Sometimes the arrange- 

 ment of chromatophores is so regular that it suggests a crystalline 

 pattern; consequently, the details of the chromatophore are ob- 

 served more easily in thin sections of the chromatophore fraction 

 (Fig. 7). 



The chromatophores occur as individuals, clumps, and occasion- 

 ally as chains. These configurations can be regarded as secondary 

 associations produced during fixation since the fixatives cause the 

 progressive flocculation and precipitation of dilute suspensions of 

 either the cells or the chromatophores. There is no evidence that 

 the chromatophore is a fragment of a more highly organized, larger 

 organelle. 



The sections show that even the finest preparations are not per- 

 fectly clean. They contain a small number of larger vesicular struc- 

 tures which resemble the chromatophore; in addition, small particles 

 ( '— 100 A ) of high intrinsic electron density are present in variable 

 numbers. It is interesting to note that these impurities were not 

 noticed in the analytical ultracentrifuge. This illustrates the sensi- 

 tivity achieved by using thin sections of sedimented fractions as an 

 assay of homogeneity. 



In studying the crude cell fractions we were surprised to find 

 large cell fragments in which the contents appeared to be quite 

 undisturbed. We assume that the cytoplasm has "gelled" as a re- 

 sponse to stimuli received during the sonic oscillation at 10 kilo- 

 cycles in the cold (0-6° C), or because of the formation of nucleic 

 acid gels during rupture. One consequence of this phenomenon is 



