STRUCTURE OF PHOTOSYNTHETIC BACTERIA 91 



easily visible by phase contrast microscopy, are liberated from lysed 

 spheroplasts under conditions where the release of pigment is neglig- 

 ible. Clearly, therefore, the lysed spheroplasts could not physically 

 retain individual structural elements of the dimensions of chromato- 

 phores. 



The gaps and apparent contradictions in our present knowledge 

 about the internal organization of the cell in photo synthetic bacteria 

 have led us to undertake an extensive study of fine structure in this 

 group. The present communication constitutes a progress report. 



EXPERIMENTAL PROCEDURES 



These have been described in a recent paper (12). Here, it is suf- 

 ficient to say that we have used, throughout, the preparative technique 

 for electron microscopy of bacteria developed by Ryter and Kellen- 

 berger (13), with two minor modifications. The period of main fixation 

 has been reduced in recent work from 16 to 2 hours, since this appears 

 to give equally good, if not superior, preparations. Secondly, most of 

 the sections shown have been post-stained with lead hydroxide (14), 

 a treatment which greatly sharpens the definition of wall and membrane 

 structure. Negative staining with phosphotungstic acid was performed 

 as described by Huxley and Zubay (15). All electron micrographs were 

 taken with a Siemens Elmiskop I, operating at 80 KV. 



THE FINE STRUCTURE OF RHODOSPIRILLUM RUBRUM 

 AND RHODOPSEUDOMONAS SPHEROIDES 



Physiological studies in our laboratory have shown that environ- 

 mental conditions profoundly affect the chemical composition of the 

 cell in nonsulfur purple bacteria (16,17). In particular, the cellular 

 content of photosynthetic pigments can vary over a wide range, even 

 under conditions of strictly photosynthetic growth (anaerobiosis), in 

 response to such factors as light intensity and temperature. In faculta- 

 tively aerobic strains, pigment synthesis can be completely suppressed 

 by growth in the presence of air; if oxygen access becomes limited, 

 photosynthetic pigment synthesis resumes, even in the absence of light 

 (18). The magnitude of these environmental effects on the pigment 

 content of R. nibnan and Rhodopseudomonas spheroides is shown by 

 the representative data assembled in Table 1, 



Accordingly, our first goal in the cytological study of these bacteria 

 was to compare the fine structure of cells with different pigment con- 

 tents. Typical results are shown in Figs, 1 to 7, It can be seen that the 



