OBSERVATIONS ON THE FORMATION OF THE PHOTOSYNTHETIC APPARATUS 299 



maximum of about 2" o oxygen even though chlorophyll concentration per 

 unit volume of cells is highest under near anaerobic conditions (0-036% 

 Oo) (Fig. 2). The observed maximum in chlorophyll concentration per 

 volume of culture at about 2% is due to the rapid growth of the organism 

 at this oxygen tension where chlorophyll synthesis is only partly inhibited. 

 As the oxygen tension is raised beyond this level, there is no appreciable 

 change in the amount of growth but chlorophyll synthesis is increasingly 

 inhibited and comes to a standstill at about 8% oxygen. 



STRUCTURE OF DARK-GROWN CELLS AS REVEALED BY ELECTRON 



MICROSCOPY 



Electron micrographs of thin sections of cells grown in the dark at 

 various oxygen tensions reveal the following picture. Under highly aerobic 

 conditions the nearly complete absence of chlorophvll is accompanied by 

 a virtually complete lack of chromatophores in the sectioned cells (Figs. 15, 

 6). Cells derived from near anaerobic cultures gassed with nitrogen con- 

 taining 0-036/0 oxygen and 5",, carbon dioxide contained a high concen- 

 tration of bacteriochlorophyll (Fig. 2) and when sectioned revealed a 

 great abundance of chromatophores (Fig. 4). It was possible to isolate 

 photochemically active chromatophores from such cells (Table I) which 

 could carry out both light-induced formation of ATP* and the photo- 

 reduction of DPN in the presence of suitable cofactors. Observations on 

 cultures grown at intermediate oxygen tensions indicate a decrease in 

 chromatophore concentration with increasing oxvgen tension. It appears 

 that both chlorophyll concentration and the abundance of chromatophores 

 can be controlled w-ithin certain limits by controlling the oxygen tension 

 at which the organisms are grown in the dark. 



PHOTOCHEMICAL ACTIVITIES OF DARK-GROW'N CELLS AND OF CELL- 

 FREE PREPARATIONS DERIVED FROM THEM 



Intact cells, derived from initially chlorophyll-free cells, which had 

 developed chlorophyll in the dark at reduced oxygen tensions, show an 

 active CO., fixation (Table I). Cell-free preparations obtained from such 

 cells carry out active light-induced phosphorylation and the photo- 

 reduction of diphosphopyridine nucleotide [6, 8]. Cohen-Bazire and 

 Kunisawa [5] who have carried out similar work with R. riibriim also have 



* Abbreviations used: ADP, adenosine diphosphate; ATP, adenosine tri- 

 phosphate; Pi, inorganic orthophosphate ; DPN, DPNH, diphosphopyridine 

 nucleotide and its reduced forms ; TPN, triphosphopyridine nucleotide ; FMN, 

 FMNH2, flavin mononucleotide and its reduced form; PPNR, photosynthetic 

 pyridine nucleotide reductase. 



