134 METABOLISM AND PHYSIOLOGY 



predominance of one particular sequence to a balanced mixture of al- 

 ternative pathways. These appear to include, in whole or in part, most 

 of the major pathways now recognized, such as the reductive pentose 

 cycle, the citric acid cycle (13,24), and the glyoxylate cycle. It is 

 relevant to note that the reductive pentose cycle is only one of several 

 mechanisms by which photosynthetic bacteria can reduce substantial 

 quantities of CO2 to cell material. For example, there is some evi- 

 dence (25,26) that, in Rho do spirillum , CO2 may condense with a C2 

 fragment to form a C3 compound such as pyruvate, and it is possible 

 that under certain conditions this type of reaction is of importance in 

 net synthesis (see also ref. 27), Another type of CO2 assimilation is 

 encountered in the metabolism of Rhodopseudomonas gelatinosa. Ace- 

 tone serves as a carbon source for photosynthetic growth of this or- 

 ganism, but only if CO2 is also provided, Siegel's studies (28-32) on 

 this system suggested the following sequence of carbon conversions: 



CO + acetone ^ acetoacetate ^2 acetate ^ cell materials 



The initial step is an endergonic carboxylation and Siegel's investiga- 

 tions were particularly instructive in that they clearly indicated the 

 required energy can be supplied alternatively by: light-induced phos- 

 phorylation, dark oxidative phosphorylation, or substrate-level phos- 

 phorylation coupled with the dark anaerobic fermentation of added 

 acetoacetate. 



An important question that arises in connection with enzymatic al- 

 terations caused by variation of the carbon source is whether such 

 changes will be primarily limited to enzymes specifically concerned 

 with carbon transformations. It seems very unlikely that this would be 

 the case, since the carbon conversions which occur during growth are 

 obviously intermeshed or connected in some way with a multitude of 

 other types of reactions. These interconnections provide a potential 

 basis for an amplification of alterations, which may eventually attain 

 relatively major proportions. 



In growing cells, particularly close relationships exist between 

 carbon and nitrogen metabolism and, in the purple bacteria, the nature 

 of the nitrogen growth source exerts a profound influence on the ulti- 

 mate fate of organic compounds supplied in the environment. This has 

 been studied particularly in R. nihrnm, but the available data suggest 

 that many other photosynthetic bacteria behave similarly. Let us com- 

 pare gross metabolic events in cultures oiR. rubrum growing anaero- 

 bically in the light on malate, with either an ammonium salt or an 

 amino acid such as glutamate as the nitrogen source. With the am- 

 monium salt, the bacterium grows rapidly and, considering that it is 

 an anaerobe, the cell yield is remarkably high (33): aside from CO2, 

 metabolic byproducts are not found in the medium in appreciable 



