METABOLIC ASPECTS 131 



photosynthetic bacteria (e.g., Chromatium) can grow under both sets 

 of circumstances (see Table 2). 



The term "autoheterotrophic" (6) is suggested as a convenience 

 to designate the growth pattern when an organic compound constitutes 

 the carbon source, in cognizance of an autotrophic mechanism for the 

 energy supply and a heterotrophic carbon metabolism. Fixation of 

 CO2, liberated during the metabolic conversions of added organic com- 

 pounds, also occurs during autoheterotrophic metabolism, but with 

 most substrates the extent of this process is unknown and undoubtedly 

 varies considerably depending on the nutritional conditions. In this 

 connection, it is noteworthy that certain anaerobic heterotrophs re- 

 quire surprisingly large quantities of CO2 for optimal growth (9,10). 

 An example is given by the myxobacterium Cytophaga succinicans , 

 whose CO2 requirement has been shown (11) to be related to the energy 

 metabolism of the organism. In this instance, CO2 is essential in sub- 



TABLE 2 



Sources of carbon, reducing poiver, and energy for anaerobic growth 

 of photosynthetic bacteria 



Mode of growth: 

 "autotrophic" "autoheterotrophic" 



Carbon: CO^, organic compounds 



Reducing power: H , S" etc. organic compounds 



Energy: light light 



strate amounts because it is a precursor of oxaloacetate (through con- 

 densation with phosphoenolpyruvate), which in turn functions as a major 

 oxidant for NADH2 generated during the fermentative breakdown of 

 glucose. Accordingly, the CO2 is eventually converted to the carboxyl 

 group of succinate and in essence has served as an electron acceptor. 

 It seems quite possible (12,13) that CO2 may be used, to some ex- 

 tent, in a similar way during autoheterotrophic metabolism of photo- 

 synthetic bacteria (i.e., simply as an "accessory electron acceptor"). 

 In fact, some thirty years ago F. M. Muller (14) concluded that this 

 probably was the case during growth of purple sulfur bacteria on or- 

 ganic substrates such as succinate, acetate, and butyrate. Muller also 

 appreciated the autoheterotrophic character of the purple bacteria, 

 which is clearly evident from the following statements made in his 

 well-known paper published in 1933: 



