132 METABOLISM AND PHYSIOLOGY 



In the meantime it has to be kept in mind that the experiments referred 

 to above yielded another fact of primary importance, viz. that also in this 

 heterotrophic metabolism the cooperation of radiant energy is an essential 

 factor. We must conclude from this fact that photochemical processes 

 play a part in the conversion of the organic substrates, which is a most 

 remarkable phenomenon. Besides the purple sulphur bacteria and the 

 Athiorhodaceae , no heterotrophic organisms are known in whose metabo- 

 lism radiant energy plays such an important part. 



The potential significance of heterotrophic carbon metabolism in 

 bacterial photosynthesis was also recognized by others (e.g., see 15, 

 16,17), leading to the suggestion (16,18) that CO2 fixation may be by- 

 passed or suppressed during photometabolism of certain organic sub- 

 strates. This point of view, however, was not generally considered or 

 discussed for some time, apparently because of preoccupation with 

 the concept that CO2 reduction was a central feature of all bacterial 

 photosyntheses. Combined autotrophy and heterotrophy in the sense 

 used here is not confined to photosynthetic bacteria and may actually 

 be more widespread in nature than is commonly supposed. An inter- 

 esting example is found in the organism Desulfovibrio desidfuricans 

 (Hildenborough strain). Mechalas and Rittenberg (19) have shown quite 

 conclusively that this bacterium can use the "autotrophic" oxidation of 

 H2 with sulfate as the energy source for "heterotrophic" growth on 

 assimilable organic compounds. They suggest that other "heterotrophs" 

 which contain hydrogenase, suchsis Escherichia coli,Azotobacter, etc., 

 may also be capable of using "autotrophic" oxidation of H2 (e.g., with 

 O2) as an energy supply for "heterotrophic synthesis." 



There has been considerable discussion in the literature on the 

 fundamental differences between autotrophs and heterotrophs, and it is 

 becoming increasingly difficult to make satisfactory definitions. 

 Since the discovery of CO2 fixation in heterotrophs, many other 

 distinctions which were once thought to be decisive have diminished in 

 significance. In addition, the list of facultative autotrophs and organ- 

 isms in the "grey zone" between the extremes constantly increases 

 in length. This is really another way of saying that we are gradually 

 seeing an enlarged spectrum of similarities and this, in itself, poses 

 problems for comparative biochemical interpretations. It is interest- 

 ing that this development was clearly anticipated by B. C. J. G. Knight 

 in his early classic monograph (20) on bacterial nutrition. Knight not 

 only emphasized the arbitrariness of the classification of organisms 

 into the two categories of autotrophic and heterotrophic, but also held 

 the view that this separation had "serious disadvantages from the 

 point of view of the use of comparative physiology as a guide to further 

 investigation." 



It was noted earlier that certain photosynthetic bacteria (e.g., 

 ChroDiatiiou and R. nihriim) can grow using either CO2 or a single 

 organic compound as the sole, or primary, carbon source. There is 



