130 METABOLISM AND PHYSIOLOGY 



exploring metabolic phenomena. This is illustrated by the fact that we 

 routinely mix subcellular components isolated from very different 

 organisms in order to reconstruct certain complex metabolic proces- 

 ses; at least, to serve as models. At the same time, a number of in- 

 vestigators working in different fields of biology have expressed dis- 

 quieting attitudes on the general aims of comparative biochemistry 

 and have also criticized current tendencies in its usage. This has 

 resulted in some debate as to whether comparative biochemistry 

 should now be concerned primarily with the study of common chemical 

 principles shown by different forms of life, or with the origin and 

 nature of biochemical variability from patterns common to many 

 organisms. It can be argued that this difference in viewpoint represents 

 a trivial philosophic confusion concerning two sides of the same coin, 

 but I am inclined to believe that the debate is ultimately inspired by 

 important questions which have not as yet received the scrutiny they 

 deserve. 



One can legitimately ask whether undue emphasis on a particular 

 apparent correlation may not have the effect of obscuring (or delaying 

 the recognition of) a more profound relationship or a significant in- 

 stance of biochemical "disunity." This possibility has led Ernest 

 Bueding (3) to caution that excessive attention to biochemical unity, 

 at the expense of adequate consideration of biochemical diversity, 

 may lead to a distorted picture. In a very perceptive recent essay (4) 

 relating to this topic, Seymour Cohen concludes that "the notion of the 

 'unity of biochemistry' has been advanced in an overly simplified form 

 and reflects a primitive stage in the development of the discipline," 

 Furthermore, he cites the area of photosynthesis as providing an ex- 

 ample "in which the predilection for simplicity has impeded the de- 

 velopment of understanding." 



Research advances during the past decade or so have sharpened 

 the focus on the comparative biochemistry of photosynthesis consider- 

 ably, but it is my opinion that a really satisfactory conception has 

 not yet been achieved. The remainder of this paper is concerned with 

 general metabolic properties of photosynthetic bacteria and with the 

 discussion of several questions which are of particular significance 

 for reevaluation of the comparative problem. 



Carbon metabolism and its ramifications 



From the standpoint of carbon nutrition, photosynthetic bacteria can 

 grow anaerobically under two markedly different sets of conditions 

 (2,5). On the one hand, CO2 can serve as the sole or primary source 

 of carbon, provided an inorganic hydrogen donor is present. On the 

 other hand, many types can grow luxuriantly in a completely synthetic 

 medium containing a single organic compound, such as malate, in 

 place of CO2 and the inorganic "accessory" hydrogen donor. Certain 



