Molds as Mefabol'ic Models 99 



entities existing in nature. For example, it is a well docu- 

 mented fact that numerous species of microorganisms are 

 unable to utilize carbohydrate as their sole source of carbon 

 and energy. On the other hand, those organisms which are 

 proved capable of utilizing carbohydrate may not, and 

 indeed probably do not, have the opportunity to exercise 

 that capacity under the conditions which may exist in the 

 soil. Irrespective of the amount or the origin of soluble 

 carbohydrate, the vast majority of the microbial popula- 

 tion could well lose out in the competition for it to the 

 fastest growing minority populations in the soil. For ex- 

 ample, the bacteria would, in this respect, be formidable 

 antagonists for most fungi, except in acid soils where the 

 bacteria are differentially suppressed. 



If we examine further the capabilities of the soil popula- 

 tion, we find innumerable soil forms eminently well 

 adapted to utilize various one-, two-, and three-carbon 

 compounds as the sole source of carbon and energy. This 

 point strikes at the heart of a thesis for which ample sup- 

 port might be adduced: the synthesis of protoplasm from 

 one-, two-, and three-carbon split products derived from 

 the dissimilation of larger molecules by fast growing 

 minority populations may well account for a much larger 

 proportion of the gross biosynthetic activity in the soil 

 than hitherto appreciated. And since fungi, as a rule, ap- 

 pear to be metabolically sluggish compared to most bac- 

 teria, this ability to survive on metabolic "scraps" may be 

 widespread among, and perhaps characteristic of, many 

 molds and other microbial growths in nature. In any case, 

 I strongly suspect that in microorganisms, pathways to total 

 biosynthesis from one- and two-carbon compounds may yet 

 emerge and prove to be as significant as sugar breakdown 

 in the over-all foundations of biosynthesis in nature. An 

 analogy to photosynthesis will be perceived, especially in 

 respect to intracellular self-regeneration of appropriate 

 acceptors via cyclic mechanisms. 



