FIRST HETEROTROPHS AND ANAEROBES 417 



of this sort must have occurred in e\en the very earhest 

 hving things, it must have arisen concurrently with Hfe. It 

 is, therefore, to be found in all organisms without exception, 

 in particular in such typical heterotrophs as the bacteria 

 responsible for butyric acid fermentation, in which the actual 

 formation of butyric acid results from the reductive con- 

 densation of two acetyl residues. 



What has been said accounts for the extremely widespread 

 occurrence, not to say uni\'ersality, of coenzyme A, which 

 has been found in all organisms in which it has been looked 

 for. 



'Active acetyl ', however, is not used only for the condensa- 

 tion of two molecules of acetic or analogous acids, it can also 

 be used for bringing about the combination of these acids 

 with cOo. The very presence of acetyl derivatives of coenzyme 

 A therefore necessarily implies the possibility of the fixation 

 of CO,. 



Studies in this field do indeed show that when CO2 is fixed 

 its labelled carbon atoms always appear in carboxyl gioups 

 combined with pre-formed organic molecules containing not 

 less than two carbon atoms. In particular we may indicate 

 the following types of reaction whereby co, is fixed by some 

 heterotrophs." 



(c„ + Ci) Clost. butylicum 



Acetate -f cOo -j- h. >pyruvate + h.o 



(C3 4- Ci) M. lysodeikticus 



Pyruvate -f cOo >oxaloacetate 



(C4 + Ci) Esch. coli 



Succinate -\- CO2 + h, >a-oxoglutarate 4- HoO 



As a rule, heterotrophs cannot synthesise substances in 

 which two neighbouring carbon atoms are derived from cOo, 

 the ability to do this being peculiar to autotrophs, which 

 build the whole of the carbon skeletons of the components 

 of their protoplasm out of carbon dioxide which they have 

 fixed. 



However, K. T. Wieringa,*" and later H. A. Barker and 

 his colleagues," have succeeded in isolating bacteria which 

 require organic nutrients for their growth and development 



27 



