COMMON SOURCES OF CARBON AND NITROGEN 87 



available COg, but in the chemosyntlietic group the oxidation 

 occurs aerobically as a source of energy. 



The strict autotrophes can multiply only in the presence 

 of inorganic matter, and may be inhibited by the presence of 

 organic matter. There are, however, some organisms which 

 lead an autotrophic-like existence, in that they utilise COg 

 or bicarbonate as carbon source and ammonia or nitrate as 

 nitrogen source, but are able to obtain energy from the break- 

 down of certain organic substances. These organisms thus 

 form a bridge between the true autotrophes and the hetero- 

 trophes, and are consequently called " autotrophic hetero- 

 phants "; they do not represent any large proportion of the 

 organisms in common experience. 



An example of an organism whose metabolism is inter- 

 mediate between that of the autotrophes and that of the 

 heterotrophes is B. methanicus which obtains its energy (and 

 possibly its carbon) by the oxidation of methane. 

 CH4 -f- 2O2 = CO2 + 2H2O + 195 Cals. 



We also find organisms whose metabolism is intermediate 

 between that of the photosyrithetic autotrophes and the 

 heterotrophes. Thus the Athiorhodaceae are photosynthetic 

 organisms closely related to the Thiorhodaceae but use organic 

 acids as hydrogen-donators whereby to reduce the CO2. They 

 can be distinguished from the heterotrophes in that growth 

 will only occur anaerobically and in the light. COg is essential 

 to the growth and the organism appears to obtain most of its 

 carbon material from the assimilation of COg. 



TABLE VII 

 Common Sources of Carbon and Nitrogen 



Sources of Carbon 



Sources of Nitrogen 



CO2 

 HCO 



Na 



NH, 

 NH2OH 

 Carbohydrates : ! NOg' 



Glucose ' NO,' 



