INTRODUCTION 5 



at least eleven growth factors. (The term growth factor is 

 used here in the same sense as vitamin in animal nutrition.) 

 With regard to the amount of carbon used for the synthesis 

 of cellular material, the contribution of the organic com- 

 pound serving as a source of carbon and energy varies in- 

 versely with the number of cytoplasmic constituents which 

 the heterotroph derives preformed from the environment: 

 in a rich medium this compound may function primarily as 

 a source of energy. At one time, autotrophs were diiferen- 

 tiated from heterotrophs on two counts: firstly that hetero- 

 trophs were unable to incorporate the carbon of CO 2 into 

 organic molecules, and secondly that autotrophs live entirely 

 and exclusively at the expense of inorganic substances. 

 There is now adequate information to show that both of 

 these statements require modification [18, 19, 5]. The 

 growth of heterotrophs is in fact dependent on the presence 

 of CO 2 and they are known to possess enzyme systems 

 accomplishing its fixation: but, although essential, CO2 is 

 neither a complete nor a major source of carbon for hetero- 

 trophs. Furthermore, it has been established that several 

 organisms regarded as autotrophs can exist heterotrophic- 

 ally. For example, in the presence of a suitable H-donor, the 

 purple sulphur bacteria (Thiorhodaceae) obtain their energy 

 from light, whilst CO2 and NH3 (or N2) serve as complete 

 sources of C and N. The H-donor may be an inorganic form 

 of sulphur or an organic substance such as a fatty acid, and 

 the Thiorhodaceae can therefore be regarded as facultative 

 autotrophs. On the other hand, the green sulphur bacteria 

 use only an inorganic H-donor and appear to be obligate 

 autotrophs. The Athiorhodaceae (non-sulphur purple bac- 

 teria) require certain growth factors and usually an organic 

 H-donor, i.e. they are heterotrophs, although they too- 

 derive their energy from light. 



Synopsis of monograph 



Many organisms can derive their energy either directly 

 or indirectly from nitrogenous compounds, and examples 

 of this feature of their metabolism are given in separate: 



