4 NITROGEN METABOLISM 



of the same basic materials, namely, mineral salts, water and 

 sources of carbon and nitrogen together with a mechanism 

 providing energy in a form that can be utilized in biological 

 systems. Autotrophs are organisms whose carbon require- 

 ments are entirely satisfied by CO 2 (perhaps in some by 

 CO). On the other hand, heterotrophs require a more com- 

 plex carbon source, i.e. an organic compound, as well as 

 CO 2 . Moreover, heterotrophs usually derive their energy 

 by catabolism of the organic carbon source and are therefore 

 to be contrasted with autotrophs which obtain their energy 

 either from light (photosynthetic autotrophs) or by the 

 oxidation of inorganic substances, e.g. H2S, S, NagSaOa , 

 NHt, NOi", H2 or Fe"*""^ (chemosynthetic autotrophs). 

 Each chemosynthetic autotroph oxidizes one specific com- 

 pound, or in certain cases, a limited number of chemically 

 related compounds, and presumably part of the energy 

 released during these oxidations becomes available in the 

 form of energy-rich phosphate groups. How the light energy 

 absorbed by the chlorophyll of photosynthetic organisms 

 becomes converted into a form that can be utilized in 

 enzymic reactions is not yet known, though recent experi- 

 ments have provided some indications of a possible 

 mechanism [17]. 



All autotrophs derive their nitrogen from an inorganic 

 source and, depending on the organism, use molecular Ng , 

 NHt J nitrate or nitrite. Although one or more of the latter 

 may serve as a complete source of nitrogen for certain 

 heterotrophs, the nutritional requirements of many of these 

 organisms are not so simple. It appears that such hetero- 

 trophs are unable to synthesize one or more of the organic 

 constituents of cytoplasm and they are therefore only able 

 to grow if these substances are present in their environment, 

 i.e. they are exacting towards these substances. The ability 

 to synthesize complex organic nitrogenous compounds is 

 especially variable, and whilst some organisms are exacting 

 towards only one compound, e.g. Salmonella typhosa to 

 tryptophan and Proteus vulgaris to nicotinic acid, the nutri- 

 tion of other heterotrophs is far more complex, e.g. Leuco- 

 nostoc mesenteroides P-60 requires eighteen amino-acids and 



