CHAPTER IV 



AUTOTROPHIC ASSIMILATION WITH 



SPECIAL REFERENCE TO NITROGEN 



METABOLISM 



Given a suitable source of energy and of carbon, auto- 

 trophic organisms are able to synthesize all their cell con- 

 stituents from inorganic materials. Something has already 

 been said of the synthesis of carbon compounds and this 

 chapter must be chiefly concerned with the autotrophic 

 assimilation of nitrogen. This emphasis is justified since the 

 most characteristic components of living matter are nitro- 

 genous and since departure from the autotrophic mode of 

 life is most frequently manifest as an inability to synthesize 

 some essential nitrogenous metabolite. 



Nitrogen may be assimilated by autotrophic algae as the 

 element, as nitrate or as ammonia. 



NITROGEN FIXATION BY ALGAE 



The ability to assimilate, or 'fix', the elementary nitrogen 

 of the atmosphere is restricted to a comparatively few organ- 

 isms and has as yet been demonstrated with certainty only 

 in certain bacteria, i.e. Azotobacter spp., Clostridium spp., 

 Rhizohium spp. in symbiotic association with leguminous 

 plants, and some photosynthetic bacteria, and in certain 

 Myxophyceae.-^^' ^^^^ 



That some algae are able to fix nitrogen was first reported 

 by Frank in 1889,^°^ four years before the first isolation of a 

 nitrogen-fixing bacterium was announced by Winogradsky. 

 Frank's cultures were, however, impure and must have 

 contained numerous organisms other than algae to which 

 the observed nitrogen fi^xation might be attributed. The 

 isolation of blue-green algae in pure culture is not easy and 

 it was not until 1928 that satisfactory evidence of nitrogen 



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