50 NITROGEN METABOLISM 



bacteria (species of Rhodospirillum, RhodopseudomonaSy 

 Rhodomicrobium, Chromatium and Chlorohacterium) [19, 26 

 27, 28], Desulphovibrio [34], blue-green algae of the family 

 Nostocaceae [15, 17, 43], and Calothrix [43]. Claims for 

 fixation by yeasts and other fungi are as yet unsubstantiated. 

 The successful demonstration that nodules detached from 

 roots fix N2 was dependent on the use of nodules from 

 leguminous plants grown in the field rather than the green- 

 house, performing the experiments immediately after 

 detachment from the roots and subjecting only the soluble 

 nitrogen compounds of the nodules to isotopic analysis [2]. 

 The Leguminoseae is not the only family in which nodules 

 are formed in consequence of microbial invasion, and the 

 root nodules of the Alder {Alniis) and Coriaria japonica may 

 also be concerned with the fixation of Ng. 



Factors affecting Nz-fixation 



There are no substantiated experiments in which Ng- 

 fixation has been divorced from growth of the experimental 

 material [24], consequently care is required in assessing 

 whether the factor being studied directly affects the fixation 

 mechanism: the observed effects may be no more than the 

 result of interference with processes, such as the production 

 of energy, which are essential to metabolism in general. 

 For example, the influence of the partial pressure of oxygen 

 (pOa) on fixation by Azotohacter and by clover plants 

 appears to be entirely explicable in terms of its effects 

 on respiration, and in consequence, the availability of 

 energy [7, 41]. Similarly, although molybdenum, iron, 

 calcium and strontium have all been implicated in the 

 fixation mechanism, it is difficult to decide whether this is 

 their primary function. These difficulties cannot be circum- 

 vented by performing experiments in the presence of fixed 

 nitrogen, since the latter induces a quicker rate of growth, 

 and in all probability growth is then limited by different 

 factors from those operative when the organisms are fixing 

 molecular Ng . Whilst molybdenum undoubtedly influences 

 the growth of Azotohacter^ Nostoc, CI. pasteurianum and 

 leguminous plants, there is no direct evidence that it is 



