168 PHYSIOLOGY OF BACTERIA 



by the observation that complete removal of carbon 

 dioxide prevents the growth of all bacteria. This has 

 been asserted by many investigators and denied by many 

 others, but the work of Valley and Rettger (1927) 

 leaves hardly any doubt of the fact. 



The role which CO2 plays is entirely unexplained. 

 Since the amounts present in air are sufficient for normal 

 growth, it would seem that the carbon dioxide enters in 

 the process of cell construction rather than in energy 

 liberation. With the latter process, larger amounts 

 would probably be required to produce noticeable 

 effects. The author is of the opinion that we stand here 

 at the already opened door to an important biological 

 principle. 



(d) SOURCES OF NITROGEN 



Elementary nitrogen is used by the three well-known 

 groups of nitrogen-fixing bacteria, the Rhizohium group, 

 the Azotohacters and certain Clostridium species. Less 

 intensive nitrogen fixation has also been claimed for 

 some other bacteria, and even for yeast (Fulmer and 

 Christensen, 1925). All organisms mentioned can take 

 the nitrogen in other forms. 



Ammonia and nitrates, also nitrites in low concentra- 

 tions, are used by many molds and many bacteria. 

 Considerable discussion has taken place concerning the 

 preference of ammonium salts and of nitrates, especially 

 for the growth of molds. The results by different 

 investigators are in some cases contradictory. It seems 

 quite probable that we have the superposition of another 

 factor which was not generally recognized at the time 

 when most of these experiments were conducted, i.e., 

 the antagonism of certain salts, the change of hydrogen 

 ion concentration or the oxidation-reduction potential. 



