PART VI. BIOLOGY OF SPECIAL- 

 IZED GROUPS 



CHAPTER XVI 

 PROTOTROPHIC BACTERIA 



Introduction. The prototrophic bacteria comprise those 

 which, from a physiological standpoint, are the most primi- 

 tive. They obtain their food from inorganic sources and are 

 able to build up these simple substances into their complex 

 protoplasm. It seems probable that such organisms as these 

 must have been the original forms of life on the earth, and that 

 the metatrophic and para trophic have developed from them, 

 and, in a sense, these must be considered as degenerates. 



From this standpoint the prototrophic bacteria occupy a 

 very important place in the developmental history of life on the 

 earth. Many of them at the present time, as well as in the 

 past, are of inestimable service to man. These prototrophic 

 bacteria include the nitrifying, the iron, the sulphur, and the 

 legume, or nitrogen-fixing bacteria. 



Nitrifying Bacteria. The nitrifying bacteria oxidize 

 ammonia to the nitrites and the nitrates. There are several 

 different species which belong to two different classes : those 

 which carry on the first part of the process, that is, oxidize 



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