Chapter 37 



Metabolic Pathways of Bacterial 

 Nitrification* 



M. I. H. Aleem and Alvin Nason 



INTRODUCTION 



XXfter Pasteur's suggestion was confirmed by Schloesing and 

 Muntz some 85 years ago that nitrification is brought about by 

 living microorganisms, the process was shown to occm" in two 

 steps— first, the oxidation of ammonia to nitrite, followed by the 

 oxidation of nitrite to nitrate (37). The organisms catalyzing each 

 reaction were subsequently isolated in pure culture and named 

 Nitrosomonas and Nitrobacter. The former converts ammonia to 

 nitrite, and the latter oxidizes nitrite to nitrate. The discovery that 

 they possessed the unique ability to grow and persist indefinitely 

 in media devoid of organic substances led to the concept of 

 chemoautotrophy in microbiology (38). A quarter of a century 

 later, Otto Meyerhof described their growth characteristics and 

 the effects of various organic and inorganic compounds on the pri- 

 mary oxidation reactions brought about by these chemoauto- 

 trophs (32). 



Until a few years ago little was known about the intermediary 

 metabolism or the mechanisms of utilization of the energy lib- 

 erated from the oxidations of NHs and NO~2. Somewhat earlier 

 several determinations and calculations had established that the 

 energy released is used with an efficiency about 5 to 10 per cent 

 by Nitrosomonas and Nitrobacter (10, 32). The investigations 



* Contribution No. 361 from the McCollum-Pratt Institute. Some of the ex- 

 perimental data provided represent a portion of a study supported in part 

 by a research grant from tlie National Science Foundation and by a re- 

 search grant (No. 2332) from the National Institutes of Health, U. S. 

 Public Health Service. 



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