Metabolic Pathways of Bacterial Nitrification 393 



of Dr. H. Lees and his co-workers during the last ten years (9, 

 11, 12, 22, 28) have indicated in part the probal^le pathways of 

 ammonia and nitrite oxidation by intact cells of Nitrosomonas 

 and Nitrohacter respectively. Their studies have contributed to 

 our understanding of the biochemistry and mechanisms involved 

 in the primary oxidation reactions by these nitrifying organisms. 

 More recently Aleem and Alexander (1-3, 8) demonstrated for 

 the first time an active nitrite oxidizing system at the cell-free 

 level. Aleem and Nason (4-7, 33, *) subsequently purified and 

 characterized the nitrite oxidizing system from Nitrobacter as a 

 cytochrome-containing electron transport particle ( designated as 

 nitrite oxidase) which catalyzes the enzymatic transfer of elec- 

 trons from nitrite to molecular oxygen via cytochromes c- and 

 Oi-like components. They showed that nitrite oxidation in the 

 presence of partially purified nitrite oxidase particles was ac- 

 companied by phosphate esterification resulting in the formation 

 of ATP. Almost simultaneously Delwiche (31) also reported the 

 coupling of ATP formation to nitrite oxidation using crude cell- 

 free preparations of Nitrohacter. Soon afterwards Nicholas and 

 Jones (34) obtained a partially purified hydroxylamine-oxidizing 

 system from Nitrosomonas. Very recently Delwiche et al., has re- 

 ported the oxidation of hydroxylamine and concomitant phos- 

 phate esterification by crude, cell-free extracts of Nitrosomonas 

 (15). According to several recent reports (19, 20, 30), certain 

 heterotrophic nitiifiers also appear to play a limited part in the 

 nitrification process. There is no evidence at this time that the 

 oxidation of ammonia or nitrite is accompanied by phosphate 

 esterification in these organisms. Full attention, therefore, will be 

 devoted in this review to the metabolic studies that have been 

 conducted on the chemoautotrophs, Nitrosomonas and Nitro- 

 bacter. 



AMMONIA OXIDATION BY NITROSOMONAS SPS. 



Nitrosomonas cells derive their energy for growth and cell 

 synthesis from the aerobic oxidation of ammonia to nitrite ( 10, 

 21) as indicated in the over-all equation: 



NH4+ -1- I.5O2 -> 2H-^ + NO2- + H,0 4- 66.5 K.Cal. 



unpublished data. 



