Metabolic Pathways of Bacterial Nitrification 405 



Exp. I contained 6.7 x 10^ cpm of initially added P^- orthophosphate per 

 Warburg flask. Exp. II contained 5.2 x 10^ cpm and Exp. Ill contained 

 4.3 x 10° cpm. 



cates the formation ot ATP during nitrite oxidation. Studies are 

 now in progress to determine the sites of phosphorylation and the 

 exact site of involvement of iron in the electron transport chain. 



BIOLOGICAL FORMATION OF NITRITE AND NITRATE 

 BY HETEROTROPHS 



A number of gram negative bacilli have been reported to 

 form very small quantities of nitrite from ammonia (20). How- 

 ever, none of the soil isolates including bacteria, actinomycetes 

 and fungi are known to produce nitrite exceeding 0.5 ppm NOj- 

 N, (19) suggesting that heterotrophic ammonia oxidation, if it 

 is coupled to phosphate esterification, is not the main energy 

 yielding reaction in those organisms. Nevertheless, in view of 

 the prevalence of these organisms in natural environments, it 

 seems possible that they may play a substantial role in the nitro- 

 gen economy of nature. Several heterotrophs belonging to the 

 genera Alcaligenes, Corynebacterhim, and Nocardia which do not 

 produce nitrite from ammonia, can utilize oximes, ethylurethane 

 and hydroxylamine to yield nitrite. The transformation is inhibited 

 by hydrazine and is unaffected by thiourea or allylthourea ( 14, 

 30). These results suggest that the nitrification of pyruvic oxime 

 and hydroxylamine takes place along a pathway which probably 

 resembles the chemosynthetic hydroxylamine oxidation by Nitro- 

 somonas. The second step of nitrification, namely the formation 

 of nitrate, is apparently included in the metabolism of Strepto- 

 myces nitrif icons and Aspergillus flovtis. The latter is able to pro- 

 duce substantial amounts of nitrate in the order of 50-60 ppm 

 NO3-N from complex nitrogenous materials such as casein, pep- 

 tone and yeast extract ( 19 ) presumably by the ammonification of 

 these substrates and its subsequent conversion to nitrate. All the 

 enzymatic steps involved in the transformation of ammonia to 

 nitrate are possibly contained in a single organism like Aspergillus 

 flaviis, whereas two distinct cliemoautotrophs Nitrosomonas and 



unpublished data 



