406 Marine Microbiology 



Nitrobacter are successively involved in the sequential oxidation 

 of ammonia to nitrite, and nitrite to nitrate. Our knowledge of 

 the pathways and mechanisms of heterotrophic nitrite and nitrate 

 formation from ammonia or other nitrogenous compounds is 

 sadly lacking. A comparative study of chemoautotrophic and 

 heterotrophic nitrification in terms of electron transport systems, 

 energy coupling mechanisms and the possible intermediates 

 should prove to be highly significant. 



CONCLUDING REMARKS 



Bacterial nitrification brought about by the chemoautotrophs 

 Nitrosomonas and Nitrobacter sps. is an energy yielding enzy- 

 matic process. It involves biological oxidation-reduction reactions 

 in highly organized steps which are carried out by specific en- 

 zymes with specific affinities for their substrates. Both ammonia 

 and nitrite oxidations are apparently mediated by cytoclirome 

 systems (4, 27, *). 



During the biological transformation of ammonia, the latter 

 is first converted to hydroxylamine and then to nitrite, presumably 

 via an unidentified and unstable intermediate at the nitrogen 

 oxidation level ( + 1 ) represented by ( NOH ) . This perhaps fur- 

 ther dissociates to N2O and N-. Recently Cresswell and Hewitt 

 (13) reported the oxidation of hydroxylamine by a peroxidase 

 system of the marrow plant resulting in the formation of nitrite 

 and unidentified gaseous products. To date there is no evidence 

 which would indicate that a similar system is responsible for the 

 oxidation of hydroxylamine by Nitromonas. 



Thus far the biological oxidation of nitrite by molecular 

 oxygen has been most clearly characterized. Nitrite is first acti- 

 vated by the nitrite- cytochrome c reductase accounting for the 

 flow of electrons from NOl" to cytochrome c causing the oxida- 

 tion of the former and the reduction of the latter. The further 

 transfer of electrons from cytochromes c to a, and finally to O2 

 is catalyzed by cytochrome oxidase causing oxidation of cyto- 

 chrome c, reduction of a and finally oxidation of a and reduction 

 of O2. These oxidation and reduction processes taking place in 



unpublished data 



