NITRATE REDUCTION 183 



by means of an enzyme called " nitratase." Hydrogen may 

 be supplied by a dehydrogenase such as, for example, formic 

 dehydrogenase, so that the organisms can oxidise formic acid 

 anaerobically in the presence of nitrate : 



H.COOH + HNO3 > CO2 + H2O + HNO2. 



Both Esch. coli and 01. welchii possess an active hydrogenase 

 (p. 48) and in the presence of hydrogen, we find that the 

 reduction of nitrate proceeds further than nitrite on to 

 ammonia, with the probable intermediate formation of 

 hydroxylamine : 



H2^=^2H 



2H + HNO3 ^ HNO2 + H2O 



4H + HNO2 > NH2OH + H2O 



2H + NH2OH > NH3 + H2O 



or, the over-all reaction: 



HNO3 + ^^2 > NH3 + 3H2O. 



The interchange between ammonia and nitrate is thus rever- 

 sible, but whereas the forward reaction (c) can be carried out 

 by certain strict autotrophes only, the reduction process (d) 

 can be performed by a number of heterotrophic organisms, 

 both strictly and facultatively anaerobic. 



(e) Denitrification 



When certain species of Serratia, Chromobacteria, and 

 Pseudomonadaceae are grown in media containing either 

 nitrate or nitrite as source of nitrogen, there is a disappearance 

 of fiji:ed-nitrogen from the culture and bubbles of gaseous 

 nitrogen form in the culture fluid. The chemistry of this 

 denitrification process has not yet been worked out; it is 

 not reversible. The " Chromobacteria " thus provide the 

 final link in the cycle which starts with the fixation of atmo- 

 spheric nitrogen by Azotobacter, etc., passes through the 

 heterotrophic interchange of organic nitrogenous compounds 

 with final degradation to ammonia, through autotrophic 



