40 NITROGEN METABOLISM 



metabolism. Since it is generally believed that only inorganic 

 nitrogen in the forms of NH3 can be incorporated into 

 organic molecules, it is probable that in both cases the 

 metabolic pathways have at least the initial steps in common. 

 The end-result is different in that one leads to the assimila- 

 tion of nitrogen, whilst in the other the products are excreted 

 into the medium. The ability to reduce NO7 enables certain 

 organisms to grow anaerobically in media which would 

 otherwise only support their growth in the presence of O, , 

 and in such cases NO 7 may be regarded as replacing oxygen 

 as the ultimate acceptor of metabolic hydrogen [25]. For 

 example, Esch. colt cannot grow anaerobically on lactic acid 

 as the sole source of carbon unless the medium also contains 

 a suitable H-acceptor, and nitrate is only one of several sub- 

 stances which can fulfil this function. Serratia marcescens 

 and Pr. vulgaris behave similarly [2, 4], but other organisms 

 are known which specifically use NO 7 and are unable to 

 grow anaerobically in its absence, even though the medium 

 contains NH3 . In anaerobic conditions, the chemosynthetic 

 autotroph, Thio. denitrificans, can obtain energy only by the 

 oxidation of sulphur compounds at the expense of reducing 

 NOI: 



6KN03+5S + 2H20=K2S04+4KHS04+3N2+energy 



A number of aerobic spore-forming bacilli related to 

 Bacillus subtilis can live anaerobically only in the presence 

 of NO 7, and they have been isolated from anaerobic enrich- 

 ment cultures in media containing a high concentration of 

 KNO3 (8-10 per cent) [5, 30]. 



The first step in the reduction of NO 7 involves its con- 

 version to NO 7 by an enzyme system which is adaptive in 

 nature, and is known as nitratase [29]. The nitratase of 

 Neurospora has a prosthetic group of adenine flavin di- 

 nucleotide and will use reduced TPN to reduce nitrate to 

 nitrite [zib]. Some strains oi Esch. coli are unable to reduce 

 NO 7 any further [24], but others reduce both NO 7 and 

 NO^ quantitatively to NH3 in the presence of a suitable 

 H-donor such as glucose [2, 36]. Organisms able to reduce 



