34 NITROGEN METABOLISM 



serve as a source of COg and at the same time prevent the 

 H"^ produced during nitrification from lowering the pH to 

 a value unsuitable for growth [20]. The optimum pH for 

 the growth of a particular strain tends to be related to the 

 pH of the soil from which it was isolated, e.g. one from 

 a peaty soil will nitrify in relatively acid conditions [11]. 

 Meyerhof observed that the growth of Nitrosomonas and 

 Nitrohacter is inhibited by high concentrations of their 

 respective substrates. Furthermore, high concentrations of 

 NHt also inhibited the oxidation of NO^, and the higher 

 the pH of the medium the greater the inhibition, both of 

 growth and of nitrite oxidation. Both NHt and NO^ in 

 excessive concentrations are known to be injurious to cells 

 in general, and the effect of pH may be explicable on the 

 basis that conditions of high pH favour the formation of 

 unionized NH3 which may enter the cells more easily than 

 the ammonium ion [20]. 



The autotrophic nature of the nitrifiers, together with 

 their apparent inability to grow in organic media, led Wino- 

 gradsky to conclude that they neither required nor utilized 

 organic nutrients. There has been much controversy as to 

 whether all organic materials are toxic to these organisms 

 and whether they can in fact assimilate at least some of these 

 substances. There is now a certain amount of evidence that 

 their growth is stimulated by small amounts of peptone, by 

 yeast extracts and by a partial hydrolysate of egg albumin [9]: 

 moreover, some strains of Nitrosomonas can grow in the 

 presence of high concentrations of glucose (10%) [11, cf. 12]. 

 Some evidence that the nitrifiers may be able to obtain 

 energy in a manner typical of heterotrophs, namely by the 

 degradation of organic compounds, has been obtained by 

 Bomeke. In manometric experiments with thick suspen- 

 sions of Nitrosomonas and Nitrohacter y he found that in the 

 absence of exogenous nutrients, these organisms absorbed 

 O2 , indicating that they could obtain energy by the oxida- 

 tion of endogenous reserve materials [6]. Whilst there have 

 been many experiments purporting to show that organic 

 media support the growth of the nitrifiers, the majority of 



