84 growth: synthesis of bacterial protoplasm 



Chemosynthetio autotrophes: organisms of this group 

 synthesise all their protoplasmic constituents from CO2 or 

 HCO3' and NH3 or NO3', and obtain the energy for the synthesis 

 by oxidation of an inorganic substrate which is specific for the 

 particular organism, and by means of which the various 

 organisms can be identified. For example, there are two 

 chemosynthetio autotrophes present in soil which carry out 

 the nitrification of ammonia. The first step in the oxidation 

 is carried out by Nitrosomonas, which obtains its energy from 

 the oxidation of ammonia to nitrite : 



2NH3 + 3O2 = 2HNO2 + 2H2O + 79 Cals. 



Nitrite is then oxidised to nitrate by Nitrobacter utilising 

 this reaction as a source of energy for its synthetic processes: 



HNO2 + = HNO3 + 21-6 Cals. 



The oxidation substrate is specific ; Nitrobacter cannot oxidise 

 or grow on ammonia as source of energy, while Nitrosomonas 

 cannot oxidise nitrite; neither organism can oxidise sulphite 

 or carbon compounds. Some of the chemosynthetio organisms 

 are inhibited by the presence of organic matter; thus the 

 nitrifiers are inhibited by the presence of meat broth but 

 the inhibitory substances can be removed by extraction of the 

 broth with ether or ethanol. The growth of Nitrobacter is 

 accelerated by the addition of 0-1 per cent peptone to the 

 medium but is inhibited by the presence of asparagine, 

 gelatine, or urea. Consequently the nitrifying organisms 

 cannot be isolated by plating out on solid media containing 

 gelatine or agar-agar, but are usually isolated by growth on 

 inorganic media solidified in silicic acid gel. 



A further example of a chemosynthetio autotrophe is the 

 iron bacterium, Leptothrix ochracea, which lives in iron-con- 

 taining streams and obtains its energy by the oxidation of 

 ferrous carbonate to ferric hydroxide. The complete 

 reaction is 



4FeC03 -f O2 + 6H2O = 4Fe (0H)3 + ^CO^ + 81 Cals. 



