84 NUTRITION AND METABOLISM. 



vacuoles are usually absent in protozoa which are parasitic within other 

 animals." 



A plant character which is found in most molds and yeasts, and in 

 some bacteria, is the assimilation of nitrates and ammonium salts, but 

 this distinction is not at all definite, for a large number of bacteria 

 are exceptions. It will be unnecessary, therefore, always to make a 

 distinction between microbial plants and animals. 



ENERGY SUPPLY OF MICROORGANISMS. Microorganisms, like animals, 

 require food for energy as well as food for growth, and, as with animals, 

 one compound may often serve for both these functions. It is not always 

 possible to state whether or not a certain compound is used for growth as 

 well as for energy production, because the amount required for growth is 

 always very insignificant, but the distinction exists and can be indicated 

 by a few examples. 



Certain bacteria are known to live entirely on mineral matter; they use 

 minerals exclusively for building their cell substances, resembling in 

 this respect the green plants, and provide for the necessary energy by 

 oxidizing mineral compounds. Two typical examples are the nitrifying 

 organisms in soil which oxidize ammonia to nitrates. This process, ac- 

 cording to Winogradski, is divided distinctly into two phases: the 

 Nitrosomonas oxidizes the ammonia to nitrous acid, 



NH 3 + 3 O=HNO 2 +H 2 O 



and the Nitromonas oxidizes the nitrous acid to nitric acid, 



HNO 2 +O = HNO 3 



These oxidation processes yield a certain amount of energy which enables 

 the bacteria to build their cells from carbon dioxide, ammonia, and certain 

 mineral salts. Without ammonia or without nitrous acid, respectively, 

 these bacteria cannot grow for lack of energy; they would be like a plant 

 without light. It is evident in this case that the food for energy is also 

 used to some extent as food for growth. The nitrogen necessary to the 

 bacteria is supplied by the ammonia or the nitrous acid. 



As an example distinguishing between the food for growth and the 

 food for energy may be mentioned the hyposulphite bacterium studied 

 by Nathanson. This organism oxidizes hyposulphites to sulphates and 

 sulphur, largely following the formula 



Na 2 S 2 O 3 + O = Na 2 SO 4 +S 



Hyposulphite Sulphate Sulphur 



