Il8 NUTRITION AND METABOLISM. 



matter and when this takes place mineral products are secreted. Two 

 main processes can be distinguished, oxidation and reduction. 



OXIDATIONS are the result of the organisms seeking a supply of energy. 

 Several oxidations of minerals have been indicated previously, as the 

 oxidation of ammonia to nitrites, of nitrites to nitrates, of hyposulphites 

 to sulphates, of hydrogen sulphide to sulphur and of sulphur to sulphuric 

 acid, of ferrous salts to ferric salts. All these microbial changes are simple 

 processes and can be followed by chemical analyses much more easily 

 than organic fermentations. The organisms which cause these changes 

 do not thrive in organic substances and for this reason pure cultures 

 can be obtained only with difficulty. Their activity is of great impor- 

 tance in soil fertility. 



REDUCTIONS of minerals by organisms have not been discussed. 

 They, too, are of great significance. As a typical example, nitrates may 

 be reduced to nitrites, to ammonia, to nitrogen gas, and, rarely, to nitro- 

 gen oxides. The reduction may be performed either by the direct removal 

 of oxygen, or by the formation of hydrogen. The reduction of nitrates 

 to nitrites can be written in the following three ways: 



KNO 3 - O = KNO 2 

 KNO 3 = KNO 2 +O 

 KNO 3 + 2H= KNO 2 +H 2 O . 



The result in all three cases is the same. Many bacteria can reduce 

 nitrates to nitrites or to ammonia. A few can reduce them to nitrogen. 

 These "true denitrifiers" are found in soil and in old manure. Their 

 reducing process is as follows: 



Ca(NO 3 ) 2 - 50= CaO+ 2N. 



Nitrates are reduced through the efforts of the organism to secure a 

 supply of oxygen. The denitrifying bacteria have strong oxidizing prop- 

 erties; they take oxygen from all sources possible. If cultures of denitri- 

 fying bacteria are well aerated, as in soils with a proper moisture content, 

 they scarcely attack the nitrates, while they will reduce them in ordinary 

 liquid cultures so fast that the escaping nitrogen gas forms a froth on top 

 of the nitrate solution. Denitrifying bacteria need the oxygen to oxidize 

 organic matter. They cannot live without organic food. It has been 

 stated previously that the oxidation of ammonia to nitrates liberates 

 energy, consequently the reduction of nitrates must absorb energy, and 



