174 NUTRITION AND METABOLISM 



sulphites to sulphates, of hydrogen sulphide to sulphur and of sul- 

 phur to sulphuric acid, of ferrous salts to ferric salts. AH these 

 microbial changes are simple processes and can be followed by chem- 

 ical analysis much more easily than organic fermentations. The 

 organisms which cause these changes, do not, as a rule, thrive in 

 organic substances and for this reason pure cultures can be obtained 

 only with difficulty. Their activity is of great importance in soil 

 fertility. ' 



Reductions of minerals, too, are of great significance. As a typical 

 example, nitrates may be reduced to nitrites^ to ammonia, to nitrogen 

 gas, and, rarely, to nitrogen oxides. 'The reduction may be performed 

 either by the direct removal of oxygen, or by the formation of free 

 oxygen. The reduction of nitrates to nitrites can be written in the 

 following three ways: 



KNO3 - O = KNO2 



KNO3 = KNO2 -1- O 



KNO3 + iH = KNOa -1- H2O. 



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(N03)2 - sO = CaO + 2N. 



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

 supply of oxygen. The dem'trifying bacteria have strong oxidizing 

 properties; they take oxygen from all sources possible. If cultures of 

 denitrifying bacteria are well aerated, as in soils with a proper mois- 

 tiu-e content, they scarcely attack the nitrates, while they will reduce 

 them in ordinary Uquid 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. 



Sulphates are reduced in a very similar way to hydrogen sulphide 



H2SO4 - 4O = H2S. 



Tap-water, containing calcium sulphates, often forms hydrogen sulphide 

 if shut off from the air for some time. 



