FOOD OF MICROORGANISMS. 93 



place of fat, glycogen, or other stored substances. If the source of hydro- 

 gen sulphide ceases, the bacteria oxidize the sulphur within their cells to 

 sulphuric acid. 



S + H 2 + 3 0=H 2 S0 4 



The second process of oxidation yields more energy than the one from 

 hydrogen sulphide to sulphur. The so-called "iron bacteria" are 

 another group of mineral-decomposing organisms, oxidizing ferrous 

 salts to ferric salts. They are commonly found in running brooks and 

 small rivers, and often develop in water pipes, sometimes forming in such 

 large quantities as to fill them entirely. It is not certain, however, that 

 they really obtain their energy by the. oxidation of iron salts. 



Organisms, feeding on marsh gas, hydrogen, and carbon monoxide, 

 have been mentioned in the chapter on carbon supply. The very recent 

 discovery of bacteria which can oxidize carbon as such in its elementary 

 form may also be mentioned. 



OXYGEN. 



Oxygen is indispensable to the life of all highly developed organisms. 

 Animals especially need it to support the oxidation that takes place con- 

 tinuously in their cells and blood. Many animals die when the oxygen 

 supply is exhausted. Higher plants also cannot exist very long in any 

 atmosphere without oxygen. 



The more simply organized forms of life are less sensitive, and micro- 

 organisms may grow without any free oxgyen. In the introduction to 

 this chapter, it has been stated that some organisms obtain their energy 

 by decomposing organic compounds without oxidation. The fermenta- 

 tion of urea consists of a simple addition of water. 



CO(NH 2 ) 2 + 2 H 2 0=(NH 4 ) 2 C0 3 



Ammonium 

 Urea carbonate 



Alcoholic and lactic fermentations are other examples. The fermen- 

 tative change of alcohol to acetic acid, however, is a process of oxidation. 



CH 8 CH 2 OH+ 2 O = CH 2 COOH+ H 2 O 



Alcohol Acetic acid 



Oxygen is required for this process. Similar oxidation processes are 

 the formation of nitrates and decomposition of thiosulphate and of 



