26 THE MICROBE AND ITS ACTIVITIES 



the building up of their cells, including carbon, hydrogen, 

 oxygen, nitrogen, phosphorus, potassium, sulfur, and a few 

 others. 



With the exception of the algae, all the microbes of the soil 

 are devoid of chlorophyll. They are thus compelled to derive 

 their energy either from the oxidation of simple inorganic sub- 

 stances, as in the case of the limited groups of autotrophic bacteria, 

 or from complex organic substances, as in the case of the majority 

 of bacteria or the heterotrophic organisms, and of all fungi and 

 protozoa. The great majority of soil organisms are thus dependent 

 upon the complex organic substances of the soil for their carbon and 

 energy supply. The algae and autotrophic bacteria can obtain their 

 carbon from the carbon dioxide of the atmosphere, the former using 

 by means of their chlorophyll the energy of the sun, and the latter 

 using energy which is liberated in the oxidation of simple inor- 

 ganic substances, such as ammonia, sulfur, or hydrogen. The 

 elements hydrogen and oxygen are present in sufficient amounts 

 in water and in the gases of the atmosphere to supply all the needs 

 of the microbes. Frequently, however, conditions arise (such as 

 deficiency of elementary oxygen) under which microbes obtain 

 their oxygen from inorganic compounds of nitrate or sulfate. 



One of the most important elements in the nutrition of microbes 



is nitrogen. This is utilized by 



i\ the microbe either in the form 

 w C of complex organic substances, 



^ \ ^ W such as proteins and amino 

 I ^ J acids, or as simple inorganic 



X ^ compounds, such as nitrates or 



ammonium salts. Certain 

 limited groups of bacteria are 

 capable of utilizing the gaseous 



atmospheric nitrogen. The 

 Fig. 13. — Nitrite-forming bacterium, , , , . i . • i i_ 



^r.. , f. „r. other elements are obtamed by 



JSIitrosomonas europea (alter Wmo- "^ 



gradsky). tlie microbes from the minerals 



of the soil, phosphorus being 

 used in greater quantities than any of the others. 



In the utilization of the organic materials incorporated in the 

 soil, the microorganisms make use of an elaborate store of enzymes, 

 or organic catalysts. These may be active in attacking the 

 insoluble protein or carbohydrate molecules, or in oxidizing. 



f-v 



