156 TRANSFORMATION OF MINERAL SUBSTANCES 



cell and which are frequently found in the soil in considerable 

 amounts, are used extensively as sources of energy by certain 

 specific types of bacteria. These bacteria are usually designated 

 as autotrophic bacteria, since they are able to synthesize their cell 

 substance from mineral materials, using carbon dioxide as their 

 only source of carbon. This process of carbon assimilation which 

 is referred to as chemosynthesis, because the energy is obtained 

 from chemical substances, is similar in many respects to the car- 

 bon assimilation by green plants which is spoken of as photosyn- 

 thesis, wherein the energy of the rays of the sun is utilized. The 

 utilization of hydrogen and of compounds of nitrogen (ammonia 

 and nitrite) as sources of energy by specific bacteria has been 

 referred to previously. In this group belong also bacteria which 

 oxidize elementary sulfur and its incompletely oxidized com- 

 pounds (sulfides and thiosulfates) to sulfate, organisms oxidizing 

 ferrous to ferric compounds, and manganous to manganic com- 

 pounds. 



Use of Inorganic Salts as Sources of Oxygen. — Under 

 conditions which favor the activities of anaerobic bacteria, as where 

 the supply of free gaseous oxygen is excluded, many bacteria are 

 capable of using certain inorganic salts rich in oxygen as sources of 

 oxygen. The reduction of nitrates, sulfates, and phosphates are 

 cases which can illustrate this group of processes. The reduction 

 of nitrates gives rise to nitrites, ammonia, nitrous oxide, and even 

 elementary nitrogen, as discussed previously. The reduction of 

 sulfates gives rise to sulfites and sulfides, including hydrogen sul- 

 fide, as shown later. The reduction of phosphates gives rise 

 to phosphites, hypophosphites, and even phosphine. 



Interaction of Insoluble Inorganic Salts with Inor- 

 ganic AND Organic Acids Produced by Microbes. — The trans- 

 formation of mineral substances as a result of their interaction in 

 the soil with organic and inorganic acids formed by microorgan- 

 isms is of common occurrence. These acids are produced in the 

 soil by numerous bacteria and fungi. The acids interact with the 

 insoluble carbonates, phosphates, and silicates, making them 

 partly or completely soluble. Certain portions of the rock frag- 

 ments in soil are dissolved, leaving the more resistant silicates. 

 Minerals of the alkalies, such as sodium and potassium, are more 

 readily acted upon than minerals of the alkaline earths, such as 

 calcium. Magnesium is affected to much the same degree as cal- 



