CHEMICAL CHANGES 149 



ganic substances in building up their own food. They 

 are not the ones most commonly studied and found in the 

 laboratory, however. Certain bacteria in the soil are 

 known to oxidize ammonia into nitrous acid, using the 

 energy thus secured for building up their own food and 

 protoplasm. Still other bacteria oxidize nitrites into 

 nitrates making similar use of the energy secured. Water 

 in springs containing a considerable amount of hydrogen 

 sulphide usually abound with so-called sulphur bacteria 

 which can oxidize hydrogen sulphide to free sulphur and 

 finally to sulphuric acid, utilizing the large amount of 

 energy thus secured for their own growth. These sulphur 

 bacteria also occur in the soil and are doubtless effective 

 in oxidizing the hydrogen sulphide developed during the 

 decomposition of organic substances. Bacteria have been 

 discovered also which can oxidize hydrogen gas, and 

 others which can oxidize carbon monoxide, thus utilizing 

 them as sources of growth energy. Certain water bac- 

 teria oxidize ferrous iron to ferric iron, and manganous 

 manganese to manganic manganese. 



It is probable that in all cases organisms belonging to 

 the prototrophic group use a considerable proportion of 

 the energy secured in building up carbon compounds from 

 carbon dioxide and water. The fact that they are inde- 

 pendent of organic materials and of sunlight makes them 

 apparently among the most primitive of living organisms. 



MetatropJiic Bacteria. To this group belong the yeasts, 

 the molds, and the most of the bacteria. These organisms 

 require organic compounds of carbon, and by oxidation of 

 these compounds they are able to secure growth energy. 

 They also use these compounds in building up their proto- 

 plasm. They are in many cases able to utilize inorganic 

 sources of nitrogen. Some forms, for example, can fix 

 atmospheric or free gaseous nitrogen. As a result of the 

 energy secured by the oxidation of carbohydrates many 



