388 PRINCIPLES OP SOIL. MICROBIOLOGY 



nutrients, neutralization of waste products and amount of total avail- 

 able energy utilized by the organism in connection with carbon assimi- 

 lation. These are the only microorganisms (except the algae), which 

 are capable of doing work in the true sense. The autotrophic bacteria 

 can be classified on the basis of the elements and compounds from 

 which they are able to obtain energy: 



1. Nitrogen and its compounds. We do not as yet know whether 

 the nitrogen-fixing bacteria are capable of utilizing any energy which 

 may be liberated in the process of transformation of the elementary 

 nitrogen into the first compound of nitrogen. Only two groups of 

 organisms, which are capable of obtaining energy from the oxidation 

 of nitrogen compounds are known: (a) those that oxidize ammonia or 

 ammonium ion to nitrite, and (&) those that oxidize nitrite to nitrate; 

 both of these are usually classified together under the term "nitrifying" 

 bacteria. 



2. Sulfur and its compounds. The latter include the reduced and 

 partially oxidized compounds of sulfur, H 2 S, sulfides, thiosulfates 

 and, perhaps, sulfites and hyposulfites. The organisms obtaining 

 their energy from the oxidation of sulfur and its compounds, especially 

 the sulfides, belong to morphologically different groups. 



3. Selenium and its compounds. Our information as to the energy 

 utilization in the oxidation of these substances is rather indefinite. 



4. Iron compounds. The most important of these is ferrous 

 carbonate. As shown elsewhere, not all organisms which are capable 

 of precipitating ferric hydroxide are autotrophic bacteria. 10 



5. Hydrogen and simple hydrocarbons. 



6. Carbon and its compounds. These include carbon monoxide, 

 methane, higher hydrocarbons, and alcohol. The autotrophic nature 

 of these is questionable; they may even be considered as heterotro- 

 phic. It is not definitely known whether any organisms are capable 

 of obtaining energy from the oxidation of elementary carbon. 



Energy utilization from the oxidation of nitrogen compounds. Energy 

 is obtained by certain well defined groups of microorganisms from the 

 oxidation of ammonium salts to nitrites (nitrite formers) or from the 

 oxidation of nitrites to nitrates (nitrate formers). In the case of these 



10 Various organisms are capable of accumulating metals within their cells, 

 without the metal taking part in the chemical processes of the organism; this 

 has been demonstrated for gold by Plotho: O. von Plotho. — Der Einfluss kol- 

 loidaler Metallosungen und seine Ursachen. Biochem. Ztschr., 110: 1-32. 1920. 



