402 PRINCIPLES OF SOIL MICROBIOLOGY 



uct of the reaction, it was found that normal growth took place even 

 in concentrations of 0.25 molar, while 0.5 molar did not completely 

 prevent growth. In general, the metabolism of the Thiobacillus thio- 

 oxidans approached, in its energy utilization, very closely that of Nitro- 

 somonas and Nitrobacter. 



Energy utilization from the oxidation of iron compounds. Certain 

 bacteria are able to utilize the energy obtained from the oxidation of 

 certain iron compounds and live autotrophically. According to Lieske, 36 

 Spirophyllum ferrugineum can grow in inorganic media free from or- 

 ganic matter and oxidize ferrous carbonate to ferric hydroxide. The 

 process supplies the organism with the necessary energy for the chemo- 

 synthetic assimilation of CO2: 



2 Fe C0 3 + 3 H 2 + £(0 2 ) = Fe 2 (OH) 6 + 2 C0 2 + 29.8 Cal. 



One mol of ferrous carbonate thus liberates only about 15 Calories, 

 and 1 gram of the substance oxidized liberates 0.12 Cal. The amount 

 of energy obtained is small in comparison with that liberated in the 

 processes of oxidation of the nitrogen and sulfur compounds. The 

 organism has to oxidize, therefore, large quantities of iron to obtain 

 enough energy to assimilate the necessary carbon chemosynthetically. 

 This results in the accumulation of large quantities of iron hydrate in 

 the bacterial bodies; to assimilate one part of carbon, about 750 parts 

 of hydrate must be formed. This figure is, however, far from accurate, 

 as stated by Lieske. Other ferrous salts will not take the place of 

 the carbonate. Much organic matter impairs growth and may finally 

 stop it altogether. 



Some organisms, like species of Leptothrix, can live without iron 

 compounds, but can utilize them either in the form of ferrous carbonate 

 or as soluble organic iron salts. Other bacteria will use certain soluble 

 organic iron compounds, but cannot utilize inorganic iron salts. We 

 have here a series of transition stages from pure autotrophy to pure 

 heterotrophy. Harder 37 divides the iron bacteria, as regards their phys- 

 iological activities, into three groups: 



1. Those that precipitate ferric hydroxide from solutions of ferrous bicar- 

 bonate and use the carbon dioxide liberated and the energy produced during 

 oxidation for their life activities (autotrophic). 



36 Lieske, 1911 (p. 95). 



37 Harder, E. C. Iron depositing bacteria and their geologic relations. Prof. 

 Paper 113, U. S. Geological Survey, Washington. 1919. 



