96 PRINCIPLES OF SOIL MICROBIOLOGY 



cannot live without it (obligate autotrophic), others, like Leptothrix, 

 can live without any iron compounds, but, if they are present, can use 

 either ferrous bicarbonate (or manganese bicarbonate) or soluble 

 organic compounds (facultative autotrophic). Others, such as the 

 various lower bacteria, will use the organic radical of certain soluble 

 organic iron salts when present but cannot utilize any inorganic iron 

 salts; in other words, the accumulation or incrustation of iron is purely 

 mechanical and these bacteria should not be considered as iron bacteria 

 at all. Among the latter organisms we would probably include the 

 form (similar to Bac. subtilis), 90 which precipitates ferric hydroxide from 

 solutions of iron salts, then reduces the hydroxide anaerobically to bog 

 iron. 



Bacteria obtaining their energy from the oxidation of simple carbon 

 compounds. Methane bacteria. Methane may be produced in appre- 

 ciable amounts in volcanic eruptions, around oil mines and as a result of 

 different chemical processes. It is also produced in the anaerobic 

 decomposition of cellulose, of other carbohydrates, organic acids and 

 proteins. 91 Swamps, 92 manure heaps and low-lying meadows also 

 contribute large amounts of methane to the atmosphere. 



Although the chemical oxidation of methane has been demonstrated 

 in various instances, it is primarily a phenomenon accomplished by 

 microorganisms. According to Harrison and Aiyer, 92 the soil film con- 

 tains bacteria and algae capable of oxidizing methane and hydrogen and 

 assimilating methane and C0 2 , increasing the oxygen output. B. 

 methanicus (Methanomonas methanica Orla-Jensen) was isolated from 

 the soil by Sohngen. 93 It was found to be a short, motile rod, 2 to 

 3 by 1.5 to 2fx in size, and could transform methane partly into organic 

 compounds and partly into C0 2 . In older cultures the organism 

 became nearly spherical. 



90 Mumford, E. M. A new iron bacterium. Jour. Chem. Soc. 103: 645-650. 

 1913. 



91 Omeliansky, W. De la mise en liberte tie methane au cours des processus 

 biologiques naturels. Arch. Sci. Biol. St. Petersbourg, 12: No. 2. 1906. 



92 Harrison, W. H., and Aiyer, P. A. S. The gases of swamp rice soils. II, 

 Their utilization for the aeration of the roots of the crop. Mem. Dept. Agr. 

 India, Chem. Ser. 4: 1-18. 1914. 



93 Sohngen, N. L. tJber Bakterien, welche Methan als Kohlenstoffnahrung 

 und Energiequelle gebrauchen. Centrbl. Bakt. II, 15: 513-517. 1906. 



