PRINCIPLES OF MICROBIAL METABOLISM 369 



microorganisms, capable of utilizing free nitrogen gas, in the absence 

 of available compounds of this element, stand as a group by themselves. 



The minerals, chiefly phosphates and potassium salts, but also iron, 

 magnesium, sulfur, calcium and traces of other elements, are utilized 

 by all microorganisms either in the form of simple inorganic compounds 

 or are obtained from complex organic substances in the process of their 

 decomposition. The minerals may often be obtained from insoluble 

 inorganic materials, especially if the organism produces acids which 

 tend to make them soluble. 



In the utilization of nutrients by heterotrophic microorganisms, two 

 general stages are observed. (1) The dissimilation or decomposition 

 stage, in which organic matter is broken down by the agencies of hy- 

 drolysis, oxidation and reduction. (2) Assimilation stage, or syn- 

 thesis, whereby the cells of microorganisms are built up out of the 

 substances previously broken down. 



The metabolism of autotrophic bacteria consists only of the syn- 

 thesizing stage so far as organic substances are concerned. The auto- 

 trophic microorganisms utilize for their synthesis the products of dis- 

 similation of the heterotrophic organisms, such as the various minerals, 

 nitrogen compounds and even energy sources such as ammonia, hydro- 

 gen sulfide, etc. The heterotrophic microorganisms utilize for their 

 dissimilation stage the products of assimilation of the autotrophic 

 forms, namely the complex organic substances synthesized by these 

 cells. 



HjO"* NH3^ RjS CO z organic acids other compounds 



Autotrophic organisms Heterotrophic organisms 



Only the autotrophic organisms actually produce work, in the true 

 thermodynamic sense, as shown later, while the heterotrophic forms 

 may simply build up the new protoplasm out of the constituents of 

 the medium. This explains the considerably greater assimilation and 

 the more extensive protoplasm produced for the same amount of 

 energy available. 



Chemical reactions in the microbial cell. The microbial cell may be 

 considered as an osmotic system. The absorption and liberation of 

 substances by microorganisms leads to a series of chemical reactions 



