ENERGY TRANSFORMATION 387 



be obtained from the process; this is a better measure than the heat 

 of reaction, which includes also the diffused energy which serves to 

 increase the entropy of the system. 



The figures given in the following pages refer to the heat of reaction 

 and are based upon one mol of the reacting substances. It would be 

 more correct and would present a truer picture of the process if the 

 free energy values were utilized, considering also the heat of solution 

 and heat of dilution, especially in the oxidation of substances in a solid 

 phase; by considering the energy of ionization, the values will frequently 

 be distinctly different, as in the case of oxidation of free ammonia or 

 of the ammonium ion. Unfortunately practically all the data recorded 

 in the literature dealing with the transformation of energy by micro- 

 organisms are calculated on the basis of heat of reaction and may be 

 thus provisionally recorded. 9 " These data are based upon the work of 

 Thomsen, Berthelot, Ostwald and others. 



The thermochemical results are usually presented as follows: 



H 2 (g) + |O a \g) = H 2 {l) + 68.4 Cal. 



The last figure indicates the heat of reaction per mol of the reacting 

 substances. 



On the basis of energy utilization, the soil microorganisms can be 

 divided into four distinct groups: ' 



I. Microorganisms obtaining their energy autotrophically: 



1. Photosynthetic utilization of energy — algae. 



2. Chemosynthetic utilization of energy — autotrophic bacteria. 



II. Microorganisms obtaining their energy heterotrophically, or from the de- 

 composition and transformation of organic matter: 



3. Organisms which utilize atmospheric oxygen and obtain their energy 



by oxidations— aerobic bacteria, fungi, actinomyces, etc. 



4. Organisms which obtain their energy without the intervention of at- 



mospheric oxygen, or by true "fermentation-^ plkcesses — anaerobic 

 bacteria. *\ 



4 



Energy transformations by autotrophic bacteria. Since autotrophic 

 bacteria are capable of assimilating carbon from CO2 and of deriving 

 their necessary energy from the oxidation of simple substances, accurate 

 information can be obtained on the amount of chemical energy re- 

 quired to build up complex organic substances from minerals and 

 carbon dioxide. The simple composition of the nutrients helps to 

 differentiate between structural and functional energy required in the 

 metabolism; it also throws light on the influence of concentration of 



9a See Baas 4 Becking and Parks. Physiol. Rev., 7: 85-106. 1927. 



