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pounds to sulphate, and finally iron bacteria which in all probability 

 oxidize not only ferrous ions but also metallic iron (Table III). 



TABLE III 



Dissimilation processes of some autotrophic micro-organisms 



i. NH 3 +iV 2 2 -> HN0 2 +H 2 



2. HNOa+VaOa-^HNOg 



3. H 2 S+V 2 2 ->S+H 2 

 4- S+iV 2 2 +H 2 0-^H 2 S0 4 



5. Na 2 S 2 3 +H 2 0+20 2 -> Na 2 S0 4 +H 2 S0 4 



6. 4FeC0 3 +0 2 +6H 2 -> 4Fe(OH) 3 + 4 C0 2 



7. H 2 4-V 2 2 ->H 2 



The biggest surprise of all, however, is that the microbe world con- 

 tains exceptions to the rule which holds for higher organisms and with 

 which we all are familiar, namely that the presence of free oxygen is 

 an essential condition for the maintenance of life. Since the classical 

 studies of Pasteur we are acquainted with the existence of micro-organ- 

 isms which can thrive in the absence of oxygen and for which, in cer- 

 tain cases, this gas even has a harmful effect on the activities and 

 proliferation. We all know that Pasteur at once felt the need for an- 

 other energy-yielding process as a substitute for respiration, and that 

 he did not hesitate to indicate the sugar fermentation process as such. 



Alcoholic fermentation is by far the best known of these sugar fer- 

 mentation processes, but gradually several other types of anaerobic 

 sugar dissimilation have been found to be the energy-yielding pro- 

 cesses in certain bacterial groups (Table IV). 



These processes are usually designated by the main product formed 

 out of the sugar, as lactic acid, propionic acid, butyric acid, butanol, 

 butanediol and other fermentations. 



Even a superficial acquaintance with fermentation processes will suf- 

 fice to show that such a process presents a peculiar type of chemistry: 

 the number of final products can be large, whilst the quantities of the 

 various products do not have a stoichiometrical relationship; more- 

 over, they vary considerably with the external conditions. This so ap- 



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