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which initially were confined to aerobic conditions it must have been 

 a constant temptation to use these reducible ions as a substitute for 

 oxygen at those spots, where this gas was lacking. 



For the denitrifying bacteria the situation is, indeed, that they can 

 thrive either with oxygen or with nitrate, which means that these 

 primarily aerobic organisms can eventually extend their zone of life 

 to those nitrate-containing soil layers in which the air does not enter. 

 The cases of sulphate- and carbonate-reducing bacteria must appar- 

 ently be seen as a further step in this development, since these bacteria 

 are strictly anaerobic, which implies that free oxygen is no longer a 

 suitable hydrogen acceptor for them. 



There are a few more types of dissimilatory reactions which I should 

 like to bring to your attention. My fellow-countryman Wieringa dis- 

 covered in 1936 an anaerobic bacterium which had the remarkable 

 ability of converting a mixture of carbon dioxide and hydrogen into 

 acetic acid. This means that the hydrogen acceptor function of carbon 

 dioxide is not restricted to its conversion into methane, but under 



TABLE VIII 



Survey of fermentation depending on the formation of acetic acid out 



of carbon dioxide 

 General equation: 



4 H 2 A+2C0 2 -> 4A+CH 3 .COOH+2H 2 



Clostridium aceticum: 



4H 2 +2C0 2 ->CH 3 -COOH + 2H 2 (Wierinj 



C 6 H 12 6 +2H 2 -> 2CH 3 -COOH+2C0 2 +8H 

 8H+ 2 C0 2 -> CH 3 COOH+2H 2 



C 6 H 12 6 -> 3CH 3 - COOH 



Clostridium acidi-urici: 



C 5 H 4 3 N 4 + 7 H 2 -» 5 C0 2 + 4 NH 3 +6H 



6H+i.5C0 2 -^o.75CH 3 -COOH+i.5H 2 



C 5 H 4 3 N 4 + 5 . 5 H 2 -> o. 75 CH 3 .COOH+4NH 3 +3.5C0 2 



Butyribacterium rettgeri: 



CH 3 CHOH-COOH+H 2 0-^CH 3 COOH + C0 2 +4H 

 8H+2C0 2 ^ CH 3 •COOH+2H 2 

 4H+2CH 3 •COOH^CH 3 .CH 2 •CH 2 •COOH+2H 2 



456 



