( 71 ) 



already called attention to it as an excellent source of carbon for 

 Azotobacter in my papers of 1902. 



It must be allowed that the amount of fixed nitrogen in these 

 pure cultures is not considerable, about 1.5 m.g. for each gram of 

 oxidised malate, but perhaps here too, will be observed a greater 

 production it' only the very young cultures are examined; then, 

 however, only little of the salt can be oxidised and the absolute 

 quantities will of course be small. 



It seems not supertluous here to call to mind that it is by no 

 means the same whether a known amount of calciummalate be 

 absorbed from a dilute solution or from a more concentrated one. 

 In the latter case the malate will be more easily assimilable for the 

 Azotobacter cells, which will induce a stronger oxidation and 

 thus an increased oxygen assimilation in equal limes, so that the 

 tension of the oxygen in the liquid will be less than in the less 

 concentrated solutions. As the growth of Azotobacter seems favoured 

 by this lower tension, and in any case, a rather strong concentration 

 of the carbon food proves favourable to the process of nitrogen fixation 

 in absolute quantity this circumstance lias been taken into con- 

 sideration in all the experiments. Further, we did not always 

 wait for the moment at which the malate had disappeared from 

 the medium, but commonly it was much earlier subjected to the 

 analysis for the reason mentioned above. 



The observation that calciummalate can, glucose, cane-sugar and 

 mannite on the other hand, cannot form the starting point for nitrogen 

 fixation in liquid pure cultures, while yet the said carbon hydrates are in 

 the crude cultures much more productive and may even give gains 

 of nitrogen of 7 m.g. per gram of decomposed sugar, gives rise to 

 the supposition that these carbon hydrates must previously be 

 changed by other bacteria into organic acids and that these, at the 

 moment of their production, serve as carbon food for Azotobacter 

 and primarily cause the fixation of the nitrogen. 



Of course it cannot be malic acid which hereby originates from 

 the sugar; but the important growth of Azotolxtcter to which also 

 the acetates, the propionates and lactates may give rise, suggest the 

 question whether perhaps the acids of these salts may be first pro- 

 duced from the carbon hydrates and then govern the nitrogen fixation. 



It is to be remarked that as well in the malate as in the lactate 

 cultures slight amounts occur of a volatile acid, which will perhaps 

 prove to be acetic acid, although it is has not been positively demon- 

 strated by means of Behrens' uranylanatrium-acetate reaction. It is 

 of importance to know that this volatile acid is not only found 



