1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 233 



serve as well where the release of C0 2 is slow and takes place in 

 the solution. 



There is perhaps no ultimate difference between saying that 

 a bacterium produces carbonates and finding carbonates formed 

 by the neutralization of C0 2 , yet there is considerable actual 

 difference when the intimation goes with it that organic salts are 

 "fermented" to form carbonates instead of C0 2 and H 2 0. If 

 we are to consider carbonates as products of metabolism then 

 we must consider that H 2 CO» and not Cd and H 2 are the pro- 

 ducts of respiration of most organisms and that it is only the 

 environment which breaks up the acid into C0 2 and H,0. 



In considering the case of certain fungi using organic acids, we 

 find these organisms growing in a medium too acid for the fixation 

 of COi. As the course of active metabolism nears a close the 

 solution rapidly becomes alkaline (see experimental results), 

 yet not even traces of carbonates can be detected by ordinary 

 methods. The reasons for this are probably two-fold. The fungus 

 has used the free acid first, then the acid salts, leaving only the 

 alkaline salts which, owing to the alkalinity of the solution, are 

 metabolized with difficulty and the C0 2 given off by the fungous 

 mat falls to a very low figure, and probably almost stops when 

 even moderate alkalinity is reached. The C0 2 that continues to 

 be given off is that resulting from the autolysis of the fungous 

 mat but this is in large measure passed off into the air. So slowly 

 is the CO 2 given off at this stage that the gradient through the 

 stopper to the outer atmosphere is probably sufficient to prevent 

 the accumulation of any considerable C0 2 tension in the atmos- 

 phere of the flask. Weakly alkaline solutions do not greedily 

 absorb C0 2 from the air. Undoubtedly the use of a sensitive 

 apparatus for the determination of small quantities of COi and 

 carbonates, such as that used in determinations with blood, 

 would show considerable CO* evolved from these alkaline cultures 

 upon the addition of acid in comparison with that which could 

 be detected in the original solution. It is equally obvious that the 

 amount of C0 2 present in the culture solution of a fungus which 

 has ceased to consume the carbohydrate, due to the increasing 

 alkalinity of the medium, will never equal the amount produced 

 by bacteria growing in the solution and continuing active de- 



