TRANSIENTS IN ACID PRODUCTION 455 



As in the case of the acid gush we are forced to consider not only 

 carboxylation reactions as possible causes for the initial uptake and 

 dark gush but also a multitude of other metabolic reactions which 

 are known to produce or consume other acids besides C()2. These 

 might affect the external pH by a direct excretion of the acid in- 

 volved (8) or, more likely, through changes in intracellular bicar- 

 bonate. 



Among the reactions involved in the current picture of photo- 

 synthesis which might be expected to rapidly produce or take up acid 

 to an extent sufficient to explain the initial uptake and dark gush, 

 the most evident are those concerned with the formation and utili- 

 zation of PGA and inorganic phosphate. 



A reduction of the PGA pool largely to neutral triose before the 

 concentration of the CO2 acceptor has a chance to build up offers 

 a possible explanation for the initial acid uptake. The dark gush might 

 be the result of the rapid conversion in the dark of the ribulose 

 diphosphate pool to PGA (6) resulting in the formation of two car- 

 boxyl groups for each CO2 used. It should be noted that if there were 

 very little intracellular bicarbonate, these interna! acidity changes 

 might not affect the external medium at all, in which case only one 

 change would be observed : a CO2 uptake on darkening due to the car- 

 boxylation of ribulose diphosphate. 



While PGA pool changes may play a role, preliminary measure- 

 ments of ATP levels during the initial uptake and dark gush suggest 

 that changes in phosphate esterification may account for the bulk 

 of the pH changes. We found that the formation of ATP, measured 

 by the method of Strehler and Totter (9) , during the first moments 

 of illumination is of the same order of magnitude as the acid taken 

 up in the initial uptake. During the dark gush the ATP falls to or be- 

 low the level before illumination. 



Further studies are required before the contribution of various 

 factors to the initial uptake and dark gush can be quantitatively 

 evaluated. 



Discussion 



Strehler: liitciestingly, the amount of ATI' in tliese l)actena is niauy times 

 hifiiher, perhaps 10 to 100 times higher, tliaii what you find in the Cftlorella; and, 

 although there is onlj' about a 50% change in the total ATP content that we detect 

 here, it is quite an enormous change in comparison with what Chlorella does with 

 the same amount of light. 



