THE CARBON DIOXIDE FACTOR 



1889 



beginning as early as at 2% (7 X 10"^ mole/1.) CO, in saturating light, and 

 somewhat later in weaker light. When the rate was thus " [COJ limited," 

 bicarbonate was found able to serve as substitute for the deficient carbon 

 dioxide. According to Gaffron, this cannot be attributed to removal of dif- 

 fusion limitation in an insufficiently stirred vessel, because of the shape of 

 CO2 curves obtained at different light intensities (c/. under (6) below). Fig. 

 37D.1 represents a "CO2 curve" obtained in dilute bicarbonate solution, 



Fig. 37D.1. Carbon dioxide curve of Chlorella (determined with pH meter) in 2 X 

 10-5 M bicarbonate, in near-saturating green light (after Gaffron and Rosenberg 1953). 

 CO2 saturation at 1.2%. Sharp initial rise of the curve indicates contribution of bicar- 

 bonate. Suspension density, 0.9 volume percent. 



which shows at [CO2] ~ a "residual" rate attributable to bicarbonate. 

 This residual rate increases with increasing bicarbonate concentration, until, 

 in 0.01 or 0.1 M bicarbonate (i. e., in the usual carbonate buffers), the "CO2 

 curve" becomes practically fiat down to [CO2] values as low as 0.1% (3 X 

 10-^ mole/1.), or less, as found by Emerson and other earlier observers. 

 (However, this observation does not explain the finding by Emerson and 

 Green, and by Whittingham, of CO2 curves that remained flat down to very 

 low [CO2] values also in acid media, containing no bicarbonate.) The 

 contribution of bicarbonate to the yield was found by Gaffron to depend 

 on the previous history of the cells. An extreme case is illustrated in fig. 

 37D.2. It shows that cells incubated in 50% CO2, and then brought into 

 an atmosphere of 0-5% CO2, at first show no capacity to utilize bicarbonate 



