CARBON DIOXIDE CURVES 891 



carbon dioxide exhaustion could occur even in the solutions which contained no bicar- 

 bonate ions. He also noted that, with 10 X 10 ^ mole/1, of HCOs" present, growth was 

 as much as twenty-five times faster than with 10 X 10 "« mole /I. of carbon dioxide; and 

 concluded that Scenedesmus quadricauda uses bicarbonate ions (for growth, and thus 

 presumably also for photosynthesis) twenty-five times more efficiently than free carbon 

 dioxide molecules. Later (1950'- 2) Osterlind found that Chlorclla pyrenoidosa does not 

 use bicarbonates; since he found no difference in the carbonic anhydrase content of 

 the two species, he suggested that their cell membranes arc different. 



Pending further analysis concerning the role of carbonate ions (a ques- 

 tion which the above-described experiments have reopened), we will pro- 

 ceed on the old assumption that the rate of photosynthesis is primarily a 

 function of the concentration of the molecular species CO2 in the immediate 

 surroundings of the cells, and that the main effect of the presence of HCO3- 

 ions is to prevent this concentration from depletion during photosynthesis. 



Figures collected in Table 27.1, apart from those given for Myriophyl- 

 lum by Steemann-Nielsen, give no indication of a large, direct contribution 

 of carbonate ions to photosynthesis. We note, for example, that Emerson 

 and Green (1938) were able to achieve carbon dioxide saturation of photo- 

 synthesis in an acid phosphate buffer when the medium contained only 

 0.7 X 10 -^ mole/1. CO2. In experiments with carbonate buffers, in which 

 each carbon dioxide molecule was accompanied by 1000 bicarbonate ions 

 (and as many or more carbonate ions), saturation usually was observed 

 either at approximately the same or at an even higher value of fC02]. 



2. General Review of Carbon Dioxide Curves 



The necessity of carbon dioxide ("fixed air") for photosynthesis was 

 discovered by Senebier in 1782 (c/. Vol. I, chapter 2). The earliest quanti- 

 tative studies of the relation of the rate of photosynthesis to the concentra- 

 tion of carbon dioxide were made by Kreusler in 1885 and 1887, Brown and 

 Escombe in 1902 and Treboux, and Pantanelli, both in 1903. Since these 

 observations showed an increase of the rate with increasing fC02] in the 

 region of low concentrations, and a decline at high concentrations, they were 

 interpreted on the basis of the then popular "optimum theory" (fig. 26.1), 

 until Blackman and Smith suggested in 1911, that they can better be ex- 

 plained by the concept of "limiting factors." Blackman pointed out that 

 no evidence existed of a "minimum" [CO2] required for the beginning of 

 photosynthesis, or of a sharp "optimum"; instead of the latter, experi- 

 ments showed a wide range of [CO2] values over which the rate remained 

 approximately constant. As described in chapter 26, Blackman claimed 

 that correctly determined carbon dioxide curves must be broken lines of the 

 shape shown in figure 26.2; and many investigations have been carried 

 out with the expressed purpose of "proving" or "disproving" this "law." 



