952 



CONCENTRATIOiN FACTORS 



CHAP. 27 



carbon dioxide equilibrium. What we wanted to know — and did not quite 

 succeed in finding out — was whether the rate of photosynthesis is deter- 

 mined uniquely by the concentration of free carbon dioxide molecules, 

 [CO2], in the immediate surroundings of the cell, or whether the carbonic 

 acid ions also contribute to photosynthesis directly (and not merely because 

 they serve as a reserve for rajiid replacement of neutral carbon dioxide 

 molecules). One of the complicating factors was that a change in [HCOs"] 

 at constant [CO2] could not, and cannot be achieved without a change in 

 [0H~], i. c, a variation of the pH; and while some algae, such as ChlorcUa, 



25U 



200 



o 

 <-) 



o 



UJ 



a. 



3 



Fig. 27.13. Effect of pH on carbon 

 dioxide reduction with thiosulfate by 

 Chromatium (after Wassink, Katz and 

 Dorrestein 1942): 5% CO2; 1% thio- 

 sulfate; 29° C; strong Hght. 



o 

 o 



< 



Q. 



700 



600 



500 



400 



300 



200 



A High light intensify 

 o LOW light intensity 



Fig. 27.14. Influence of pH on rate of 

 carbon dioxide reduction by Chromatium 

 with hydrogen (5% CO., 15% H.2) (after 

 Wassink, Katz and Dorrestein 1942). 



are capable of efficient photosynthesis over a wide range of acidities (say 

 from pH 4.5 to 10), this does not prove that the observed changes in rate 

 can be attributed entirely to variations of [CO2] and not to variations of 

 pH. Some unicellular algae (e. g., Hormidium) are definitely injured by 

 alkaline media. 



The observations in this field, described in Volume I (pages 339-340) 

 can now be supplemented by the results of Wassink, Katz and Dorrestein 

 (1942) with purple bacteria. When reductants such as thiosulfate or hy- 

 drogen sulfide (or inhibitors such as cyanide) are used, the pH effect is com- 



