906 CONCENTRATION FACTORS CHAP. 27 



these experiments. From this point of view, and from the point of view 

 of favorable ratio of surface to volume, unicellular algae offer much better 

 conditions. In brief experiments, or in weak light, they can be used in 

 .acid solutions previously equilibrated with carbon dioxide of sufficiently 

 high partial pressure (>1%; it was calculated above that a suspension 

 containing 1 volume per cent of cells will use, in saturating light, all 

 the carbon dioxide contained in water equilibrated with 1% CO2, in 1.5 

 minutes). If stronger illumination or longer duration of experiments is 

 desired, acid solutions can be used only if the carbon dioxide content is 

 continuously renewed, e. g., by stirring with a gas the carbon dioxide con- 

 tent of which is maintained by contact with an alkaline carbonate buffer. 



More efficient should be the provision of carbon dioxide reserves in 

 situ by using carbonate buffers directly as suspension media, as first sug- 

 gested by Warburg. However, a certain difficulty arises from their un- 

 physiological and variable alkalinity. In progressing from M/IO buffer 

 No. 1 (0.5 X lO-'^ mole/1. CO2) to M/10 buffer No. 11 (29 X 10"^ mole/1. 

 CO2), we find the pH declining from 11 to 8.5. Since all living cells are 

 more or less sensitive to excess alkalinity (even if Chlorella appears to be 

 remarkably resistant to it), this drop of pH could cause continued increase 

 of the rate of photosynthesis in a range where this rate is intrinsically in- 

 dependent of carbon dioxide concentration. This may explain, for ex- 

 ample, the difference between the carbon dioxide curve of Chlorella as de- 

 termined by Warburg (1919) in carbonate buffers, and the same curve ob- 

 tained by Emerson and Green (1938) in a phosphate buffer. The first 

 one continues to increase up to and beyond 9 X 10^^ mole/1. CO2, while 

 the second one is perfectly flat above 0.7 X 10"^ mole/1. CO2. 



On the other hand, observations of Ruttner (1947, 1948) and others on 

 the maximum pH reached in non-renewed media after prolonged photosyn- 

 thesis by aquatic plants (c/. above page 890), tend to discount the damaging 

 effect of alkalinity on algae and submerged phanerogams (as contrasted 

 to water mosses), by indicating the continuation of photosynthesis up to 

 pH 11 or 12; pH measurements on cell sap showed it to maintain its ap- 

 proximately neutral reaction even in such highly alkaline media. 



(For other possible explanations of the difference between the CO2 curves 

 of Chlorella as observed by Warburg, and by Emerson and Green, see 

 page 908.) 



Another pertinent question is whether the rate of conversion of HCOa" 

 ions into CO2 molecules always is high enough to provide effective replenish- 

 ment of used-up carbon dioxide. In chapter 8 (cf. Vol. I, page 175) we dis- 

 cussed the finite rate of hydration and dehydration of carbon dioxide, and 

 estimated that, in acid solution at room temperature, an H2CO3 molecule 

 lives ca. 0.1 sec. before dissociating — the monomolecular rate constant 



