890 CONCENTRATION FACTORS CHAP. 27 



(1946) enlarged these observations by determining the carbon dioxide curves of photo- 

 synthesis in solutions containing different cations. He found that, with Myriophyllurn 

 spicatum, in acid solution, the presence of sodium or calcium (in the form of chlorides) 

 had no effect on the rate of photosynthesis. In alkaline solutions, on the other hand, 

 the rate was lowest in sodium bicarbonate, higher in potassium bicarbonates and still 

 higher in calcium bicarbonate. The highest rate could be obtained in a solution con- 

 taining K+, Na+, Ca2+, Cl~, and S04^~ ions in the same proportion as the water of the 

 lake that was the natural habitat of the plants. In lake water, the rate was independent 

 of pH between 8.5 and 10.5, while in potassium carbonate-bicarbonate buffer (10 ~' 

 mole HCOs" per liter) the rate increased slowly between pH 8.4 and 10.5 and dropped 

 sharply to zero at pH 1 1 . 



These results can be interpreted in terms of Steemann-Nielsen's concept of direct 

 participation of bicarbonate ions in photosynthesis (for example, by assuming different 

 rates of penetration of different neutral molecules, MeHCOs, through the cell membrane; 

 cf. Vol. I, page 197), but they may also be of a more indirect and complex origin. Ac- 

 cording to Pratt {cf. fig. 25. 1 ) the cation effects are largely irreversible, a complication 

 not considered by Steemann-Nielsen. 



Tseng and Sweeney (1946) studied the red alga Gelidinium cartilagineum and found 

 that the rate of its photosynthesis was determined exclusively by the concentration of 

 free carbon dioxide molecules, [CO2], and not affected by the simultaneous presence of 

 a large number of bicarbonate ions, [HCOs"] > 10 [CO2]. 



Ruttner (1947) compared the limiting pH values established in water as the result 

 of prolonged photosynthesis of different aquatic plants. Elodea (canadensis or densa), 

 Photomageton, Myriophillum prismaium, Lemma trisuUa and several other aquatic 

 phanerogams continued to reduce carbon dioxide until the pH rose considerably beyond 

 pH 9; while several mosses (such as Fonlinalis antipyretica) ceased to assimilate carbon 

 dioxide when pH reached 9.0. At the latter pH, [CO2] = 0.4 X IQ-^ mole/1.; this is 

 the region in which the "carbon dioxide compensation point" was found previously 

 with land plants {cf. page 899). Ruttner suggested that the capacity of aquatic higher 

 plants to carry out net positive photosynthesis at [CO2] equilibrium values <^ 1 X 10~^ 

 mole /I., if bicarbonate is present, indicates their capacity to utilize bicarbonate ions di- 

 rectly, and not merely as source of CO2 molecules in the medium. In a second paper, 

 Ruttner (1948) gave evidence supporting the assumption that the cessation of photo- 

 synthesis of Fontinalis at pH 9 is the result of low CO2 concentration (0.4 X 10 -'^ mole /I., 

 corresponding to about 0.01 vol.%), and not of excess alkalinity. Earlier observations 

 of Shutov (1926), Bode (1926) and Dahm (1926), who found that many aquatic plants 

 can raise the pH of the medium to values as high as 11.8 (Spirogyra), can then be in- 

 terpreted as meanmg that these plants, too, can use bicarbonate ions directly as source 

 of carbon for photosynthesis (or, more exactly, as a vehicle to transport carbon dioxide 

 from the medium into the cells). 



Osterlind (1948,1949) went even further than Steeman-Nielsen and Ruttner, and 

 asserted that certain plants use bicarbonate ions more effectively than carbon dioxide 

 molecules. He noted that the alga Scenedesmus quadricauda did not grow at all at pH 

 5.5 (in a solution aerated with ordinary air). It reached a high rate of growth at about 

 pH 6.5, and increased it slowly up to pH 9. According to Osterlind, this increase is 

 not an effect of alkalinity, but a consequence of the presence of bicarbonate ions. He 

 based this conclusion on the observation that, in air containing 5% CO2, good growth 

 could be obtained even at pH 3-4. The maximum rate of growth was reached when 

 [HCO3-] exceeded 9 X 10-^ mole /I.; between 2 and 8 X lO^^ moIe/1., the rate was 

 proportional to [HCO3-]. Osterlind thought that, with the cell populations used, no 



