THE CARBON DIOXIDE FACTOR 1887 



Osterlind's first five papers (1947, 1948, 1949, 1950i'2) had been sum- 

 marized in Part 1 of Vol. II (pp. 890-891). A difference was reported there 

 between the two species, Scenedesmus quadricauda and Chlorella pyrenoi- 

 dosa, in respect to the influence of bicarbonate ions on the rate of growth. 

 The two species were found to contain different amounts of carbonic an- 

 hydrase (c/. chapter 14, p. 380 and chapter 37A, p. 1744, but the difference 

 was not great enough to account for the wide discrepancy in the apparent 

 capacity for bicarbonate utihzation. Variations in membrane permeabiUty 

 to bicarbonate were suggested as the most hkely explanation. 



In the next paper, Osterlind (19510 measured manometrically the 

 rate of photosTjnthesis (in Ught of about 6 klux) of the same two species. 

 Ten-day old Scenedesmus cultures, and all the Chlorella cultures studied, 

 showed no evidence of bicarbonate utilization, but five-day old cultures of 

 Scenedesmus proved able to photosynthesize at maximum rate in 1 X 

 10~^ M NaHCOs (pH 8.1) — a solution that contains only insignificant 

 amounts (0.02 X 10"^ mole/1.) of free carbon dioxide. At pH 4.0-4.6, the 

 same cells produced oxygen at a rate proportional to the concentration of 

 free carbon dioxide between [CO.] = 0.9 X 10"^ and [COo] = 9 X 10"^ 

 mole/1.; in this case, [HCO3-] was much too low (<0.2 X 10"^ mole/1.) to 

 provide a contribution. From these two experiments, Osterlind concluded 

 that "young" cells of Scenedesmus quadricauda assimilate HCOs" ions 

 more effectively than CO2 molecules (in agreement with the conclusions he 

 had derived earlier from growth experiments) . 



Later, Osterlind (19512) suggested that the capacity of Scenedesmus quadricauda 

 to use bicarbonate ions depends on an enzyme that requires "photactivation." At pH 

 5 ("CO2 assimilation") oxygen liberation began at full rate immediately upon illumina- 

 tion; while at pH < 7.3 ("HCOj" assimilation"), the steady rate was reached only after 

 an induction period of about one half hour. Osterlind discussed two alternative mech- 

 anisms: either HCOs" ions undergo "activation" before they can enter the cell, or their 

 activation occurs inside the cell. In the latter case, activation may mean simply dehy- 

 dration and the enzyme requiring activation may be none other than carbonic anhydrase. 



Subsequently, Osterlind (1952') studied the inhibition of photosynthesis of Scene- 

 desmus quadricauda by cyanide, at different pH values, in the hope of obtaining a clue 

 to the alternative: utilization of bicarbonate as such, or its conversion to carbon dioxide 

 inside the cell; but the results proved compatible with both interpretations {cf. section 

 2(ffl) below). 



Osterlind's suggestion — that the prolonged induction period he ob- 

 served in C02-poor bicarbonate solutions (with algae that had been grown 

 in ample [CO2]) is caused by slow photactivation of an enzyme needed for 

 bicarbonate utihzation — was opposed by Briggs and Whittingham (1952) 

 {cf. Vol. II, Part 1, p. 908). They suggested instead that cells grown in 

 high [CO2], and therefore full of metabohtes, when exposed to strong light 

 in C02-deficient medium, tend to form — as suggested in the induction 



