QUANTUM YIELD OF BACTERIAL AND ALGAL PHOTOREDUCTION 1125 



to "self -inhibition" effects, described on page 880). Experiment 8 shows 

 decreased efficiency in the presence of agar— an observation that Tonnelat 

 suggested may explain some of the low values found by McGee, DeWitt 

 et al. (who used agar to prevent the suspension from settling). 



Experiments were performed in vessels with black or reflecting bottom. 

 In the first case, t values could be too low (if absorption of light by the 

 blackened bottom was counted as absorption by the cells) ; in the second 

 case, they could be too high (because of possible escape of reflected radia- 

 tion). Figures in Table 29.VII indicate that the first effect is real (experi- 

 ment No. 6 shows a lower value of y at low cell concentration), but reveal 

 no effects due to reflection. 



Tonnelat concluded from these experiments that the energy conversion 

 factor, 6, is about 0.30, and calculated from this a quantum yield of Vg. 

 Equation (29.1) gives, however, 7 = 0.030/2.12 = 0.U5, or approximately 

 }i. Tonnelat's error was probably due to the use of an incorrect value, 

 8.9 X 10-12 instead of 7.9 X lO"^" erg/mole, for the heat effect of photo- 

 synthesis. 



3. Quantum Yield of Bacterial and Algal Photoreduction 



French (1937^), who worked in Warburg's laboratory, used the bacterial 

 species Streptococcus varians to study the quantum yield of the reduction of 

 carbon dioxide by molecular hydrogen (cf. chapter 5, page 104). The rate 

 of hydrogen disappearance was determined manometrically. The lines 

 852 and 894 m^ were isolated by filters from the fight of a cesium lamp; 

 from 17 to 58% of this light was absorbed by the suspension. The quan- 

 tum yields calculated by French ranged from 0.07 to 0.23 molecule of carbon 

 dioxide {i. e., from 0.14 to 0.46 molecule of hydrogen) transformed per 

 quantum, depending on the pretreatment of the bacteria. French con- 

 sidered these experiments proof that carbon dioxide reduction by Athio- 

 rhodaceae requires four quanta per molecule of carbon dioxide, similarly to 

 the assimilation of green plants according to Warburg and Negelein. As 

 mentioned before, the fight curves obtained by French in this work 

 were sigmoid: the y,naz- values were derived from the maximum slope of 

 these curves (reached in the inflection point). This procedure requires 

 justification. To divide the increase in yield in a certain region of the light 

 curve by the corresponding increase in light absorption, and to call the quo- 

 tient "quantum yield" presupposes that the increase in light intensity pro- 

 duces a certain additional amount of photosynthesis, characterized by its 

 own quantum yield. Wassink, Katz and Dorrestein (1942) suggested 

 that this may, in fact, be the case, if the bacteria use, in weak light, mainly 

 intracellular organic compounds, instead of the externally supplied react- 

 ants. As light intensity increases, this photochemical process is soon light 



