QUANTUM YIELD OF BACTERIAL AND ALGAL PHOTOREDUCTION 1127 



ties, and investigations of the effect of intensity and duration of illumination 

 on the shape of the light curves, could help to elucidate the situation. Un- 

 til there is proof that the sigmoid shape of the light curves actually is due to 

 an internal photochemical process resulting in no (or only little) gas con- 

 sumption; and until it has been proved that this process, having become 

 saturated in very low light, continues at a constant rate as the light grows 

 stronger, the legitimate way to interpret the light curves is the conserva- 

 tive one: to consider the sigmoid shape as evidence that the average 

 quantum yield of photoreduction, y, first increases with light intensity and 

 then decreases again. The measure of j in each point of the curve then is 

 the slope of the straight line drawn from this point to the origin of the co- 

 ordinates, and not the slope of the tangent. (Similarly, we do not attri- 

 bute the convex part of the light curves to a superposition of low-yield 

 photosynthesis upon persisting high-yield photosynthesis, but to a decrease 

 in the average yield.) 



In this way, we can deduce from the sigmoid light curves only a lower 

 limit of the maximum quantum yield (this limit being given by the slope 

 of the tangent to the curve that passes through the origin of the coordi- 

 nates). This limit, derived from French's light curve of Streptococcus 

 varians, is about jnm. = 0.11. 



In French's study of Spirillum ruhrum (1937^), the yield was measured 

 by the uptake of carbon dioxide. As mentioned above, the light curves 

 showed, in this case, no initial curvature; their slope corresponded to a 

 quantum yield of the order of 0.06 to 0.Q7. These values were termed 

 "unreliable" by French because of inexact determinations of light absorp- 

 tion. Subsequently, however, yields of similar magnitude were found in 

 several investigations by the Dutch group (Wassink, Katz and co-workers). 

 In their measurements, the initial concavity of the light curves often was 

 only slight, and did not affect essentially the calculated quantum yield. 



Eymers and Wassink (1938) measured the quantum yield of photosyn- 

 thesis by Thiorhodaceae, with thiosulfate serving as a reductant and 

 a cesium or sodium lamp as light source. The results are shown in Table 

 29. VIII. One notices that these organisms have a very strong dark meta- 



Table 29.VIII 



Quantum Yields of Cakbon Dioxide Reduction by Purple Bacteria 

 WITH Thiosulfate as Reductant (after Eymers and Wassink 1938) 



" Quanta/molecule CO2. 



^ In old cultures, I/7 values up to 175 were observed. 



