LIGHT CURVES OF FLUORESCEXCE 



1053 



influence of the reductant is much stronger than that of carbon dioxide. 

 Figure 28.34 shows the "critical" intensity, /<-, (fig. 28.27) in relation to the 

 thiosulfate concentration. It rises from 2 kerg without thiosulfate to 10 

 kerg in 0.5% thiosulfate, and then becomes more or less constant. Figure 

 27.13 indicated that thiosulfate "saturation" of photosynthesis occurs in 

 about the same concentration region. 



Wassink and co-workers noted, however, that, with hydrogen as reduc- 

 tant, at pH 7.6, the transition point of fluorescence was markedly higher 

 than the saturation point of the gas exchange. 



14 



14 



I2[- Hydrogen, 15% 

 o with CO2 

 A without COj 



xlO" 



INCIDENT INTENSITY, erg/cm^ sec 



Fig. 28.30. Effect of C0-> on fluorescence of purple bacteria in presence of 

 reductants (after Wassink et al. 1942). pH 6.3, 29° C, 



The characteristic initial bend upward of the fluorescence curves of 

 Chromatium does not quite disappear even in complete absence of reduc- 

 tants. This can be attributed to the presence of "internal reductants," 

 which in weak light suffice to prevent a complete conversion of the "photo- 

 complex" into the strongly fluorescent form. The linearity can in fact be 

 improved by preliminary starvation of the bacteria, depriving them of 

 metabolites that could serve as internal reductants. 



Figure 28.35 shows that the effect of the reductants on the fluorescence 

 of Chromatium persists even in the absence of carbon dioxide. This fact 

 must be compared with the above-mentioned observation (fig. 28.29) that 

 the removal of carbon dioxide had no effect on fluorescence in the absence 

 of a reductant. We will return to the discussion of this interesting differ- 

 ence on page 1077. 



