814 FLUORESCENCE OF PIGMENTS 7A^ VIVO CHAP. 24 



length of the exciting hght on the fluorescence of live cells. Vermeiilen, 

 Wassink and Reman (1937) found that the spectral distribution of the fluor- 

 escent light of Chlorella and Chromatium is independent of the wave length 

 of exciting radiation. The quantum yield of fluorescence (<^) also was ap- 

 proximately constant, between 442 and 624 mju in Chlorella, and between 

 450 and 750 ray. in Chromatium. However, a slow systematic decrease of 

 fp was observed in Chlorella at the shorter waves — a trend that became 

 accelerated below 424 m^u (fig. 24. 5A). In Chromatium., maxima and 

 minima of (p were observed in two or three places in the visible spectrum 

 (fig. 24. 5B). The Dutch investigators concluded from these observations 

 that the quantum yield of chlorophyll fluorescence in vivo does not depend 

 on wave length except when carotenoids interfere with the light absorption 

 by chlorophyll or bacteriochlorophyll. In green plants, this occurs only 

 below 520 mju ; the carotenoids of purple bacteria, on the other hand, have 

 absorption bands in the green, yellow and orange {cf. Table 21. IX and 

 fig. 22.27), and these bands could perhaps account at least for the first 

 minimum of the fluorescence yield noticeable in figure 24.5B at about 550 



mju. 



While qualitatively the conclusions of Vermeulen and co-workers ap- 

 pear plausible, quantitative considerations lead to some interesting compli- 

 cations. In Chlorella, for example, ^ declined, in the violet, by only 10 or. 

 20%, while figure 22.43 indicates that the carotenoids must account for at 

 least one third the total absorption in this region! It thus appears as if 

 the chlorophyll fluorescence can be excited, with considerable probability, 

 also by the light absorbed by carotenoids! This hypothesis has been 

 strikingly confirmed by experiments with fucoxanthol-containing diatoms. 



The results of these experiments, carried out by Dutton, Manning and 

 Duggar (1943), are shown in Table 24. III. They indicate that the yield of 

 chlorophyll fluorescence is the same, whether it is excited by red light, ab- 

 sorbed exclusively by chlorophyll, or by blue-green light (470 m^u), three 

 quarters of which probably is absorbed by carotenoids, mainly fucoxan- 

 thol (see fig. 22.46 and figs. 30.9B and C. Table 24.III also contains 

 new results with Chlorella, which confirm the conclusions drawn above from 

 the earlier work of the Dutch observers. These results indicate that the 

 light absorbed by carotene and luteol is almost — but not quite — as ef- 

 ficient in the excitation of chlorophyll fluorescence in green algae as the 

 light absorbed by fucoxanthol in diatoms. In striking contrast is the 

 result of the experiment with an acetonic extract from Nitzschia ; here, light 

 absorbed by the carotenoids is completely lost for fluorescence. The ex- 

 periment with acetonic solutions of chlorophylls a and h shows that the 

 quantum yield of chlorophyll fluorescence in solution does not increase with 



