ACTION SPECTRUM OF BROWN ALGAE 1171 



the ratios of the quantum yields in the bhie, violet, green and red, but, even 

 more strikingly, by the absolute value of the yield at 496 m/x, where, ac- 

 cording to their estimates, 93% of absorbed light is taken up by the carot- 

 enoids. They argued that, if all photosynthesis observed in this spec- 

 tral region were attributed to chlorophyll, the quantum yield would be 

 0.059/(1 — 0.93) = 0.84, i. e., much larger than the maximum allowed by 

 thermochemical considerations. However, this estimate was based on the 

 distribution data in figure 22.46, and therefore is subject to possible grave 

 errors. True, at 496 mn, the apportionment of energy is not very sensitive 

 to the postulated specific value of the "red shift"; it would remain almost 

 the same if a shift of 10 or 30 m/i were postulated, instead of 20 m/z (the 

 value used by Button and Manning), or if the shift of the carotenoid 

 bands — particularly^ those of fucoxanthol — were assumed to be twice or 

 three times as large as that of the chlorophyll bands. (A difference of this 

 type is indicated by some data in chapter 22; cf. Table 22. VI and page 

 706.) It may thus seem as if an extreme and unlikely assumption con- 

 cerning the enhancement of the absorption of blue-green light by chloro- 

 phyll in vivo, or the assumption of a spatial distribution of pigments strongly 

 favoring absorption by chlorophyll would be required to explain the quan- 

 tum yield observed at 496 m^t without recourse to sensitization by carot- 

 enoids. This argument, however, ceased to be quite conclusive, since 

 Strain and Manning (1942) confirmed the presence in blown algae and 

 diatoms, of a pigment with strong absorption in the blue-green, chlorophyll 

 c. According to figure 21.5 this component has an absorption peak at 450 

 m/x in methanol; in the living cell, its absorption maximum must lie near 

 470 m/x, if the shift is the same as for chlorophyll a. According to figure 

 30. 9C, at 470 mix, chlorophyll c in a methanol extract from diatoms accounts 

 for about ten times more absorption than chlorophyll a. The neglect of 

 chlorophyll c in the calculations of Button and Manning thus may have 

 shifted the ratio of the absorptions by the chlorophyll pigments and the 

 carotenoids, from perhaps about 1 to 1, to the extreme value of 9.3 to 0.7. 

 To sum up, the average 7 values found by Button and Manning sup- 

 port the assumption that the carotenoids in diatoms (and fucoxanthol in 

 particular) contribute directly to the sensitization of photosynthesis; but 

 the wide scattering of individual results called for reinvestigation with 

 material and methods giving more consistent results. Furthermore, all 

 results, and particularly the absolute yields at 496 m/x, were in need of re- 

 examination in the light of the possible role of chlorophyll c. This re- 

 examination could have conceivably brought the brown algae in line with 

 green algae — organisms in which a distinctly lower quantum yield of 

 photosynthesis was observed in the regions of the carotenoid absorption, 

 but a yield not sufficiently low to permit the assumption of complete in- 



