ABSORPTION BY CAROTENOIDS IN BROWN ALGAE 723 



and accounts for about 50% of total absorption between 500 and 450 m^; 

 below 450 m^u, the absorption by the chlorophylls again becomes predom- 

 inant. 



Figure 22.44 is an attempt to interpret the conditions in living cells. 

 It shows the transmission spectrum, log (To/T), of the cell suspension — 

 it would be better if it were the absorption spectrum, log I/S. Also shown 

 is a composite absorption spectrum of the pigments obtained by shifting 

 the chlorophyll bands, derived from the preceding figure, above 550 m^u, 

 toward the red by 10 m^, and below 580 m^, by G m^ (it would be simpler 

 to make the plot on the frequency scale and use a uniform shift!); the 

 carotenoid bands, derived from the preceding figure, were shifted by 14 m/x. 

 (The values used for chlorophyll may be a little low; cf. page 706.) 



Two breaks in intact cell curve are at points where cell suspension was 

 stirred and filters in monochromator were changed. The dotted curve 

 shows fraction of total absorbed light absorbed by carotenoids, based on 

 the curve for extracts after introduction of wave length shifts. 



The difference between the composite absorption curve for total pig- 

 ments and the — much more diffuse — transmission curve of the cell sus- 

 pensions can be due (as discussed in part B) partly to scattering and partly 

 to intrinsic changes in band shapes. A theoretical treatment, as de- 

 scribed on pages 711-714, could eliminate the scattering effect and reveal 

 more clearly the intrinsic alterations of the pigment spectra; but no at- 

 tempt was made to take scattering into account, and the proportion of 

 light absorbed by the carotenoids was calculated simply by comparison of 

 the absorption curve of the combined extracts in figure 22.44 with those of 

 the individual extracts in figure 22.43 (after appropriate "red shifts" of 

 the latter). The results, indicated by the dotted line, show 40% absorp- 

 tion by the carotenoids at 520 m^, a maximum of 75% at 500 mju, a second- 

 ary maximum of about 55% at 460 m/x and a decline to about 25% below 

 440 niM- According to these results, carotenoids contribute significantly 

 to the total absorption of plastid pigments in Chlorclla from 530 m^t down- 

 ward. 



5. Absorption by Carotenoids in Brown Algae 



Brown algae (including diatoms) contain no chlorophyll b, and should 

 thus, according to page 720, transmit more freely than green plants in the 

 region 460-540 mix. Instead, as their color shows, they absorb consider- 

 able amounts of green light (510-580 mju) — much more than do the green 

 cells. This must be ascribed to the presence of a specific carotenoid, 

 fucoxanthol. If figure 21.35A is correct, and the absorption spectrum 

 of fucoxanthol in solution does not extend toward the red any further than 



