ACTION SPECTRUM OF RED AND BLUE ALGAE 1183 



may perhaps even make that of chlorophyll superfluous, although so far no 

 chlorophyll-free red algae have been found. 



We Avill see below, however, that a different — and no less striking — 

 interpretation of these results is possible. 



In one species of Iridophycus, which bleaches to almost green color at 

 high tide, a much greater participation of chlorophyll in photosynthesis 

 was noted in rough experiments. 



Haxo and Blinks said that, in contrast to Emerson and Lewis's results on 

 Chroococcus, they found only a weak chlorophyll activity also in two blue- 

 green algae, Anaboena and Oscillatoria, whose action spectra were similar 

 to those of Porphyra perphoraia (fig. 36.11F). They suggested that culture 

 conditions may affect the relative activity of different pigments in algae of 

 the same class or even the same species. 



Haxo and Blinks noted that the saturation rate of photosynthesis of 

 Delesseria is the same in blue light (565 niju) as in red light (672 ran) '• This 

 seems to indicate that the enzymatic mechanism of photosynthesis — or, 

 at least, the rate-limiting enzymatic reaction — is the same whether the 

 quanta are absorbed by a phycobilin or by a chlorophyll. 



In chapter 24 (p. 815) we desciibed the fluorescence studies of French 

 et at. (1951) and Duysens (1951) that indicated effective transfer of excita- 

 tion energy in red algae from carotenoids and phycobilins to chlorophyll 

 a, and (in certain of them) from chlorophyll a to d (despite the low con- 

 centration of the latter). If one assumes that transfer to chlorophyll d 

 constitutes a "leak" which makes energy unavailable for photosynthesis, 

 the results of Blinks and Haxo become understandable. The question 

 remains why energy transferred to chlorophyll a from phycobilins is not 

 also lost to chlorophyll d but remains available for photosynthesis. It w'as 

 noted on p. 815 that this energy stays with chlorophyll a long enough to 

 cause its fluorescence (Avhile most of the energy absorbed by chlorophyll a 

 itself causes the fluorescence of d). Two suggestions were made there as 

 to the possible reasons for this difference in the fate of excitation energy; 

 but further study is needed for a convincing explanation. 



Duysens' observations provide the strongest argument at present in 

 favor of assuming that chlorophyll a is the one pigment directly par- 

 ticipating in photosynthesis, and that not only the carotenoids, but also 

 the phycobilins, sensitize photosynthesis by transferring their excitation 

 energy to chlorophyll a. Another argument supporting this view is the 

 observation of French and co-workers (1951) that the yield of fluorescence 

 of the phycobilins in algae shows none of the induction effects and of the 

 peculiar changes with hght intensity which were discussed at length in 

 chapters 24 and 28 (part B) and are indicative of an intimate relationship 

 between chlorophyll and the chemical processes of photosynthesis. 



