364 RADIATION BIOLOGY 



approach to it, in the region 420-520 m/i, is the optical-density curve 

 for the active component in Chroococcus (Fig. 6-86). Subtracting this 

 curve from that of the active components in Coilodesme (Fig. 6-7rfIT), we 

 obtain data that should represent the optical density of the active carote- 

 noids (see Fig. Q-7dlll). It is reahzed that this curve can be improved 

 upon with more detailed measurements, which would give a better chloro- 

 phyll a curve. 



The active-absorption curve obtained for chlorophyll a between 580 

 and 700 m/x, calculated from Coilodesme that contains no chlorophyll b, 

 has been used similarly to subtract from the curve for the active com- 

 ponents in Ulva. The result should represent an approximation to the 

 absorption curve of chlorophyll b in living algae (see Fig. 6-7c). 



In the action spectrum of Haxo and Blinks (1950) for photosynthesis 

 of Porphyra naiadum. (see Fig. 6-8a), the photosynthetic function of 

 phycoerythrin (500-570 m^u) and of phycocyanin (620 m/x) is seen to 

 exceed that of chlorophyll (430 and 670 m/x). The phycoerythrin of 

 Porphyra naiadum is different from other phycoerythrins in that its 

 490-mM band is inconspicuous. In Fig. 6-8c is the absorption spectrum 

 of the active and, by difference, that of the inactive pigments in this 

 species, as calculated from Eq. (6-9). Strangely enough, the curve for 

 the inactive pigments is composed mainly of chlorophyll but does also 

 include some carotenoid absorption around 500 m^. In some unpub- 

 lished experiments C. Yocum and L. R. Blinks (1950) have found that 

 red algae may be adapted by growth in red light so that the inactive 

 chlorophyll becomes functional. This adaptation process is reversed if 

 the algae are exposed to strong green light. An illustration of photo- 

 synthesis due to absorption of light by phycocyanin as well as by chloro- 

 phyll and to inactive absorption by a carotenoid is found in Fig. 6-86, 

 calculated from the data of Emerson and Lewis (1942). 



A red alga with a high content of phycoerythrin and a low phyco- 

 cyanin content, Delesseria decipiens, was one of the many species used in 

 the work of Haxo and Blinks. Curves for the active and inactive pig- 

 ments of this alga have been calculated from their data and are given in 

 Fig. 6-8f/. The derived absorption curve for the inactive pigments of 

 this alga seems to be a reasonable approximation to that curve so basic 

 to the present subject — the in vivo absorption spectrum of chlorophyll a. 



FLUORESCENCE SPECTRA OF PHOTOSYNTHETIC PIGMENTS 

 FLUORESCENCE SPECTRA OF THE EXTRACTED PIGMENTS 



Absorption spectroscopy is widely used for the identification and quan- 

 titative determination of biological pigments. Far less use has been made 

 of fluorescence spectroscopy, largely because of the lack of adequate com- 



